Part I: The Construction of a Model-Locked Nd³⁺: Glass Laser and Non-Linear Optical Techniques. Part II: Applications of Picosecond Laser Pulses in Chemistry: Vibrational Relaxation Times in Liquid Alkanes and Alkenes Citation Patumtevapibal, Saon (1975) Part I: The Construction of a Model-Locked Nd³⁺: Glass Laser and Non-Linear Optical Techniques. Part II: Applications of Picosecond Laser Pulses in Chemistry: Vibrational Relaxation Times in Liquid Alkanes and Alkenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/sska-f577. https://resolver.caltech.edu/CaltechTHESIS:02172022-193613745 Abstract PART I. The construction and qualitative explanation of the pulsed, mode-locked laser are described: the generation of a train of picosecond 1.06μ pulses is achieved by properly aligning a saturable absorber in the Nd 3+ : glass laser cavity. The pulsewidth, being on a picosecond time scale, has to be measured' by a special two-photon method. In order to make the laser more chemically useful, second harmonic generation of the fundamental (1.06 μ) pulses is necessary. A phase-matched KDP crystal is employed in this process. Some non-linear optical techniques, such as stimulated Raman scattering and self-phased modulation, which generates continuum light from a monochromatic pulses, also enrich the usage of the laser. Azulene experiment is tried with our laser set-up. PART II. The dephasing times and vibrational lifetimes of C-H stretching vibrations are studied systematically in a series of liquid alkanes and alkenes, using the Raman effect. The results indicate that the vibrational energy loss takes place primarily through the methyl groups in these molecules. A preliminary result of the methylene C-H stretch vibrational lifetime is conducted in liquid CD 3 -CH 2 -CH 2 -CD 3 Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Chemistry) Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Robinson, G. Wilse Thesis Committee: Unknown, Unknown Defense Date: 28 June 1974 Record Number: CaltechTHESIS:02172022-193613745 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:02172022-193613745 DOI: 10.7907/sska-f577 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 14501 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 18 Feb 2022 00:08 Last Modified: 05 Aug 2024 23:16 Thesis Files PDF - Final Version See Usage Policy. 31MB Repository Staff Only: item control page

PART I : THE CONSTRUCTION OF A MODE-LOCKED Nd 3+: GLASS LASER AND NON-LINEAR OPTICAL TECHNIQUES PART II: APPLICATIONS OF PICOSECOND LASER PULSES IN CHEMISTRY: VIBRATIONAL RELAXATION TIMES IN LIQUID ALKANES AND ALKEN ES Thesis by Saon Patumtevapibal In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 91109 1975 (Submitted June 28, 1974) -ii- 11 I REGARD NO MAN AS POOR WHO HAS A GODLY MOTHER" - Abraham Lincoln This thesis is dedicated to my mother. -iii- Acknowledgements First of all, I would like to thank my family and my best friend for their love and care, for waiting so patiently, and also for all the loving letters that have brightened my gloomiest days. I wish to thank my adviser, Professor G. W. Robinson, for his help and encouragement, and for teaching me to have fun with science. I am also greatly indebted to Dr. Paul R. Monson who has started the picosecond spectroscopy work and taught me all the techniques in the laboratory. Part II of the thesis owes much to Dr. Kenneth J. Kaufmann, with his enthusiasm and humor, for useful and constructive discussions. The laboratory works owe a great deal to Mr. W. Scheulke and the instrument shop, Mr. E. segal and the glass shop, and also the electronic shop, all with friendliness and willingness. I wish to thank Professor H. B. Gray and his group for the facility in using the Cary 14 and the cold-temperature dewar. I thank C. I. T. for the GTA, GRA, research and library facilities, and the swimming pool. The scholarship from the Thai Govern- ment for both my undergraduate and graduate studies are gratefully acknowledged. The late Professor Satang Mongkolsuk, Professor George Jura, and the late Dr. M. J. Jorgenson, have contributed much to my science education, and led me the way to chemical resear.c h. And to all my teachers, this thesis is also dedicated. Last, but not least, I can only say that "I got by with little help -ivfrom my friends," With them, my Caltech years have been quite enjoyable. My very special thanks to Sister Gabriel Mary Hoare for providing my "second home" and many beautiful things. Her friend- ship has made my staying in the U. S. much more meaningful and less painful. -vAbstract PART I The construction and qualitative explanation of the pulsed, mode-locked laser are described: the generation of a train of picosecond l .06µ pulses is achieved by properly aligning a saturable absorber in the Nd 3+ : glass laser cavity. The pulse- width, being on a picosecond time scale, has to be measured' by -a special two-photon method. In order to make the laser more chemically useful, second harmonic generation of the fundamental (1.06 µ) pulses is necessary. employed in this process. A phase-matched KDP crystal is Some non-linear optical techniques, such as stimulated Raman scattering and self-phased modulation, which generates continuum light from a monochromatic pulses, also enrich the usage of the laser. Azulene experiment is tried with our laser set-up. PART II The dephasing times and vibrational lifetimes of C-H stretching vibrations are studied systematically in a series of liquid alkanes and alkenes, using the Raman effect. The results indicate that the vibrational energy loss takes place primarily through the methyl groups in these molecules. A preliminary result of the methylene C-H stretch vibrational lifetime is -vi- TABLE OF CONTENTS Page Acknowledgement iii Abstract v PART I: The Constructfon of a Mode-Locked Nd 3+:Glass Laser and Non-Linear Optical Techniques A. Introduction B. The Laser Theory: 1. 1 A General Description of a Solid 2 State Laser 2. Mode - Lacking of the Laser 9 3. Q-Switching Principles 12 4. Saturable Absorption and Passive 14 Mode-Locking Experiment: C. Picosecond Pulse Generation Two-Photon Method as a Pulse Width Measurement Experiment and Theory D. E. 20 35 Second Harmonic Generation Theory 43 Experiment 48 Two-Photon Method of Two Different Optical Frequencies 54 F. Synchronizatton of Two Optical Paths 59 G. Experiment on Radiationless Transitions of Azulene 65 -vii:page H. Generation of Continuum from 1. 06 Laser Pulses 71 References 78 PART II: Applications of Picosecond Laser Pulses in Chemistry: Vibrational Relaxation Times in Liquid Alkanes and Alkenes. A. Introduction 81 B. Historical Review of Vibrational Lifetime Works C. 83 Theory Dephas ing Time 84 Vibrational Relaxation Time 85 Stimulated Raman Scattering 85 Radiation Transfer Equations 93 D. Experiment Materials 98 Dephasing Time Measurement 99 Vibrational Relaxation Time Measurement at Room Temperature 101 Vibrational Relaxation Time Measurement at Low Temperature 102 E. Results and Discuss ions 105 Temperature Effect 112 -viii- Page References 115 PROPOSITIONS: Proposition 1 118 Proposition 2 121 Proposition 3 124 P ropos it ion 4 128 Proposition 5 138 -1- A. Introduction A typical solid state laser consists of an open optical resona- tor, which consists essentially of a pair of opposing flat or curved reflectors, and some laser gain medium within this resonator [l]. The arrangement of reflectors and the medium (a ruby or a Nd glass rod) constitute one form of Fabry-Perot interferometer. Standing light waves may be set up within the cavity at a series of discrete optical frequencies over which the gain of the laser medium exceeds the resonator losses. These frequencies are known as the Fabry-Perot modes, and the laser output consists of radiation at a number of closely spaced frequencies with variable durations (microsecond or less). The total output is dependent on many parameters, such as the amplitudes, frequencies, and relative phases of all of these oscillating modes. If there is nothing to fix these parameters, the random fluctuations and nonlinear effects in the laser medium will cause them to change with time and the output will vary in an uncontrollable way. If the oscil- lating modes are forced to maintain equal frequency spacings with a fixed phase relationship to each other, the output as a function of time will vary in a well-defined manner. 11 mode-locked or 11 11 phase-locked 11 The laser is then said to be [2]. The technique of mode-locking the laser is done by employing a sa turab le dye inside the laser cavity to 11 Q-swi tch II the laser (i.e., to cause the laser to emit a short high power burst of energy). The properties of the dye must be such that the relax~tion time is very short compared to the round trip time within the cavity length of the original laser pulse. By employing the right dye solution, one can -2- obtain the laser output consisting of a number of short, intense pulses of light, the duration of which is on the order of 10- 12 sec, and the peak power is on the order of l0 9watts. B. The Laser Theory l. A General Description of a Solid-State Laser A laser oscillator consists of an active medium, having a complex refractive index as given by n = n0 - ik (where n0 is the index of refraction of light in vacuum, and k = 2;), which is inserted between a pair of coaxial planes of spherical mirrors, separated by distance L, as shown in the diagram in Fig. l. The flash lamp, when triggered by a power supply, acts as an exciting source, inducing the population inversion in the upper energy state of the amplifying medium. There are in general two types of energy level structures: a three-level and a four-level type (shown in Fig. 2). Excitation is supplied to t he solid by radiation of frequen- cies which produce absorption into the broad band [4] (consider the four-level type as an example). Most of the absorbed energy is trans- ferred by fast, radiationless transitions into the intermediate sharp level [3]. heat. The energy difference is given up to the crystal lattice as The emission of radiation, associated with the spontaneous re- turn from level 3 to 2 , is ordinary fluorescence. cence will take place even at a low level of excitation. Such fluoresWhen the exciting radiation is sufficiently intense, it is possible to obtain -3- c 4 l~ LAH? ;J ,. L • nt::===::::r-------,_.,...-------.. ~ ~ 1 1 - -..,. Li AH1'LIF't1NG MATERIAL IOOf.R. ?ARTIAL HIRROR MtRROR Fi9ure1. A Simple lasev- Cavity LASER OUTPUT -4- .SPONTANEOUS SPONTANEOUS OECA~ OECA~ \ EXCITATION ~ @ SPONTANEOUS EXCITATION @ ANO 5,;IHULATfD SPOt-JfANEOu, AND STIHULATED EHI.SSION !Hl5SION I ,_,_..........,_______ © A THREE - L.e.VEL C.NeRG~ OIAlllAH Fi9uv-e 2 _..__..__ _ _ _---.,.--© _ _ _ _ _ _ _ (i) A FOUR-LEVEL E:NER6Y l>IAGRAH SiK1plif ied E~crgy Level Dia9r-aW1s of Solid, State Lasers -5- more atoms at level 3 than are left at ground state. The spontane- ously emitted photons travelling through the crystal will stimulate additional radiation, and emission thus induced is superposed on the spontaneous emission. This emission is the laser action. The laser light then travels back and forth inside the laser cavity (Fig. l). is y In each passage, the loss factor of the intensity per pass, due mainly to reflection losses, whereas the gain factor is eaL' a= gain per unit length = where k(v) 0 n' r ,r2 are reflection coefficients of the mirrors, 1 k(v) is the absorption in the unexcited laser material 0 at frequency v , n' is the relative population inversion l 91 = No (92N2- Nl) g ,g are the multiplicities of levels l and 2 (ground 1 2 and excited states), respectively N ,N 2 are the populations of energy levels l, 2 1 N is the total population. 0 If al > y , the intensity of radiation of the proper frequency will build up rapidly until it becomes so large that the stimulated transitions will deplete the upper level and reduce the value of a. FA&R't-PEROT RESONANCES ~IIH 1)114 1)....,. ' .. ~~1 ~... . ,,' ~+f .{ ·y ---r-· . .. O" THRE.SHOLD CAVlT'l' LOSS Fi9ure. 3 1\..i'l. "'•+rt l l·...., -.l··...··J . J ~~ 1)111-t GAIN i----. J_J _J_j _J_ I I 0) -7This is the dynamic situation that in most solid lasers gives rise to pulsations. On the other hand, if al< y , the intensity of radiation does not build up; so the threshold of the laser oscillation is required a L = y m (1) The resonances of such a system are determined by the spectral linewidth of the laser transition and by a number of half wavelengths existing between the two mirrors which form a type of Fabry-Perot interferometer. The gain of a laser line profile superimposed on the reson- ances of a Fabry-Perot interferometer is shown in Fig. 3. (2) where vm is the interferometer resonant frequency m is an integer representing the number of half wavelengths stored between the two reflectors L is the resonator optical length or optical distance between the two mirrors n 0 is the index of refraction inside the cavity which is assumed constant. In general, the optical paths between the mirrors is inhomogeneous (amplifying material and air); it is advantageous to introduce the optical distance L L" = f n (z) dz 0 integrated along the cavity length. Then (2) becomes -8(3) Since the threshold condition is met only in a restricted frequency range, laser oscillations will occur only for a few discrete frequencies vm which satisfies (3) and which lie in this restricted range. The separation between modes is given by f = C (4) 2[T so the round trip transit time is just, (5) Since the solid state laser resembles a long cylindrical cavity with conducting walls and parallel end surfaces, it is advantageous to introduce transverse electromagnetic modes to the laser wave inside the resonator. The longitudinal modes are those along the axis of the resonator, whereas the transverse modes refer to the one on the plane perpendicular to the axis. In the ordinary case the resonator output contains a number of longitudinal resonator modes (10 6 or more) [3]. An appropriate resonator must be designed for the laser to operate at a single longitudinal mode. For example, the laser resonator should be sufficiently short so that c/2L' gain of the laser material. However, for each longitudinal mode num- is greater than the bandwidth of the ber, there exists a set of transverse modes. For a curved mirror resonator with rectangular symmetry, the cross-sectional amplitude distribution is given closely by: -9- 2 2 2 m,n [Hm(-/2w x) Hn(/2wy)Jexp(-(x +y )/w ) A (x ,y) = A where (6) x,y are the transverse coordinates Am,n is a constant whose value depends on the field strength of the mode w is the radius of the fundamental mode (m=O,n=O) at 1/e maximum amplitude is the a th order Hermite polynomial with argument b Ha(b) m,n are called the transverse mode numbers, often referred to as TEM mn . Theoretically, the higher the TEM modes are, the more the energy of the light wave gets distributed into little modes away from the axis. Since the reflectors have finite dimension, some of the beam energy will not get intercepted and hence will be lost. Also, it turns out that for a good mode-locking pattern, it is best to operate at TEM 00 or as low TEM modes as possible. Hence, it is necessary to use some device which will give high losses to all transverse modes except the desired one. Since the higher order modes spread further from the axis, the easiest way to obtain the single TEM mode is to use an aperture whose size is large enough to let TEM 00 mode through, but small enough to increase substantially the losses of the higher order modes. 2. Mode-Locking of the Laser In general, a solid state laser is an inhomogeneous [5] laser. The oscillating modes (the ones that lie in the region where the gain curve is above the loss line in Fig. 3) may vary in frequency spacing, -10- rel ative phases, and amplitude. This causes the intensity of the laser output to fluctuate randomly and greatly reduces its usefulness. In order to fix the relative phases or phase-lock the laser, a modulator is needed inside the laser cavity. It is reasonable to assume that one resonant frequency v m. will be nearest the peak of the laser gain profile and will therefore be the first to oscillate. ing at a frequency the frequency If an electro-optic amplitude modulator [6] operatf is inserted in the laser feedback interferometer, vm wi 11 be amplitude-modulated at a frequency f , such that the time dependent intensity output will be of the form [6] Io 1 -I . ~ -2 [l + M sin 2nf t] (7) 1 where E2 Io is the output intensity a: I. is the input intensity a: M is the modulation index ( i . e. , the degree of modulation) 1 0 E~1 From (7) E02 a: E~(t)[l + M sin 2nf t] 1 (8) Now (9) ( l O) In general, M << 1 using Taylor s expansion (10) becomes, 1 ( 11 ) -11- By expansion of Eq. (11), the equation of a wave with a simple sinusoidal amplitude modulation can be written in the form (12) If the modulating frequency f with the axial mode frequency spacing is chosen to be commensurate f = 6f (Fig. 3), superposition of the upper (vm+f) and lower (vm-f) side bands of the amplitude modulated light beam with the adjacent resonances, couples the (vm+f), (v m-f), and vm axial modes with a well-defined amplitude and phase [7]. As the (vm+f) and (vm-f) oscillations pass through the modulator, they also become amplitude-modulated; their sidebands, in turn, couple the (v m+2f) and (v m-2f) axial modes to the previous three modes. This process continues until all axial modes falling within the oscillating linewidth are coupled. In order to compute the peak power of the pulses, the phases are made equal to zero and, writing the total optical field in the simplest form, E(t) = (N-1)/2 I e 2ni(v +nf)t m ( 13) - ( N- l )/ 2 = where e iw t n sin Nwt/2 sin wt/2 ( 14) N is the number of modes locked together; the average laser power output is proportional to E(t)E * (t): -12- 2 P(t) cr sin (Nwt/2) s i n2 (wt/2) thus the peak power is ( 15) N times the average power. The above di s cuss i on i s ca 11 e d act i ve mo de - l ock i ng II experiment a "passive mode-locking 11 11 technique is used. • In our They have, how- ever, the same principle, as will be seen later in Section 4. Since the 11 passive modulator" which is utilized in generating picosecond pulses serves both as a Q-switch and an amplitude-modulator, it is appropriate to first discuss the Q-switching principles [8]. 3. Q-Switching Principles It has been shown in Eq. (1) that the laser reaches the thresh- old of oscillation when a Cl= where the quantity a reaches the value ( 16) y/L is proportional to the population inversion N=N In order to relate lifetime (photon lifetime) ~-N 2 92 1 Q to the loss of the resonator, a decay tc of a cavity mode needs to be defined as follows: ( 17) where £ is the energy stored in the mode. tensity loss per unit time as We can also write the in- -13- (18) From (17) and (18), (19) The quality factor is defined as [8] w E, d £ I dt where (20) is the rate of energy dissipation. By comparing (19) and (20), we obtain ( 21 ) (22) or (23) The Q-switching technique is done by keeping the Q factor of the resonator very low during pumping, which will increase the loss rate of a system (Eq. (22)) so that the population inversion can build up to a very high value without oscillation. energy reservoir. This high inversion serves as an When the inversion reaches its peak, the restored suddenly to the (ordinary) high value. Q is The gain per pass in the laser medium is now wel_l above (ordinary) threshold (Eq. (23)). This causes the extremely rapid build-up of the oscillation and a -14- simultaneous exhaustion of the inversion by stimulated 2➔ transition. This process converts most of the energy that was stored by atoms pumped into the upper laser level into photons, which are now inside the optical resonator. All this happens in an extremely short time, resulting in very short pulsewidth (nanoseconds), high power, and intensity. Q-switching may be accomplished by changing the reflectivity of one of the mirrors, by inserting or moving the diaphragm, by changing the path of rays between the mirrors, and also by changing the transparency of the material within the laser cavity. For the efficient production of the single large pulse, it is essential that the Qswitching process be fast compared to the photon-decaying time in the cavity, and that the time of switching from low Q to high Q be chosen so as to assume the accumulation of the greatest possible inversion of the material. Most typical pulsewidths are 10 to 50 nsec, with the pulse peak on the order of megawatts. 4. Saturable Absorption and Passive Mode-Locking [9] Mode-locking a laser oscillator with an active modulator requires the critical adjustment of mirror spacing and modulatlng frequency. In order to avoid this problem, a passive modulator which simultaneously serves as a Q-switch and as a~ amplitude modulator is utilized. Saturable Absorption [10] If a material is normally absorbing at w due to the transition between levels ll> and 12> , the rate of absorption may become so -15great at high intensities that a sizable population the excited level N builds up in 2 12> • Since the absorption rate is proportional to the population difference (N 1-N 2 ), the absorption coefficient will decrease at high intensities. Thus, at optical frequencies, the material · acts as a bleachable filter, i.e., it absorbs at low intensity and becomes transparent at high intensity. Such an absorber turns out to be well suited for the role of passive modulator in mode-locking lasers. However, certain properties are required: 1. That it have an absorptioD line at the laser wavelength; 2. That it have a linewidth equal to or greater than the laser l i newi dth; 3. That the dye-recovery time be shorter than the loop-time delay (the time it takes for a pulse to travel back and forth between the two reflectors) of the laser. The simple mode-locking laser set-up is shown in Fig. 4. Consider a pulse propagating back and forth in the laser cavity. As the pulse propagates through the absorber (assuming a two-quantum level system), the leading edge (which is the low-intensity part) of the pulse is heavily absorbed and molecules make transitions to the upper state. As a result, the absorber tends to saturate (i.e . , becomes transparent only at high intensities). opening of the passive modulat~r. This process represents the After the pulse has propagated through the system, the atoms make transitions back to the ground state which corresponds to the shutting of the modulator. As the pulse is reflected back and forth between two mirrors, more and more of the low -16- e J PA1tTIAL HIRROR ---.\.----~s~---r-or--•• OUTPUT LASER MEDIUH .SATURA&LE Aa50R&E~ Figure 4 A Hod.e-Locked. Laser -17intensity parts of the pulse get absorbed, until the sharpness of the pulse continues to increase to the point that harmonic content of the pulse equals the overall bandwidth of the system. the pulse cannot become narrower. When this occurs, This pulse is then let out of the cavity through the partial reflector. When viewed from the phase relationship of the modes in such a pulse, the simplified explanation of the mode-locking of the pulse can be given with the aid of the diagram in Fig. 5. When the laser action begins, several axial modes are inevitably excited in the resonator, and the beats (or couplings) between them give rise to intensity fluctuations with a characteristic time f\, T/m , where T is the time necessary for the light to travel through the resonator; and m is the number of the excited modes. , In the region of the linear development (the ordinary absorption of the dye), natural mode selection by the amplifying medium takes place, owing to the preferred amplification of the modes near the center of the gain line. frequency If an optical carrier v0 (which is the mode at the center of the gain line in this case) along with two sidebands v ±f 0 are superimposed, an amplitude modulation of the light results at a frequency f = ~f with some peak I [see Figs. 5b and 5c]. to peak variation M, and a peak intensity 0 When this beam is passed. through a saturable absorber having the typical characteristic illustrated by Fig. 5a, and the relaxation time is much shorter than T/m , then the absorber builds up intensity fluctuations in the nonlinearity region . As a result, the most intense fluctuation parts become stronger and are compressed much more rapidly, and the peak to peak excursi-0ns of the fluctuations will be increased, -1 8- lWTENSIT~ (a) SATUAA&e A&sOR&e~ SAiU~A&L£ A&SOR.l>ER (C) Figure 5 -19i.e., M2 > M1 , and the time duration of the fluctuation will be shorter as a result of the nonlinear transmission characteristics of the saturable absorber. With the sharpening of the amplitude varia- tion, additional sidebands are added to the spectrum. cavity of length In the optical L = c/26f , this process is repeated over and over again by reflecting the light beam back and forth between the two mirrors. The fluctuation will continue to sharpen until a discrete pulse is circulating in the cavity. The laser media provide gain to compensate for the residual saturation loss of the absorber and the mirrors. The circulation rate 6f is given by c/2L . will eventually acquire a steady-state width bandwidth of the laser media. 6T The pulse detennined by the The repetitive output pulse train emit- ted from the laser mirrors will have discrete spectral components defined by the Fabry-Perot resonances of the cavity extending on either side of v 0 for a bandwidth 6V , ±m6f as illustrated in Fig. 5c. The frequencies are all multiples of 6f, and the narrower the pulse width 6T , the larger the bandwidth required for reproducing the repetitive pulse, i.e., The magnitude and characteristic of pulse sharpening depends mainly on two factors: 1. The relaxation time of the saturable absorber Tb compared to the pulse width 6T is, the shorter and more sharpened as The shorter the relaxation time the pulse will be. When the bleach- able absorber relaxes to the initial state within the time interval -20- Tb << 6~ , the absorber is predominantly bleached by the most intensive peaks. As a result, the pulse width is reduced (all other parts of the pulse being absorbed) and a significant intensity discrimination of large peaks against the background of numerous weaker peaks takes place. On the other hand, when Tb~ 6T or when the inertia of the absorber becomes appreciable, the absorber does not have time to one trailing edge of the pulse. 11 collapse on 11 As a result, absorption occurs only on a leading front, and the trailing edge of the pulse remains practically undeformed, as has been shown experimentally by [9d]. For the best non- linear absorbers that are available for ruby and Nd-glass lasers, the values of Tb are: for Kodak dye #9740 = 35 psec; for Kodak dye #9860 = 6 psec; DDI = 14 psec; cryprocyamine = 22 psec [11 ,12]. 2. The number of passes of the pulse through the absorber in the laser cavity. Experiment The Nd 3+:glass laser rod is used as the laser amplifying medium in our experiments. The Nd 3+:glass Picosecond Pulse Generation. energy levels and laser lines are shown in Fig. 6. The laser operation in these transitions depends on the suppression of the favored 4F ➔ 4r 1112 transition by proper choice of frequency dependent mir312 rors. Moreover, the 0.918 µm line cannot be observed above 77°K. The main advantages of glass as a laser host as opposed to the crystal (YAG or v3A1 5o( 2 ) for example) are: l. flexibility in size and shape and excellent optical activity; 2. flexibility in refractive index, which may be varied from -21- , 10 ~ ~ 0 0 0 ~ ~ ~ t1"' ~ ~ fl) ...; ~ ~ <r 0 Cl w 2 5 "' Figure fo • Levels aMt! Groups of Ntf·: Glass LaseY' Lmes < Eack Level Skown ReprescMts a Group of Close I~ Spaced, Levels). -22approximately 1.5 to 2.0 by selection of the glass. It is possible to adjust the temperature coefficient of the index of refraction so as to produce thermally stable optical cavities, which is very critical in mode-locking the laser. 3. Glass hosts produce broader spectral lines than crystal. This is useful because it provides for absorption of a greater position of the pumping radiation. Also for the mode-locking operation, the broader spectral lines (containing more modes) lead to the shorter pulse width (initial T/m or final 6T ~ l/6v as discussed above). The disadvantages are: l. The low thermal conductivity of glass which imposes limitations on continuous operation or high repetition rate operations. 2. The broad spectral lines make it more difficult to attain threshold because a larger' inversion is required to compensate for the lower peak of the lineshape. In this laboratory a Nd 3+:glass rod, 8 inches long and 1/2 inch in diameter is employed. The rod is constituted of 3.1 wt.% of Nd 3+ ions which gives 2.83 x 10 20 ions/cc glass. (1/3) is 300 microsecond. 1.4 joules. The fluorescence decay time The threshold required for this laser is Both faces of the rod are polished with extreme care (a scratch on the face may cause great damage to the rod when high-powered laser pulses travel back and forth through the rod in the cavity), and cut at Brewster angle: 57°16 1 . The rod must be cleaned by using a dust- free microduster each time before operation. The excitation source or the laser head is a helical xenon flashlamp (by Korad Laser Systems, Union Carbide) which is connected to a 5 KV power supply (Korad, model -23K-l~ The laser rod is placed inside this laser head and is cooled by filtered water (chemical ion-free)--at the rate of about 2 gal/min (ITT self-pumping pump; the rate is controlled by a Variac set at 120V out for 120V in). A 99.99% R (at 1.06µ) lOM radius mirror and a 55% R (at 1.06µ) wedged mirror (both from Optical Coating Division) are used as the back and front reflectors for the laser cavity, respectively. The Q-switch dye solution is Kodak #9860 which is light- and acid-sensitive. The solvent used to dilute the dye down to the appropriate concentration for the laser cavity is acid-free dichloroethane (provided with the #9860 dye). A 40 mm diameter; 1 ,0 cm thick (Pyrex Opticell) is used as the dye cell. The cell is placed at the Brewster angle, 55°41' to the laser light so as to eliminate the unwanted reflection back to the rod. At the Brewster angle, the optical path length of the cell becomes 1.229 cm (see Figs. 7 and 8). 70% Tat 1.229 cm path length of the dye solution is made up using dichloroethane as solvent. The absorption is checked by Cary 14 (infrared region adapted) at only one wavelength of 1.06µ . Result (from the first run as an example): absorption at 1.0 cm path length= 0.125 at 1.229 cm path length = 0.125 x 1.229 = 0.1533 which is 70.30% T The #9860 solution is very strong; in order to obtain 100 ml of the desired solution, only 1.35 ml of the dye is needed. Figure 9 shows the result for this run from Cary 14. Aligning the laser cavity. A He-Ne laser (at 6328~, 1.0 mwatt, Astrophysics model 133) is used to align the cavity as shown in Fig. 10 .... -l ... ....i, ... Figure 1. ,-ru; . . . Jr', ..... . .1.. . . ...... # Tkc Nd.~•: Glass Laser Roc.L Placed. at Brewster A~9le I I N +:::, -25- I I I I , Figures. . , 8& : 55 41 AT 1.0'1)-(, A Saturable Absorber Cell, Filled. wit~ Kodak Dye# qse:,o i~ Dic~lometkaa,,e., at Stewster Av\gle. ( 8s) . - 26 - . 05 0.4 0.3 0.2 --, --· .. - A . . . . _A.. ..-~- ,a . .. - ..... .........__. --.,, .. --.- ..a.a ~ ·--· --- • .A .....L..-..JUi. _ _.. V ~ 'II ... - ,,.. .. ~,... ~..A. 0.1 0 .& - • . ~ . - ... . .... Figure 9 . Absorption Spectrum of # 9860 Dye So l ution from Cary 14 set at 1.06µ. -27- A - - - U S E R CAVJTV - - - - . . . Figuye 10. T~e Laser Cavit~ Aligt1me'Ylt -28- It is to be noted here that all our optics are mounted on magnetic bases (from Rutland Company, Japan), which are placed on a 4' x 9' steel board on top of a wooden table. A piece of foam is in- serted between the board and the table in order to absorb the vibration from the floor. The center of the laser rod is 5-3/16 above the steel 11 board, which becomes the height of our laser beam. The alignment is such that the reflections of the He-Ne laser from the mirrors are on top of the laser beam itself at the laser output (point A in the diagram). The He-Ne laser is bolted on one corner of the table; the laser beam is aligned by moving the laser and also mirror M until the beam 1 becomes level. One must check, also, whether the He-Ne beam is on the axis of the mode-locked beam. target for 1.06µ pulse. on the photograph. This is done by using a photograph as a The high-powered pulses will make a burnt spot Then unblock (the He-Ne beam is always blocked when the laser is fired) the He-Ne beam and see whether they are superimposed. This should be done at far field in order to increase the accuracy. If " they do not superimpose, the correction can be made by moving ways. M1 sideThis part is very critical, since we will be using the He-Ne beam to align all the optical systems . The threshold of the power input depends on how well the laser cavity is aligned. It is always better to operate the laser at the threshold or very close to the threshold, in order to generate a good mode-locking pulse train or good pulse structure, for some analytical purposes. In doing so, an extra mirror may be placed at the very far end of the table, to make this alignment more sensitive. First alig n t he refl ect i on spot from the mirror on t he He- Ne la ser , split t his -29- reflected beam by the beam splitter BS (see Fig. 11) and then align the cavity mirrors so as to get the reflected spots from them on the spot from the far-field mirror. An interference pattern will occur if the three spots are superimposed. After the cavity alignment, we look at the threshold of the conventional mode laser first (i.e., without the dye solution). The values observed are in the range of 2.2 - 2.6 KV (power supply), which was quite low. In order to check whether it lases, we put a photograph as a target and looked for a burnt spot. Then we put the 70% T (for example, the concentration of the dye is about 70% T to 65% T) dye solution in, realigned the cavity because the dye solution at Brewster angle changed the laser direction. The threshold was 3.0 KV (the range is from 3.0 to 3.90 KV, depending on the concentration of the dye solution). At the threshold, usually there is only one TEM mode which can sometimes be observed from the burnt spot. Operating at higher volt- ages, many TEM modes are produced and that may distort the pulse train. The pulse train is observed by means of the oscilloscope, Tektronix 519 model, which has a rise time of 0.35 nsec. fastest time resolution available so far . This is the The photodetector is a pin photodiode HP . 5082 (Hewlett-Packard) which is an ultrafast light detector for visible and near infrared radiation. Suitable filters (e.g., Corning 1-64) together with a ground glass are needed to attenuate the incident radiation on the diode. The pictures are taken by Polaroid camera type C-17, using 410 Polaroid film. A typical good mode-locked -30- pulse train is shown in Fig. 12a. If the pulses are not well mode-locked, or the diode is overloaded, the baseline of the pulse train does not stay at the same level as shown in Fig. 12b. Also, if the dye solution cell is not placed at the Brewster angle, the picture will show more than one pulse train (the additional ones are called satellites 11 at the cell window back to the laser rod. These trains do not arrive 11 at the oscilloscope at the same time. ) due to the reflection The separated distance depends on the position of the dye cell inside the cavity [la]. Or, if the laser rod is overpumped, the pulse patterns are shown in Fig. 13b. The deviation from an ideal picture [la] in this case is due to the nonlinear losses in the optical media--due to powerful radiation [9d]. The change in the refractive index of the laser glass matrix re- sults in self-focusing of the radiation and spectral broadening. The influence of the nonlinear loss is suggested in the following way. Beginning from a certain level of intensity, amplification of the most intensive ultrashort pulses decreases, i.e., there occurs a limitation on the pulse power. At the same time, less intensive USP can be ampli- fied and reach the same power level as the most intensive pulses. This should result in an increase in the number of satellite pulses at the end of the train. The wide pulses with shoulders probably contain many substructures of picosecond widths as shown in Fig. 14. Some of the very intense and powerful spikes cause the self-focusing and hence "hot spots" in the optics, and damage the111. ?ATTERN H, INTERFERENte lfili+I---- -- Figure 11 A .. Far Field" Mctl,,ocL of .-.---LASER. CAYIT'l'----• Ali9t-1me11t. M2. &Sr::=================================--'\ ME-NE USE!t ~l I w I -..J -32- Figure 11 a. A Well- Hode-Lockecl Pulse Train (from s1q Oscilloscope) at 2onsec/div. Figure 12 b. A Bad Mode-Locked Pulse TraiVl at so v.sec/ div. -33- 08 -s: $t Figure 13 ct. - □ SATELLITE PULSE A Pulse Trai-r1 witl1 Satellites. ' 11 ' ~ ,, •. ', ·,·,·,..,·t: ,, ,, ' ' . ' ..\, . 1•,1,1,l,,,.":,r,~~·/.'/~~. Figure 13 b. Ar1 Overpumped Mode-Locked Pulse Train. -34- 3r---r----,-----,,-------r-------ULTRASHORT PULSE ~f=45cm- 1 -2 ~ (.9 ___. er w 5 0 CL 0 -------~1.,.-;:s.-__,1,....:,__---1,:::.,______::....--1..____x=--~~~:.::s -10 -5 0 5 10 TIME (ps) Figure 14 A Calculated, Mode-Locked Nd: Glass Pulse, Shape with Substv-uc'ture, ere£ .31 > -35C. Two-Photon Method as a Pulse Width Measurement Since the direct electronic techniques are not yet capable of measuring time durations down to 10- 12 sec, a special technique is required in order to measure the picosecond pulse widths. The first technique developed for measuring ultrashort light pulses is based on two-photon absorption [13]. Excitation of a molecule under very in- tense irradiation takes place by absorption of two photons, provided that the molecule has an excited energy level corresponding to where v 1 is the frequency of the exciting light. 2v 1 Such two-photon absorption takes place via a virtual state of essentially zero lifetime and can occur only when the two photons are absorbed simultaneously. If the resulting excited state fluoresces, one can monitor the extent of two-photon absorption by following the emission study. The method is done by directing a mode-locked train of pulses through a cell containing a suitable solution; the resultant trail of two-photon excited emission can be photographed and will appear as a bright streak. If the pulse train is reflected back on itself by a 100% R mirror, the instantaneous light intensity at the mirror and other points where the pulses overlap will be higher and a brighter fluorescence spot will be observed. Thus, the length of the bright spot is a measure of the length of the pulse . For a 1~d 3+:glass laser, rhodamine 6G (fluorescence is centered at 55ooR) has proved to be one best detector for the TPF method. Two methods have been conducted as follows: 1. The correlation of one pulse with the succeeding pulse (cross correlation). From the picture of the pulse train (Fig. 15), one can -36- . I D .. TELESCOPE A '--D'tE CELI. DICLICTRIC MIRROR tANERA Figure 15. Cross Correlatiol'l Two PhotoV\ Method... -37see that there are more than 50 pulses. These pulses are separated from the succeeding ones by twice the round trip time in the cavity. A mirror (usually a dielectric 100% Rat 1.06 µ is used) is then placed at a distance of roughly the cavity length away from the cell of Rh 6G. The telescope is put in to reduce the size of the beam, in order to give a better depth of the field of the pulse train to the camera. The distance x must satisfy the following condition, in order to observe the TPF at the middle of the cell. Assume the cell is 5 cm long: ( 1) where L = optical path length of the cavity cR = c/nR , nR is the index of refraction of Rh 6G c = velocity of light= 3 x 10 10 cm/sec Note: The distances must be measured and calculated accurately in order to observe TPF picture. 2. The correlation of the pulse with itself (autocorrelation). This seems to be experimentally easier than the first method; the measurement is not as critical. In this method the laser pulse is split into two equal parts by a 50% R beam splitter (at 45° to the beam). Then the two pulses are reflected to meet in the Rh 6G cell (see Fig. 16). The pictures are taken by Polaroid film type 47, speed 3000, and by transparent type 46L. The transparent pictures are read by the -38- cAt1ERA LASER Figure I". T£Lf.SCOPE Autocorrelatiov, Two Photon Hetviod. -39- Jarrell-Ash microdensitometer. The length of the bright spot is the width of the pulse which is read from the half-width of the curve from the densitometer. Some results are shown in Fig. 17. The calculation is rather simple: for example, from Fig. 17: The densitometer is run at 1 mm/min The scale is 32 sec/1.25 cm The half-width readings a re 3. 7 - 3. 9 cm 32 = 1.58 min, or 1.58 mm 3.7 cm corresponds to 13.7 _25 x 60 6T = d X nR (2) C is equal to 1.344 nR 6T = 1. 58 X 10 -1 X 3xl010 l. 344 = 7.015 psec The curve from the microdensitometer is not sharp--there are many shoulders. This is due to the glow around the bright spot. The half-width is 'then read from the adjusted curve, taking the highest peak in the curve. However, the results are probably within ±2 psec. The factors involved in obtaining good TPF pictures are: 1. The two pulses are exactly superimposed. 2. The background is very dim . 3. The camera is not overexposed and is in focus. The intensity ratio between the TPF and the background is which is close to the theoretical value of 3 . 2.5 -41Theory If we consider the center of the cell to be the origin (z =O) and denote the pulses approaching the cell from the left and right as and E (t - ~) sin(kz-wt) 1 respectively, the intensity I ( 3) in the dye cell will then be given by w If we assume that the fluorescent intensity is proportional to the intensity squared: IF a: I~ (or IF a: Iw' Iw" for TPF from two dif- ferent frequency pulses), then + 4[E 3 (t-~) E (t+~) + E (t-~) E3 (t+~)cos 2kz} C C C C 2 2· 1 1 (5) The time-and-space averaged through the photographic process of IF is given by I S(i:) a: Et(t-f)dt + -oo Letting energy 2ww 00 00 00 I -oo E (t) = E (t) 1 2 is given by at E~(t+z)dt + 4 f E~(t- ~) E~(t+z)dt -oo z = 0 , the second harmonic pulse ( 6 ) -42(7) -oo The normalization of S(T) gives S (J) ex: l + 2G (T) (8) where G(T)' the autocorrelation function of the pulse energy, is given by 00 J E2 (t) E2 ( t-T) dt G(T) = -oo 00 f =l at T = 0 (9) when T=0 ( l O) E4 (t)dt -oo > S(T) _ 3 -w-- This intensity ratio is also the degree of mode-locking of the laser as suggested by J. R. Klauder, et al. [14]. If the intensity ratio is 3, that means it is mode-locked; if 1.5, it implies the free running laser (randomly phased modes). Our results are then interpreted to be quite well mode-locked. In order to narrow the pulse width and increase the peak power, the following techniques may be considered: l. Decrease the %T of the absorber dye so 1uti on. There is no theoretical way of predicting the %T of the dye solution in order to obtain the narrowest pulse width or well modelocked pulse train. 2. This varies from one system to another. Decrease the %R of the front mirror, to obtain higher powered pulses. -433. et al. [32] Decrease the path length of the dye cell. D. J. Bradley, reported that the shortest pulses and the most reliable mode-locking were achieved when the saturable dye was placed in contact with one of the cavity mirrors and the dye cell length was reduced to"' 30µ. 4. D. Better alignment of the cavity. Second Harmonic Generation (SHG) The mode-locking laser , in spite of the fact that it produces very short pulses in picosecond time scale, seems to be of limited usage with only one wavelength in the infrared (l.06µ). However, in 1961, Franken, et al. [15] reported the second harmonic generation 0 experiment by focusing the ruby laser beam at 6940A on the front surface of a crystalline quartz plate. One of the emerging beams was found to have the frequency twice that of the input . Later Giordmaine [16] and Maker [17] independently reported the method of index-matching of the KDP crystal in order to obtain the best output of the SHG. Theory SHG is a nonlinear optical phenomenon which has been observed in some crystals, such as quartz, ADP (ammonium dihydrogen phosphate) and KDP (potassium dihydrogen phosphate), etc. A suitable material for this process must be relatively transparent to the fundamental optical frequency and the desired harmonics. In addition, lack of an inversion symmetry of such crystal is a prerequisite for the SHG crystal , in order that the second order (P ex E2 ) nonlinear optfcal -44- properties can be displayed. In general , a schematic expression for the dependence of the optical polarization P as a function of applied electric field E is: (l ) where X is the linear optical polarizability, of order unity a.l is the nonlinear coefficient. is of interest, since it gives rise to the SHG. The second tenn Consider the nonlinear coupling of two optical fields, the first,having its electric field of frequency w1 along the j-direction, is given by iw 1t w iw 1t = e + C. C.) (2) Re(E;l e ) E/ (t) = (Ej ½ the second field at W2 along the k-direction is w W2 iw 2t ) E/(t) = Re(Ek e (3) If the medium is nonlinear, the presence of these field components can give rise to polarization at frequency and m are any integers. nw 1+ mw 2 , and n Taking the polarization component at (4) W3 d .. k is a third rank nonlinear susceptibility tensor and has lJ the following properties: where (l) It is nonvanishing only in noncentrosymmetric crystals. -45(2) It is independent of frequency, s i nee the material is trans- parent over the region which includes w ,w , and w . 1 2 We can then 3 write P.1 ( t) = d1..J kE J. ( t) Ek ( t) (3) (5) Since no physical significance can be attached to an ex- change of Ej and Ek in (5), it follows that therefore can replace the subscripts k,j dijk = dikj . We by a single symbol and hence reduce the full fonn of d tensor from (3 x 3 x 3) to a contracted one wh i ch is ( 6 x 3) The contracted d .. 1J tensor obeys the same symmetry restriction as the piezoelectric tensor , and in the crystals of a given point group symmetry it has the same form. For KDP, the symmetry is 42m, and the tensor is d .. = 1J 0 0 0 dl4 0 0 0 0 0 0 dl4 0 0 0 0 0 0 d30 ( 6) where the full equation of p is E2 dll PX p y pz = d12 dl3 dl4 dl5 dl6 d21 d22 dz3 d24 d25 d26 d31 d32 d33 d34 d35 d36 and the components of the nonlinear polarization are X E2 y E2 z 2Eiy 2E/x 2Eiy (7) -46p X = 2dl4 Eiy Py = 2dl4 Eix p = 2d36 z (8) Eiy In order to obtain an expression for one power of the second hannonic, Maxwell equations are the starting point: (9) where ( 10) and ( 11 ) The derivations are given in Ref . [18], and the power output of the second harmonic in cgs units is p (2w) TT area = 2cn where 3 • 2 2 2 2 2 sin 8 w3 dijk Eli Elk W3 = 2w1 , ( 12) w,= w2= w is the fundamenta 1 frequency W3 is the second harmonic frequency n3 = index refractive of second harmonic in crystal e = the angle between the j direction of the second harmonic and the direction of propagation L = length of the crystal -47- and ( 13) Index-Matching Technique. From (12), it is clear that in order to get appreciable SHG, the condition if the i and ~k = 0 must be satisfied, and k components of w1 propagate with the same phase velo- city, then ( 14) or, using kw= wnw(e)/c , we obtain ( 15) where we recognize that in a uni axial crystal (i . e., the crystal in which the highest degree of rotational symmetry applies to no more than a single axis), the index of refraction pends on the angle (optic) axis. (where nw0 n~ n for a given frequency de - e between the direction of propagation and and z KDP is a negative uniaxial crystal so that n: < n~ is the index of refraction along the extraordinary ray , and is that along the ordinary ray , for a birefringent crystal). Index matching can be obtained if the fundamental beam is an ordinary ray, while the second harmonic is an extraordinary beam, both beams propa .... gating in a direction making an angle em with one z-axis, so that w n;w(em) = no (Note that, once we choose an appropriate optical axis, n 0 (ordinary ray), but not ne (extraordinary ray) is independent of the propagation direction . This subject can be seen in detail in Ref. [18], -48- Chapter 18). A plot is shown in Fig. 18. Experimentally, the crystal face has to be adjusted both horizontally and vertically with respect to the incident fundamental beam until the maximum second harmonic is obtained (i.e., the incident beam is now in the propagation direction, making em angle with the optic axis). Once this condition is satisfied, both the fundamental and second harmonic beams emerge from the KDP crystal (in our case) at the same time and in the same direction, but with cross polarization. Experiment The experiment is shown in Fig. 19; a fraction of the incident 1.06µ , which was our monitoring beam, was split off by the beam splitter BS 1 and fed into a fast photodiode filter with appropriate output. The 0.53µ was also sent into the fast photodiode with 1 .06µ 6ut-off and neutral density filters. Both photodiodes were connected to a dual beam oscilloscope, Tektronix model 555. Since the fastest time scale on this oscilloscope is 100 nsec/div, no fine resolution of the pulse trains was observed. corded. Only envelopes of the pulse trains could be re- The ratios were taken from the maximum points of both pulse train profiles. The l inch-long KDP is placed in a doubler holder filled with oil. This whole piece is then placed in an adjustable holder (verti- cally and horizontally). The best positions of KDP were obtained from the maximum ratio of the output intensity of 0.53µ to that of the incident 1.06µ. -49- Z(OPTIC.) AXIS 1W vie l8) z.w n. - - - ' , . ... . .. '. ' I ' \ •, w ~ - , - - - ll'lo I I ·-......... --- Figure l8. Normal Undex) Surfaces for the Ordinar~ ancl Extraordi nar~ Rays in a Negative Urtiaxial Cr~sta I ( from ref. 18). L~ER Figure 1q. RG·l,AS (f'\LT£~) 1.0'-,.U. l<Ol' , a&,,li (FILTER) t0-53,U. 1 6S 1 I r J KDP Cr~stal Experiment. I DUAL Sl:AH OSCILLOSC.OPE Tne Second Harmoviic Phase Matching of l-01.}A, \ti t t ,s. I 0 I c.n -51The ratios are shown in the table and the plots on Figs . 20 and 21 from various vertical and horizontal settings, respectively. vertical settings horizontal settings (horizontal position is at 2. 50) (vertical position is at 2 .18) setting ratio setting ratio 2.25 2.20 2. 19 2.18 2. 16 2. 15 2. l 0 2.05 l. 95 l.85 l. 75 l.65 4.43 8.89 10.478 21. 818 15.33 13.53 5.45 2. 361 0.60 0.336 0.200 0. 15 2.90 2.80 2.70 2.60 2.55 2. 52 2.51 2.50 2.49 2.45 2.30 2.20 10.62 13.78 12.00 10. 91 10.00 11. 26 9.54 17 . 00 16. 15 15.00 9.00 9. 14 The best position of the doubler for index-matching is vertical reading: 2. 18 horizontal reading: 2.50 From the plot, one can see that the vertical setting is much more critical than the horizontal. this point. l. Hence, one must be very careful at If the doubler is not index-matched: The l.06µ and 0. 53µ do not leave the doubler at the same time (n~ f n~(e)). 2. The 1.06µ and 0.53µ do not travel the same direction. This is very critical at the far field when the superposition of the beams 16 . 0::: <t: 0 I- > > I- I_J _J 0 0 w w (.9 (.9 <t: <t: 0 X 2.15 I 2.45 2.55 Figure 2.0. 2.65 2.75 _ _.:r.====-1~~:d:I=.~-= f x-x -I ~ "'-x I 2.25 2.35 I \ Results From the Phase Hatching Experiment. I I_\.:) CJl X VERTICAL MICROMETER SETTING 2:05 1.95 1.75 * 1.85 -Jf_,/1 ox I X X Jx \ I 2 4 6 8 10 12 14 18 L{) . 0 r0 tD ::l. ::l. 20 24---------------------------.--------22 X 0:: <( > > 0 ...... 0 ~ 6 ...... <( <( <..9 <..9 w W 0 L!) . 0. <..O :::l ::i. r<) 8 IOr 12 14 16 18 2.1 X X \x/x /x Figure 11 . \ 2.9 X Results From the Phase Hatching Experiment. 2.2 2.3 2.4 2.5 2.6 2.7 2.8 HORIZONTAL . MICROMETER SETTING x/ / xi x/ X ' ; C)l I C,-> -54is required in analytical work. The l .06µ polarization is vertical, whereas that of 0.53µ is horizontal. E. The conversion factor is 5-10%. Two-Photon Method of Two Different Optical Frequencies TPF from Pi cosecond Pulses of Tvw Different Optical Frequencies. We use the same technique for the measurement. However, the method in setting up the experiment was quite different, following Rentzepis and Uuguay's, as shown in Fig. 22. We made use of the difference in the indices of refraction of the two frequencies, assuming that the doubler was index-matched, i.e., 1 .06µ and 0.53µ left the doubler at the same time. After passing through a 10 cm cell of bromobenzene, 1.06µ pulse led 0.53µ by 27 psec (calculated from the table of observed dispersions as reported by Topp and Rentzepis [18]. The pulses then entered a 5.0 cm cell of BBOT in methyl cyclohexane with a dielectric mirror placed at the end of the cell. This mirror is 100% Rat l .06µ which was aligned to reflect th e leading l.06µ beam back into the cell, exactly along the same direction, in order to meet the 0. 53µ beam and create the TPF in the solution. The picture was taken using Polaroid film speed 3000. A filter (Corning 7-59) was placed before the camera to eliminate the scattering of 0.53µ light. In order to atten_uate the intensity of 0.53µ when entering the cell from causing too bright background (2 photons of 0.53µ absorption in BBOT solution), a Corning 1-64 filter was used. The 1.06µ and 0.53µ pulse trains were checked by Tektronix 519 scope. The pictures are shown in Fig. 23. LASER Figure 22. 1ELESCOPE I OSCILLOSCOPE .. TO Sl'I t &BOT REfl,f;C.TING- I.Olofl', 1>1£L£CTRIC Hll~RO~ o.nµ. ,.of.JI'- &ROH O&ENZEWE 8':,~li (FILTER) 0-5~,.U, ~ gs 1.01oµ. ar1d. o.s3_µ, Two PnotoV1 Experiment. OSCILLOSCOPE ., TO 51'1 l<DP 1-0b},l, ~ I (FILT£R) U-"45 1.011,,.u, as I I CJ1 CJ1 -56- Figure 2.~a. 1.01r,_µ. Pulse Train ( frovn 51q 0£cilloscope) at so visec/ div. Figure 13 b. 0.53_µ., 'Pulse Train ( lrom s1q O.sc.illoscope.) at 50 visec/ div. -57The pulse widths were read by the microdensitometer and yielded the pulse duration in the range of 4-8 psec (±.. 2 psec in each reading); see Fig. 24, for example. We had a lot of trouble with this experiment, and it was learned that: l. If the vertical setting of the doubler is off, the two pulses do not travel in the same direction, i.e., they are not superimposed. Hence, the TPF are not observed. 2. If not enough intensity of 0.53µ pulse enters the BBOT cell, the TPF will be too weak to be observed. 3. Weak 0.53µ light is always due to weak 1.06µ, which means the dye solution is degraded and a new fresh solution must be made. One should check the burnt spots periodically, to get an idea of how high the power and the structure of pulse are (from the sound and the shape of the burnt spot). 4. If the alignment of the dielectric mirror is off, then 1.06µ and 0.53µ are not superimposed. 5. After the first four points have been corrected and still no TPF is observed, then check whether the reflection from the dielectric mirror goes right back into the laser cavity or not. If it does, this will ruin the outgoing mode-locking beam and no output is to be expected. The reflection must be pushed off a little. This last point is actually very important in all experiments having to do with reflecting the beam by mirror or by other optical pieces. It may not ruin only the pulse train but also may damage the -58- Figure 14. I.OloJJ- al'ld 0.53 µ, Two Photon l=luoy-es- c.ertce in BBOT Solution, av,d the HicY-od.ensitovnete.r Trace (5 mm/min.) -59- laser rod. Care must be taken not to let the reflection go right back into the rod. F. Synchronization of Two Optical Paths Most of the picosecond spectroscopy experiments that have been applied to chemistry are involved in the studies of time dependent reactions of various molecules. In general, two light beams are used as the exciting and the probing pulses. The time delay between the two beams has to be known in picosecond time scale. There are two methods that have been of use in many experiments: 1. The cs 2 optical shutter method [23]. The fact that power- ful optical pulses can be used in lieu of electrical pulses to induce a birefringence in liquid traditionally used in Kerr cells, is employed in this technique. The experimental diagram is shown in Fig. 25. The l .06µ pulses are used to induce the Kerr effect in cs 2 liquid in cell C. Such an effect in cs 2 is due to a field induced reorientation which has the relaxation of about 2 psec. The cs 2 cell is placed between cross polarizers which are in the 0. 53µ light path. through the first polarizer and the cs 2 cell. The 0.53µ pas~es If there is no Kerr effect in the cell, the light will be blocked by the crossed polarizers and not observed in the picture (camera placed behind the polarizer). However, if the 1.06µ and 0.53µ light paths are synchronized (by means of an optical delayer), the l.06µ pulses will arrive at cs 2 , i nduc ing the Kerr effect which will rotate the polarization of 0.53µ beam by 90°. The rotated beam can then pass through the crossed polarizers. LASE~ Fi guv-e 2s. 1.0loJA- KDP I &G,-1~ cr4,qs I.CW,_µ. Method ( P1 avid Pz. are Cv-o.ss Polarizers). \)\.~,v'-\¢"- y~o-10Y< Sync~ronization B~ CS1 Optical Shutter O.Slf- r.s VAatAal.E DELA¥ I I 0 m -612. The two-photon method. Fig. 26 . The experimental setup is shown in The l . 06µ path can be adjusted using a variable delay which consists of a prism placed on a micrometer which has a very fine scale on the order of 10- 2 millimeters . The variable delay is such that the 1.06µ pulses will lead the 0.53µ into the BB0T (2,5 bis[5-terbutylbenzoxazolyl-(2)]thiophene) solution. After being reflected by the mirror (99 . 9% at 1. 06µ ), the 1.06µ will meet 0. 53µ (if the variable delay is placed at the right position) and the two-photon fluorescence spot can be observed by means of a camera. Knowing the distance of the bright spot from the mirror, one can calculate the position of the variable delay that will synchronize the two light paths. Experiment Assuming 1.06µ pulses are ahead of 0. 53µ by a distance y , the TPF-mirror distance is x , and a vari able delay is set at posit i on 3. 00 (see Fig. 27). The time 1. 06µ travels in the cell : (BB0T in dichloroethane) (48 . 6 rnin + x) x 1 · 412 + (l . 8 mm) x 1 · 465 C C The time 0. 53µ travels : + (l . 8 mm ) x 1. 476 y + ( 48 . 6 m1. n - x ) x 1. 447 C C where n0.53µ in di ch l oroethane = 1.447 (interpol ation from the valu es given in t he Inte rn ation al Critica l Tabl e, Vol. Vi l (from Re f. [22]) n1.06µ in di ch l oroethane = l . 412 LASE'll Figure '.U,. 1-0",M, M ReJlects Haxi mu"' at o.s~.,u., Wke.-, 'Placed. at 45°] o.siJJ-,, When Place.ct. at 4s• to the. btcidevit. &eam, C&.S Trartsmits Haximum at t.OC.J-L-, Re-Elects Maximum at S~vidn·o~iz.ation b~ Two Pl1otovi Met~oci. H R.EF1.ECTIN6 AT l-Of-JA- DIEL8Cfa1c HIUOlt. 1 160T l I t-.:i m -63- Dlll.&CTRIC Nl~ROll 11.tFLEC.TING AT 1.01.j-L . Figure 2.7 BBOT Cell -64n0 _53µ in pyrex = 1.476 n1 _06µ in pyrex = 1.465 Now the time taken by both light beams until they meet at x must be the same, so (48.6 + x) x l .412 + (1.8) x l .465 = y + 48.6 - x) x 1.447 + (1.8) X and y can be calculated once x is known. 1.476 Then we know that the variable delay has to be set at the position (3.00 - f). A result from one variable delay position is not sufficient, however. This is due to the fact that the 99.9% at l.06µ reflector also reflects some 0. 53µ The two-photon spot observed can then be from the reflected 0.53µ meeting the 1.06µ in the cell. A simple check can be done as follows: l. Move the variable delay ahead, i . e., shorten the 1.06µ path source. If l .06µ is leading by y' , y ' > y and to the mirror. x should move away On the other hand , i f 0. 53µ is leading, then x wil l move closer to the mirror . 2. Move the variable backward, the opposite effect should be observed. For example, some results are shown in Table I . Once the proper variable delay setting is obtained, a timedependent experiment can be operated. -65- TABLE I Variable delay position (moving backward down the row) (cm) proper setting 3.00 1.25 19.91 8.00 0.96 20.76 13.00 0.60 20.60 18.00 0.30 21.40 X The proper setting (average) :: 20.70 min G. Experiment on Radiationless Transitions of Azulene Azulene exhibits anomalous fluorescence in that the emission originates not from the lowest excited singlet (s 1 + S0 ), but from the second excited singlet (S 2 + 5a). Furthennore, the first excited singlet state lies substantially lower in energy at~ 14400 cm-l (s 2 lies at ~28000 cm- 1 ) than in other chemically similar systems. The energy diagram is shown in Fig . 28. Rentzepis [19,20] reported the experiments on measuring the radiationless transition--vibration rela xation in the first electronically excited state, and the emission from the singlet and triplet state of azulene [20] . From the diagram, the radiation from 0.53 laser pulse (v 2 = 18863 cm- 1 ) photolyses azulene, raising to approximately the fifth vibrational level of the first excited singlet. of the laser fundamental, v 1 = 9431 cm- l The subsequent arrival which acts as the interrogating -66- EXCITED Si_ SINGL._T ·~ . .,, -, 5XCl'tEJ:> &fNGLE.T Si PLUOREStENCE. I . T1<TRIPI.ET) PHOSPHORESCENCE SINGLET GltOUll/1> So c::=:==:::::=:=:===::t:::=:==::::i Figure 2.1. Energy- Level Diagram for Azulevie (w, =-1.0'7,,U, , w1 ~ o.s3_µ.,) ( ref. 2.d.) -67or probe beam will raise azulene molecules up to s2 singlet state, provided they are still in the vibrationally excited state when the l .06µ pulse arrives. The fluorescence produced as a function of the delay between the arrival of two pulses provides a direct measure of the lifetime of the vibrational level . This can be done using the TPF technique. In our experiment we looked for the absorption of 1.06µ (a probe pulse) as a function of time delay. in Fig. 29. The experimental set-up is shown Right after KDP, a special dichrometer beam splitter (transmitting maximum at 1. 06µ , reflecting maximum at 0.53µ ) was placed to separate 1.06µ and 0.53µ beams into two different paths, with appropriate filters to purify the wavelengths in each path. The two beams were rejoined again at ss 2 (also a dichrometer BS) and travelled coaxially into the sample cell. A small amount of the light (~4%)was deflected out by a glass plate (8S 3 ) and fed into the fast diode with two RG715 filters as a 0.53µ cut off. This 1.06µ signal was a monitor- ; ng beam for the experiment and wi 11 be referred to as 11 b 11 • The rest of the light, after passing through the sample cell, was also fed into the same photodiode as oscilloscope. 11 b 11 , which was connected to a 519 Tektronix By adjusting the path length of a (the interacting 11 11 1.06µ pulses) such that a would arrive about 2-3 nanoseconds after 11 11 b 11 , 11 a double pulse train picture can be obtained as shown in Fig. 30. The decrease in the ratio a/b as a function of time delay (by moving the variable delay backward so as to delay l .06µ from 0.53µ) would yield the decay curve for the 5th vibrational excited state of of azulene. s1 state A ,,. • LASER FILTltl I SAHPLE as, KDP o.nµ. e.s'l. 1.01o,u- DElAY VARIABLE (L,L 2 and L.L 3 are Telescopes) Azule.vte Experimer1t. OSCI LLOSC.Ol'E ;a.,to 1-06.)A- L1 II Figure 2.q_ 7. '"•• ss~ t.O"J-'- l I 00 0) -69- a Figure 30. b b-a Pulse Train Irom Azule~e Experiment< s1q Oscilloscope set at 2.orasec/div.) -70The experiment turned out to be much more difficult than expected, and many problems were gradually realized and corrected, as follows : l. The He-Ne laser beam and the mode-locked beams did not always travel exactly in the same direction. The problem was due to the fact that all systems had been aligned by means of the He-Ne beam. The correction was made by adjusting mirror M (diagram in Fig . 1 10). 2. The l .06µ and 0.53µ pulses were not superimposed . It was checked by the burnt spots of the two reduced beams at the far field . This problem was caused by the index-mismatching of the KDP crystal, resulting in 0.53µ not travelling coaxially with 1.06µ. This accounted for difficulties in doing the experiment (bad TPF, no effect from the azulene experiment, etc . ) The correction was done by adjusting the doubler (vertically) until the two beams were superimposed (the other optical pieces had been well aligned). 3. The l .06µ (interrogati ng) beam was of higher intensity than the 0.53µ (photolysis) beam. be observed. The effect was then not marked enough to Stronger absorbing filters were needed to attenuate the l .06µ beam. 4. The l .06µ beam was of the same or slightly larger size than that of the 0.53µ beam. Corrections were needed to be made (e . g., reduction of the l .06µ beam by lenses) so that the l .06µ pulse would 11 see 11 only the molecules excited by 0.53µ pulse. -715. The 0.53µ could be too intense and caused two-photon absorption in azulene. 6. It is important to realize that the high accuracy in the measurement of distances is required for this type of experiment. One psec in air corresponds to l mm. Hence, we are working with the ac- curacy in the range of l or 2 mm. Besides, doing the experiment at the far field makes the measurements become extremely critical. A result was finally obtained. is shown in Fig. 31 The decay curve where A refers to the ratio of a/b normalized to the maximum absorption of 1.06µ The maximum absorp- tion was observed when the variable delay was set at 14.00. From the curve (1/e) point was at variable position 15.90, corresponding to the time (15.90- 14.00) x 2 x 3.3 = 12.5 psec cm psec/cm The maximum absorption was only 16% and each point on the curve was t he average of four readings. This result, however, did not agree too we l l with Rentzepis' work, which was reported to be 8 psec. H. Generation of Continuum from l .06µ Laser Pulses A very important technique, which has become an important tool in many picosecond spectroscopy experiments, is the generation of continuum from l .06µ or 0.53µ pulses in BK-7 glass [24], water and heavy water [25], toluene, benzene, cc1 4 , liquid N2 , and the laser amplifier - 72- VARIABLE DELAY POSITION 14 15 16 17 -<( -0.5 C 0 u -1.0 0 ~ ~ C -1.5 Figure 31. Results from Azulene Experiment itself [26]. -73The l .06µ pulses generate the continuum from l .06µ to 4000~, whereas 0.53µ generates the whole visible region 4000-7000~ Two mechanisms have been proposed for this nonlinear phenomenon. 1. Broadening of stimulated Raman emission [25,27]. First the stimulated Stokes-Raman (SSR) and the stimulated anti-Stokes-Raman (SASR) emissions are built up by the pumping laser. In addition the Stokes and anti-Stokes waves are coupled individually to the field of the pumping light, causing the gain as well as a broadening from both SR and ASR [28]. Simultaneously, this coupling leads to broadening of the original pump li~ht. Some of the excited molecular vibrational levels are not populated at room temperature and have to be excited through the process of SSR emission, the SASR wave should be retarded relative to the SSR. Rentzepis [27 ]. 2. This has proved to be the case by Varma and The retardation time is 6 psec. Broadening due to phase modulation effect of SSR and SASR and the pumping laser [29]. From the structure of the frequency spec- trum, the pattern can be explained by phase modulation through the intensity-dependent refractive index (n = n0 + n2 (E 2 ) , where n2(E 2 ) is the stationary value of the intensity-dependent refractive index) , which is the result of the inhomogeneous phase velocity within a pulse. The change in phase velocity leads to the reduction in fre- quencies in the peak front and an increase in frequency in the tail of the pulse. Experiment In our laboratory, BX-7, toluene, H20, and o2o were tried as the continuum generating liquids. o2o turned out to be the best . The -74set-up was quite simple as shown in Fig. 32. focused into a 10 cm cell of o2o. The 1.06µ light was The emerging light was collected and recollimated by a couple of convex lenses. After attenuating the 1.06µ intensity (down to ~2%), the light was sent into a spectrograph (2M CIT, with the resolution of 8~/mm) and the spectra were obtained on the Kodak IN and IM (hypersensitized) photographic plates as shown in Fig. 33. We observed the continuum from 0.06µ + 0.4µ . It is very important that a spike filter for 1.06µ was placed before the o2o cell, in order to prevent the flashlamp light and continuum that could have arisen from the laser rod itself, from getting into the spectrograph. The duration of the continuum was shown by Vanna and Rentzepis [27] to be on the order of picoseconds. The continuum emitted by o2o with excitation by a train of 0.53µ pulses were also observed in our laboratory. The anti-Stokes and Stokes sides were collected separately due to the lack of an appropriate filter. The 0.75M Jarrell -Ash 5 x 7 spectrograph was used 11 and the spectra were recorded on a Polaroid film, type 57 in Fig. 34. as shown -75- LASER 0 10 f2. I ,.~,u-0 I t s;:ctR.o6RAPH tCONTINUUH Figure 31. Continuum GevteratioVl from I.0bJJ,, 'Picosecond. Pulses ( F, is a Spike FilteY' at 1.01oµ. ; F2. is a 1.0<,p: cut Off " Filter). Figure 33. Spectral Li~es ( Recorded. on Kodak 1-N and 1-H Photographic. Plates). Sections 0£ Continuum from 1.0"µ. Picosecond -Pulses, with Hg I I a:, -.J -77- Figure 34. Contirwum from 0.53)A, tkrou9h an Echelon CTec.orded on PolaYoid Film T~pe 51) -78References l. See, for example, A. Yariv, Introduction to Optical Electronics (Holt, Rinehart, and Winston, Inc., 1971), Chapter 4. 2. The review articles on this subject are: a. A. J. DeMaria, et al., Science 156, 1557 (1967) b. A. J. DeMaria, et al., Proc. of the IEEE~, 2 (1969) c. P. W. Smith, Proc. of the IEEE 58, 1342 (1970) d. P. M. Rentzepis and C. J. Mitschele, Anal. Chem . 42, 20 (1970) e. A. J. DeMaria, Progress in Optics, Vol. IX, Wolf ed., (North Holland, 1971), page 33. 3. H. Kogelnik and T. Li, Appl. Opt. i, 1550 (1966) 4. H. Kogelnik and W. Rigrod, Proc. IRE 50, 220 (1962) 5. "Inhomogeneous" here refers to the broadening of the linewidth of the system. This is due to the fact that the individual atoms are distinguishable, each having a slightly different transition frequency v 0 , and different linewidths; the total of these together constituting the inhomogeneous linewidth of the system. 6. See for details, Section 9;3 of Ref. 1. 7. M. DiDomenico, Jr., J. Appl. Phys. 35, 2870 (1964); A. Yariv, ibid l§_, 388 (1965). 8. The quality factor Q is a measure of the property of optical resonators in building up large field intensities with moderate power inputs, and is defined by the relation energy stored by resonator Q = w x field power dissipated by resonator 2 Q = WE: EV 4P where w is the frequency of field inside the resonator P is the power input, E is the peak field, V is the resonator volume and E: is the dielectric constant. -799. The references on this topic are: a. b. c. d. A. J. DeMaria, et al., Proc. of the IEEE~. 2 (1969) A. J. DeMaria, et al., Science 156, 1557 (1967) V. S. Letokhov, Soviet Phys. JETP 28, 562 (1969) P. G. Kryukov and V. S. Letokhov, IEEE J. Quant. Elect. QE-8, 766 ( 1972) 10. Bloembergen, Am. J. Phys. 35,989 (1969) 11. J. W. Shelton and J. A. Armstrong, IEEE J. Quant. Elect. QE-3, 696 (1967) 12. M.A. Duguay and J. W. Hausen, Opt. Comm. l_, 254 (1969) 13. J. A. Giordmaine, et al., Appl. Phys. Lett. ll_, 216 (1967) 14. J. R. Klauder, Appl. Phys. Lett. }l, 174 (1968) 15. P.A. Franken, et al., Phys. Rev. Lett.]_, 118 (1961) 16. J. A. Giordmaine, Phys. Rev. Lett. ~' 19 (1962) 17. P. D. Maker, et al., Phys. Rev. Lett.~, 21 (1962) 18. A. Yariv, Quantum Electronics (John Wiley and Sons, Inc., 1967), pp. 348-355. ~. 19. P. M. Rentzepis, Chem. Phys. Lett. 20. P. M. Rentzepi s, Chem. Phys. Lett. 1, 717 (1969) 21. M. R. Topp' et al . , J. Appl. Phys. 42 , 341 5 ( l 971 ) 22. M. R. Topp, P. M. Rentzepis, J. Chem. Phys. 56, 1566 (1972) 23. M. A. Duguay and J. w. Hansen, Appl. Phys. Lett. ]2_, 192 (1969) 24. R. R.Alfano and S. L. Shapiro, Phys. Rev. Lett. 24, 584 (1970) 25. w. Werneke, et al., Opt. Comm.!, 413 (1972) 26. N. N. Il 'ichev, et al., Zhetf Piz. Red. ]2_, No. 4, 191-194. 27. C.A.G.O. Varma and P. M. Rentzepis, J. Chem. Phys.~ 5237(1973) 28. M. J. Coll es, et al., Ch. Phys. Lett. _i, 621 (1970) 117 (1968) -8029. For example: a. R. J . Joenk and R. Landauer, Phys. Lett. 24A, 228 (1967) b. F. Shimizu, Phys. Rev. Lett. ]2_, 1097 (1967) c. S. A. Akhmanov, et al., Sov. Phys. USP 93, 609 (1968) d. K. Shimada, Z. Physik 234, 293 (1970) 30. See, for example: a. P. M. Rentzepis and M. R. Topp, Ann. N. Y. Acad. of Sci. 33, 284 (1971) b. P. M. Rentzepis, Adv. in Ch. Phys . .£l_, 189 (1973), with references therein. 31. M.A. Duguay, et al., IEEE J. Quant. Elect. QE-6, 725 (1970). 32. D. J. Bradley, et al., Opt. Comm. ~. 41 (1970). -81- PART II Applications of Picosecond Laser in Chemistry: Vibrational Relaxation Times in Liquid Alkanes and Alkenes A. Introduction With the advent of ultrashort and high power laser pulses, chemistry on the picosecond time-scale can be studied. P. M. Rentzepis and co- workers have contributed a great deal in the area of Picosecond Spectroscopy, both instrumentation and chemical studies. Fast decay times (electronic, fluorescence, intersystem crossing, vibrational, reorientational) and energy transfers in many chemical systems have been studied by various groups [l]. For wider applications, techniques in generating various wavelengths other than l .06µ and 0.53µ are necessary. Some of them are 1. Optical Third Harmonic Generation in Organic Liquids [2]. The wavelengths between 0.373µ to 0.232µ have been observed in liquids suc h as BBOT, etc., from ~he high power fundamental beam (1 .06µ). 2. Fourth Harmonic Generation. By passing 0.53µ into the phase - matched KDP crystal, 0.265µ light can be generated. 3. Stimulated Raman Scattering. have been observed with high yield [3]. The Stokes lines in many liquids By employing 0.53µ, many wave- lengths in the red can be chosen from various liquids or gases (2900 cm-l shift from ethanol, acetone, etc.; 4000 cm-l shift from H gas; 2300 cm-l shift from N2-gas). From these Raman-shifted wave 2 lengths, second harmonic generation technique will give the desired light in the ultraviolet region. -82- 4. Continuum Generation in Liquids. As discussed in Part I H, the continuum between 0.4-0.7~ from 0.53v or 0.4-l.06v from l .06µ can be easily obtained from o2o liquid. 5. Mode-locked Dye Lasers. The mode-locking of organic dye laser can be achieved by pumping the laser with the second harmonic pulse train from a Nd laser or a Ruby laser, depending on the dyes employed. The wavelengths between 0.56-0.59 from the mixture of Rh 6G and RhB using a Nd laser have been reported by [4]. However, with the available l .06µ and 0.53µ lights, direct observations of vibrational decay times in the ground state can be studied in many appropriate chemical systems (i.e., the compounds with the excited electronic levels lying in the ultraviolet region or higher). B. Historical Review In 1966, F. DeMartini and J. Ducuing [5] reported the first direct observation of vibrational lifetime in Hydrogen gas using a Q-switched Ruby laser. Gaseous H2 was first excited by Q-switched laser pulse (the duration of which was 20 nsec) v1hich produced the excess population in the first vibratiOnal excited state by stimulated Raman scattering . The sample was then probed with an ordinary ruby laser (a train of spikes lasting about 500 ~sec), the power of which was below the threshold for stimulated scattering. The time evolution of spontaneous anti-Stokes scattering from the probing laser yielded the vibrationa l lifetime of hydrogen to be about 60 ~sec. The vibrational lifetime of an optical phonon on a picosecond -83- timescale was first reported by R. R. Alfano [6]. The lifetime of 1086 cm-l optical phonon in calcite was measured directly to be 12.5 ± 5 picosecond, using two picosecond laser beams operating at a wavelength of 0.53µ and 1 . 06µ . The stimulated Raman was produced by 1 .06µ pulses, whereas spontaneous anti-Stokes scattering came from 0. 53µ pulses. As for liquids, direct measurement of the molecular dephasing time was accomplished by D. von der Linde, et al. [7]. In their work, molecular vibrations were coherently excited by a first short light pulse (1 .06µ) and the rise and decay of the vibrational amplitude was measured with a second light pulse (0.53µ) properly delayed with respect to the first one. Both light pulses were of high enough intensity to produce stimulated Raman scattering . The stimulated anti-Stokes signal from the 0.53µ pulse was collected at 0° scattering angle. The authors pointed out the fact that when the laser pulse wid t h was much longer than the dephasing time , the anti -Stokes curve as a function of time gave direct information on the shape of the probe pulse [8] . This was found to be in the case of ethy l alcohol (Rama n shift at 2928 cm- 1), the decay time was the pulsewidth. As for CCT 4 , the dephasing time was obtained directly to be 4.0 ± 0.5 psec . (Raman line at 459 cm- 1 ). From the same laboratory , vibrational lifetimes we re al so obtained for the CH 3 valence bond vib ration in 1,1,l trichloroethane and ethyl alcohol I9]. respectively. The results were 5 ± 1 psec and 20 ± 5 psec Th.e incohe rent or spontaneous anti -Stokes from the -84- probing 0.53µ pulse was collected at 90° scattering angle (discussed be l ov1 in the theory section). In order to establish the molecular-vibration decay route in a liquid, R. R. Alfano and S. L. Shapiro [10] did the study on methyl vibrations in alcohol . By measu r ing the decay curves and intensities of spontaneous anti-Stokes scattering at 2928 cm-l and 1464 cm-l vibra tion, together with the delay between the 2928 cm-l and 1464 cm-l signals, the authors were led to believe that the decay route of methyl stretching vibration was due to a splitting into daughter vibrati.ons at half the frequency . Recently , A. Laubereau, et al [11], interpreted from the Raman study of CH 3cc1 3 , the mechanism of vibrational relaxation as intermolecular (a trimolecular interaction) . They concluded this fact from the dependence of the vibrational lifetime on the concentration in the binary liquid mixture of CH 3CC1 3 and CC1 4 . The vibration-vibration energy transfer mechanism was also confirmed ; the near energy resonance occurs in CH 3cc1 3 when the methyl stretching vibratio n (2939 cm- 1 ) decays to 1450 cm- l bending modes . In the solution of CH CC1 and co oo , however , the 2939 cm-l was found to decay into 3 3 3 -1 2227 cm of co 3 stretching vibration and 713 cm - 1 of C-Cl vibration . C. Theory Before going further to describe the experiment , it is useful to discuss some aspects of the dephasing times , the stimulated Raman scattering, and the vibrational lifetimes. 1. Dephasing times. Consider a number of molecules , vib rating in the vibrational excited state , after i nteracting with no n-abso r bing -85- light. These molecules will start out vibrating in phase with each other. In a very short time (picoseconds) after, the vibration will dephase, due to two possible mechanisms: a. Irreversible Dephasing. among molecules. This is due mainly to the collision Each collisional event will 11 wipe out the 11 phase memory of the molecules and hence the in-phase vibration cannot be restored. b. Reversible Dephasing. The molecules can vibrate out of phase from each other by slight molecular movement (rotation) or due to the different environment for each molecule. The loss of phase memory by this mechanism, can, however, be restored by certain non-linear optical methods (such as phonon echo [12]). Generally, in liquids, the irreversible mechanism seems more likely due to the rapid interaction among the molecules. In gas and solids, the second mechanism seems more probable, as has been studied by I. D. Abella, et al. [12]. The dephasing time can be observed by means of stimulated or coherent Raman scattering. 2. Vibrational Lifetime. This corresponds directly to the depopulation of the vibrational excited state, and can be obtained from the spontaneous or incoherent Raman scattering. The mechanisms of the vibrational decays have been studied as described in the previous section [10, 11]. 3. Stimulated Raman Scattering [13]. The stimulated Raman scattering has the following properties which make it different from -86- the ordinary Raman effect: a. It occurs only above a certain intensity threshold of the exciting beam. b. The scattered intensity increases non-linearly as the input intensity becomes very large, yielding the conversion in the order of 10-20%. c. The scattered beam has directionality properties which are similar to that of the exciting beam. d. The frequencies of higher order Stimulated Stokes lines are exact multiples of w (the fundamental frequency shift) away from the laser frequency: w -2w, w -3w, etc. 1 1 The Stokes intensity is high enough to take over the role of the laser beam and stimulate in turn radiation at the second Stokes frequency w -2w. 1 In comparison, the higher Stokes lines from spontaneous radiation are always very weak and not equally spaced, since these transitions are due to the slightly anharmonic molecular vibrations . In order to understand the non-l i near increase of the scattered light, it is appropriate to describe the Raman effect using quantum mechanics. For a molecular vibration to be Raman active , there must be a change in the induced dipole moment resulting in polarizabil i ty change of the molecule. This can be written as J.J· l = o.E (l) -87where µ.1 - induced dipole moment of molecule i a - the polarizability which is a tensor of 2nd rank E - the electric field strength of the incident beam. For a vibrating molecule, coordinate of vibration X. changes as a function of a normal a The series expansion of a(X) can then be written: (2) We can write the perturbation Hamiltonian that corresponds to the Raman effect as 1 H Raman = - ( ~ ax ) • XE 2 (3) 0 Using creation and annihilation operators, we have and (5) where wi,ws - frequencies of the incident laser and stokes respectively at ,a - are photon creation and annihilation operators at,a - are harmonic oscillator creation and annihilation V \/ operators The rate of Stokes photon emission is therefore proportional to -88- Wemiss a!<n 1 -l, ns+l,l I a1 a!a:I n1 ,ns,0> = n1 (ns+l) final initial (6) Or we can write (7) where n1 ,n 5 are the occupation numbers of the incident laser mode (w 1 ) and the Stokes mode (ws) respectively. A is an expression depending upon the scattering medium. From Eq. (7) (8) ns = (n ) Jl s O An 1 t +Ant (9) JI, In general, for an ordinary Raman scattering, the intensity of the incident beam is small and An 1 t << l, so ns =Ant JI, ( l O) The Stokes intensity varies linearly with the incident light. If n1 is large enough such that An 1 t > 1, then An 1 t ns ~ (ns) Jl (11) 0 and the Stokes intensity grows exponentially with that of the incident 1aser. From another point of view, electromagnetic treatment of Raman scattering wi 11 show us the di recti_ona l i ty property of the Stimulated Raman scattering. We can start by considering the potential energy -89- of a molecule with polarizability a in an electric field E(r,t): l -;t+ W= - 2 a(x)Jt.(r,t)J 2 ( 12) If we again write a(x) = a0 + a1x (in the lowest order approximation) ( 13) Hence, the force driving such vibrations will be ( 14) A sinusoidal varying force, F cos wt, will give the following differential equation for the vibrational coordinate: .. • 2 mx +Rx+ w x = F cos wt where ( 15) Risa phenomenological damping coefficient w is the resonance frequency Fis the effective driving force. This force includes the interaction between the induced dipole moment of the adjacent molecules. If the polarizabilities of the two molecules are a 1 and a2 , then their interaction energy is approx imately 2a,a 2/d E where d is the intermolecular distance. The ( 3) 2 corresponding force driving the vibration of molecule 2 is 3a 2 3 2 (2a 1/d ) ( ~)E . Since both forces have the same dependence on E , 3 they will combine into an effective force F = fE 2 . -90- The electric field E for the• system is now composed of the shifted (Stokes) and the unshifted (the incident laser) lights, the total Eis then t = tcos(w"t-(r} + EScos(w. S t-1<S -r +"'TS J· ~ where the subscript ~ £ ~ ( 16) refers to the incident laser beam and the sub- script s refers to the Stokes field. k is the propagation vector of the light wave. The resonant condition is when wt-ws = w . Solving for x in Eq. 15 leads to (17) Here, we can see that the phase of the molecular vibration is (kt-ks)·r which reflects that the molecular vibration is driven by the product of the electric fields with the frequency wt and ws. From Eqs. (1) and (2) µ = ~ • ax xE (18) and the rate of energy exchange between the dipole moment and the component of the field of frequency ws is given by We have so far left out the anti-Stokes part of the electric field. In order that the anti-Stokes radiation be amplified the total field is considered in the form -91- E = ER,cos(w.R,t-k.R,r) + Escos[(w.R,-w)t- - ks·r+~s] (20) Using Eqs. (15), (17l, (18), (19}, we obtain, _ f da wt+w PAS - 4R dx w { -(Et ·EAS) 2 - (Et ·EAS)(Et ·Es) cos [ (2kt-ks -tAS) . r+~s +~AS]} (21) If Es> EAS (which is the case}, EAS will be amplified if 2tt - its - t AS = o (22) and cos(~s +~AS)< 0, the maximum will be at (23) ~s +~AS= 1r Equation (22) is the ph.ase matching requirement. It turns out that from Eq. (22), the anti-Stokes radiation is emitted in the form of a conical shell with a half-apex angle s defined by [14] (24) where all the n s are indices of refraction of the medium. 1 This angle turns out to be very sma 11, in th.e order of a few degree. As for the Stokes propagation directions, it was found to be along the laser axis [15]. This can be explained qualitatively as follows: first, the process is initiated by that portion of the spontaneous Stokes radiation which passes through the focal volume and is amplified -92- there by a Raman gain. Since the focal volume resembles a long thin cylinder, the gain will peak at nearly forward angles, thus making the stimulated Stokes emission predominatly along the axis. At this point, it is of interest to compare this directionality properties of the stimulated Raman to that of the spontaneous Raman. From Placzek's Theory of Raman intensities, the final form can be written as (25) where C is the velocity of light N is the total number of molecules in the exciting beam \) It is the laser intensity µ is the reduced mass of the oscillator and v 1 is the transition frequency and the laser frequency respectively 0 a is the mean value of the isotropic part of the polarizability tensor, defined as -a= 3l Ia' XX+ a yy I+ I a ZZ J (26) a = aa/aQ 1 y 2 is the mean value of the anisotropic part of the tensor: -93y' = ay/aQ From [qs. (25), (26), and (27), it can be seen that I is the absolute intensity radiated in all directions by the sample. From these facts, one can distinguish experimentally the dephasing time from the vibrational lifetime. Since the former is characterized by the stimulated Raman, the coherent anti-Stokes signals can be collected 11 head-on 11 • The spontaneous or incoherent anti-Stokes signals are then collected at 90° scattering angle. 4. Radiation Transfer Equations. In order to obtain the expres- sion for the anti-Stokes signal as a function of time, we have to apply the electromagnetic treatment to the problem, in a slightly different manner from the previous one. We start out writing the Hamiltonian in the form [3c] (28) where Q 1 is the operator representing the oscillator displacement is the coordinate Q H0 represents the total Hamiltonian of the molecule in the absence of the field Eis the applied electric field If we consider a two-level Raman system, we define t-.. = n0 - n where n 0 and (29) n are the amplitudes of the ground state, and the vibrational excited state respectively. -94- Let N be the molecular density, then N~ represents the popula- tion difference density between the ground and excited states. We will call operator Q Qav =<Q'> the expectation value of the displacement 1 • Again, we write down the mechanical forced vibration equation similar to Eq. (15), 3~ 2 · ciLl . 1 .. ... ,., 2 ---=a_v + r --~_v + w Q a"' E at2 at av = 2m clQ where r tw (30) A LI is the phenomenological damping constant and is equated to which is the halfwidth of the resonance curve. The non-linear polarization can be written as p 3a (31) NL = N aQ. Qav E The equation of motion of t is then (32) where r' which is another damping constant corresponding to l /T,, in the Bloch equations, is the inverse lifetime of the excited vibrational state. In comparison, r then corresponds to l/T1 in the Bloch equa- tions. Finally we need one of the Maxwell's equations {_33) where z is the propagation direction. We will assume the form of tne total electric field as in Eq. (16) but without a phase constant: (34) The expectation value Qav of the molecular displacement will then have also a sinusoidal form: Q av =-Qs i n (wt + kz + <P ) (35) and w = wR, - ws (36) Assuming Q , Et , Es' and 6 to be slowly varying functions of z and t, if we substitute Eqs. (34} - (36) into (30) - (33), we obtain, (37) a6 -_ l2h Els Q + r' ( l - 6 ) at (38a) (38b) -962 = .-•:-;-:- s where n~ Et Q (39) vs is the phase velocity of the Stokes wave Ys represents the finite attenuation constant for the Stokes wave Here, we assume n0 ~ const '=" l ~ '=" at so .L (1 - n) '=" - 2-!!. at at From Eq. (37), T1 is the dephas i ng time which governs the decay of the coherent vibrational fieldQ . Also from Eq.(38b), T2 represents the vibrational lifetime of the changing n. What we have neglected so far is the anti-Stokes field equation which is of interest here. In the experiment, the stimulated anti- Stokes field depends on the changing coherent vibrational field, the electric field of the probe laser, and also the directionality of the k's. Using the similar form as Eq. (20) and the derivations of E5 , we can write . 2 2TilW aE AS N aa ix~k ~ (x t') = __,2==--- aQ e at ' e kAS where wAS = w£2 + w kAS = k£2 + k (40) -97- E12 is the probe laser electric field t 0 is the delay time between the exciting (E 1 ) and the probe (E 12 ) beams The coherent anti-Stoke signals is In Eq. (41), we have a phase-matching condition ~k. On the other hand, the incoherent anti-Stokes can be derived from Eq. (38). The signal depends on the changing w (with time) of the vibrational excited state and the probe laser electric field; the beam cross-section and the solid angle have to be considered as in spontaneous Raman scattering process. Thus, we can write wh.ere A is the beam cross-secti.on n is the solid angle of acceptance and p is the density. -98It is to be noticed tnat there is no phase-matching requirement here and s(t 0 I_ can be collected at any scattering angle. 1nc D. Experiment Alkanes were picked to be studied for an experimental reason: they have the most intense Raman li ne at the 2900 cm-l shift (C-H stretching mode) which is quite far enough from the incident laser. The second hannonic laser beam (0.53µ) which is used as the probe pulse, is so fo tensed that it gets scattered over hundreds of wavenumbers in the spectrometer. The farther and more intense the Raman shift is, the less difficult it will be in attenuating the 0.53µ intensity and observing the anti-Stoke signal. Very good "cut off" or interference filters together with a spectrometer {which can be left off if one has good enough interference filters) are required in the experiments. Furthermore,many alkanes are liquids which make it easy to work with and are more promising that the dephasing and vibrational lifetimes will be in the order of picoseconds or even subpicoseconds. l. Materials. Decane, Tri decane, Pentadecane, 1-hepene, 1-decene, were bought from the Aldrich Company, 99 +%grade. Heptane was of spectroquality from Matheson, Coleman, and Bell Company; isodecane and l, 6 heptadiene were made by Chemical Sample Company, also 99% grade. Finally, co 3-(cH 2) -co 3 gas was obtained from Merck-Sharpe and Dohm. All these compounds were used without further purification. 2 -99- 2. Dephasing Times of .Heptane, Decade, Tridecane, and Pentadecane. The experimental set-up is shown in Fig. (35). The fundamental 1.06µ pulses was used as the exciting pulses, 0.53µ pulses as probing pulses. They were both focused by a 25 cm lens and sent coaxially into the cell. The anti-Stokes signals were collected at 0° scattering angle. Beyond the cell, Corning 4-98 filter was used to attenuate 1.06µ pulses, Baird-Atomic interference fi'lter together with a Dietrich cut off at 11 11 0 5000 A filter were used to minimize the 0.53µ intensity and maximize the anti-Stoke signal. Since the cone angle of the anti-Stoke radiation was a few degrees off the axis, the beam was not focused directly into the middle of the spectrometer slit, but a little off sideway, in order to attenuate the 0.53µ intensity even more. The spectrometer used was a 1.8 M. Ebert set at 4594 A (2900 cm-l Raman shift). The detector was an Amperex 56 TVP photomultiplier connected to a Tektronix 585 oscilloscope. A small part (~4%) of the original light was deflected out of the normal path by a glass plate into a fast Hewlett-Packard photo diode [16] biased with +45 Voltage (DC). The reference signal was fed into al microsecond delay line before being sent to the oscilloscope. A half wave plate was placed in the 1.06µ path to reorient the polarization of 1.06µ beam by 90° (and hence was the same as that of 0.53µ light). The synchronization of the two light paths was done at position A in the figure, using BBOT and two-photon method as described in Part I. The purpose of the synchronization was to help us set the variable delay as close as possible to the correct position (which had to be checked experimentally by observing the anti-Stokes signal). LMH. .-----.LO'"JAl(DP 116-li as Figure 35. +O.H,M, UM,µ. HONITO~N& &HCTlltOHETER flLTEAI ½API.AT£ as 1.0(:.,1,A, -'AHP&.E (TWO·fMOTOtJ PO&ITION) C).S~ Dephasi ng Time Experiment. PHOlOHULTl,Ueit \!I &EAH 1.01,,u, tt~s VARIA&L& DELA't I .... I 0 0 -101- Since the threshold was required in generating Stimulated Raman light, the laser had to be operated such that high enough power pulses were produced. This was achieved by using higher 0.0. for the saturable dye concentration and changing the dye solution frequently enough that the high power could 6e maintained. The results of the four dephasing times are shown in Fig. (36). 3. Vibrational Lifetime Studies at Room Temperature. mental set-up was slightly rearranged as shown in Fig. (37). synchronization was done at position Bin the figure. The experiThe This time only 0 one filter (Dietrich "cut off" at 5000 A) was used to attenuate some 0.53µ light that got scattered off the side of the sample cell. When the intensity got fairly low, an amplifier (EG + G Model AN 101) was used to help enhance the anti-Stokes signal before feeding it into the oscilloscope. The results were the same when the experiment was performed both with and without the half wave plate. These results confirm that the times we measured are due to the vibrational relaxation time and not the reorientation time of the molecules [17]. 4. Vibrational Lifetimes Studies at Low Temperature. Some preliminary low temperature studies were conducted in a quartz dewar with optical windows. The desired temperature in the dewar was obtained by regulating one flow of dry nitrogen through a copper coil immersed in liquid nitrogen. The temperature of all the samples was monitored by a thermocouple placed inside' the sample cell. is shown in Fi g . ( 38) . The diagram -102- _J <t z (9 CJ) -C Cl> "Cl> ..c. 0 u CJ) w ~ 0 .,_ CJ) I 1- z <t -10 -5 0 5 10 15 20 25 30 35 40 45 DELAY TIME (ps) 50 55 Figure 3". Dephasing Times i-n n-Alkanes • {Temp .• 2.oc): •- Heptane, o- DecaYle, a-Tridecane, o 'Pentad.ecane.. 60 L.ASe-~ KOP r-, 1.0l.,U.. RG·IA5 0.4~µ, ~ Sf'aCTROH~ • o.s~µ, 1.0&.,u, 5AMPLE POSITIOIII) S rf1 (TWO-PHOTON ........ , , , ~ 0 0 6EAM .,_HOIIIITORIN& (i.llPLATE) &s Vibrational Liletime Experiment. MHi I' as -◄•----~---11-◄•-t o.s~_µ. r.o"f',- Figure 3.,. I.O"'J-" VAA.IA&i-E Z>ELA'i I vj 0 .....I P!WAR. . COLD Na GAS 1" WATElt TO HIC~OVOI.THRTER Cold. Tempev-ature Set-Up. COLO TEMNRATUtle: - te Figure 38. HWA._ coma COIL UtlllONt. WAR.H "a.•AS I ~ I ..... 0 -105- E. Results and Discussions First, heptane, decane, tridecane, and pentadecane were studied at room temperature. The vibrational lifetime results showed the increas- ing lifetimes with increasing number of carbons in the molecule. When we normalize the relaxation times to the ratio of the number of CH 3 groups to the total number of carbon atoms in the chain, a reasonable correlation is obtained for these four alkanes. This seems to show that the vibrational energy loss takes place primarily through the methyl groups in these molecules. In order to check the hypothesis, isodecane (with 3-methyl groups), 1-heptene and 1-decene (with l methyl group J, and l , 6 heptadi ene (with no methyl group) were studied at room temperature. The results show that isodecane relax about 2/3 as fast as decane whereas 1-heptene and 1-decene do at about twice as slow rate as the corresponding n-alkanes. The results and normalization of all the experiments are shown in Table I. These results lead us to believe that the C-H stretching relaxation time depends in the main on the proportion of the methyl groups in the molecule, or more accurately stated~ on the proportion of time the vibrational excitation spends on the methyl groups. The relaxation time of l, 6 heptadiene, which is also listed in Table I, is considerably longer, further confirming the above notion. Because the Raman spectra of these types of molecules have three distinct maxima (instead of a broad band in n-alkanes) in the 2900-3000 cm-l region (the CH stretching from the -HC=CH 2 groups peak at about 3000 cm whereas the CH from a single bond carbon peaks about 2900 cm- 1) [18], a -l , 60.0 20 l,6-Heptadiene 3.35 1 :10 33.5 20 1-Decene 3.00 l :7 21.0 20 l-Heptene 3.27 3.20 2: l 0 2: 15 24.5 20 n-Pentadecane 3.24 3.14 (a X b) Normalized Lifetime 3:10 2:7 Ratio methyl carbons to total carbons 3.23 20 N-Tridecane b. 2:13 16.0 21.0 20 -25 n-Decane 21.0 10.8 20 Isodecane Lifetime 11.0 14.0 16.0 a. 20 -25 -70 Temp. °C n-Heptane Compound TABLE I I 0) I .... 0 -107check was made to see if the measured relaxation time depended on detecting frequency. The relaxation data were found to be insensitive to the position of the l .8 M spectrometer setting within a range of wavelengths corresponding to the Raman shifts of 2900 cm-l to 3100 cm- 1 . In order to obtain some preliminary measurement in the C-H stretching relaxation time of the methylene groups, co 3-cH 2-cH 2-co 3 which is a gas at room temperature was studied. had to be made for this purpose. with a break-off seal. A special small cell The cell was connected to the gas bulb Before letting the gas into the cell, it was cleaned by flushing N2 gas through several times (shown in Fig. (39)). The N2 gas and water vapor was then pumped out until the pressure in the cell read ~10- 6 Torr. The cell was then sealed off from the vacuum line and placed in a small dewar. The study was conducted at -42°C (in order to fill the cell up with the liquid from the amount of gas we had). The relaxation time was ~72 psec. The relaxat1on data are plotted in Figs. (40)-(42). It is to be emphasized that these are only raw data and the lines through the data points in these figures are purely for visualization. It was, however , possible to reproduce the data in different experiments over a period of months. This indicates that despite one inherent error in doing picosecond studies with an entire pulse train, the method has apparently proved to be fairly reliable. In order to obtain more accurate relaxation times, a single pulse apparatus is needed. Although the pathway for decay of the C-H stretching vi bra ti on has not Been investigated (due to the limitation of the apparatus), the -108- t,ltlAI< OFF IIALS TO &E .S~AL&D Ol'F -109- __J <l: z (.!) -(f) C Q) ~ Q) ..c 0 CJ C (f) w ~ 0 1(f) I 1- z <l: -10 -5 0 5 Figure 40. 10 15 20 25 30 35 40 45 50 55 DELAY TIME (ps) VlbratioVlal Lifetimes of n-Alkanes (Temp.c2.o•c) : •- Heptane, o-Decane, ■ -TridecaVle, a PcntadecaVle. 60 -110- _J <x: z <.!) U) ---C Q) L. Q) .c 0 u C U) w ~ 0 1U) I 1- z <x: -10 -5 0 5 Figure 41. IO 15 20 25 30 35 40 45 50 55 DELAY TIME (ps) Vibrational Lifetimes of C7 HydroearbOYlS (Temp.• 2.o•c): e-,i.-Heptavte, o- 1-HeptcVle, 0-1, ro Heptadievie. 60 -111- _J <l: z (.9 (/) C Q) "- Q) .c 0 u C (/) w ~ 0 t(/) I t- z <l: -10 -5 0 5 Figure 42. 10 15 20 25 30 35 40 45 50 55 60 DELAY TIME (ps) VibratioVlal Lifetimes of C,o Hydrocav-boVls (Temp. • 20 ·c) : o - Isod.ccaVle, •- ri- Decavte , □- 1- Decene. -112- vibrational excitation most likely decays by way of two CH 3 bending at 1450 cm-l, as has been observed by Alfano and Shapiro [10] and Laubereau et al. [11]. The mechanism of this splitting into two lower frequency modes is probably the well-known Fermi-resonance effect. Since the second order of the CH 3-bend is around 2900 cm-l which lies close to the CH stretching frequency, the Fermi-resonance will mix the two states together, giving the linear combinations of the states. This new linear combination can then interact with the 1450 cm-l mode with a non-zero transition moment [20], and the vibrational-vibrational energy transfer is possible. The vibrational-vibrational energy transfer can go via intramolecular or intermolecular mechanism. molecular collisional interaction. The latter can be bi- or tri- In the case of CH 3-cc1 3 , it was reported to be trimolecular interaction from the binary liquid mixture study [ll]. Temperature Effect The temperature dependence has not been studied over a wide enough range to suggest its exact origin. It seems,however, that the vibrational relaxation does not occur via purely intramolecular mecha~ism. The intermolecular collision also plays a role in de-exciting the vibrational energy. As has been discussed by many authors [21] the vibrational relaxation time by collision depends inversely on collision frequency and collision efficiency per collision, which in turn are both temperature dependent. At lower temperature, molecules have smaller speed wftich will lower the collision rate. Futhermore, th.e -113- energy per molecule becomes low and low energy collisions are very ineffective. Hence, the participation in the relaxation mechanism of intermolecular collisions would give rise to a temperature effect similar to the one observed. It is interesting to note that proton spin-lattice relaxation (T 1 decay time) in long chain hydrocarbons seems also to take place most effectively from -CH 3 groups, both at room temperature (22] and at low temperature [23, 24] Furthermore, K. van Putte [25] has found that T 1 increase, with the chain length in paraffinic chains. Further experiments with a single pulse apparatus can be conducted in order to understand the mechanisms of this problem. A dilution experiment (for example, 1% of heptane in deurated heptane solution) may shed some light on the vibrational transfer mechanisms (whether it is intra- or inter- molecular). The 1450 cm-l bending mode relaxation time and intensities can be obtained and compared to those of the 2900 cm- 1 stretching mode in order to identify the decay route. Furthermore, if precise enough data were available, it might be possible to extract out the -CH2 - relaxation times from the n-alkane results by taking the total rate to be the sum of -CH 3 and -CH2 - rates normalized to the proportion of each such group in the molecule. The present data are too crude for this. Vibrational relaxation times can become a very important tool in the understanding of molecular interactions in liquid. Many -114- ~ystematic studies can be done (for example, benzene and its derivatives) in order to understand more the effect of the substituents on the molecular interactions and vibrational energy transfer mechanisms. Dephasing times, together with the photon echo technique, may promote the mechanism of dechasing phenomenon in different liquids, crystalline liquids, and solids. These experiments are extremely difficult, but with the development of the engineering art in the picosecond laser field, they may become possible. -115References l. P. M. Rentzepis, Adv. in Chem. Phys.~' p. 189 (1973), and references therein. 2. M. R. Topp, et al., Opt. Comm. l, 264 (1971). 3. M. J. Calles, Opt. Comm: l, 169 (1969}. 4. W. H. Glenn, et al. Appl. Phys. Lett . .!£, 54 (1964). 5. F. DeMartini and J. Ducuing, Phys. Rev. Lett . .!1_, 117 (1966). 6. R. R. 7. D. van der Linde, A. Laubereau, and W. Kaiser, Phys. Rev. Lett. 26, Alfano, Bull. Am. Phys. Soc.~, 1324 (1970). 954 (1971). 8. D. van der Linde and A. Laubereau, Opt. Comms. l, 279 (1971). 9. A. Laubereau, D. von der Linde, and W. Kaiser, Phys. Rev. Lett.'{]_, 802 (1971}. 10. R. R. Alfano arid S. L. Shapiro, Phys. Rev. Lett . .?1_, 1655 (1972). 11. A. Laubereau, L. Kirschner, and I~. Kaiser, Opt. Comm. 2_, 182 (1973). 12. I. D. Abella, N. A. Kurnit, and S. R. Hartmann, Phys. Rev. 141,139 (1966). 13. See,for example a. A. Yariv, Quantum Electronics (John Wiley and Sons, Inc., 1967), Chapter 23. b. E. Garmire, F. Pandaresse, and C. H. Townes, Phys. Rev. Lett. · 'll, 160 (1963) . c. M. Maier, H. Kaiser, and J. A. Giordmaine, Phys. Rev: 177 580 (1969). d. N. Bloembergen, Am. J. Jf Phys. 1§_, 989 (1967). -116- 14. See Ref. [9a] p. 398 and p. 405. 15. R. W. Minck, R. W. Turbune, and C. C. Wang, Proc. IEEE 54, 1357 ( 1966). 16. The diode is made from PIN diode type HP 5042, followed the instruction gi'ven by G. H. McCall in Rev. Sci. Inst. 43, 865 (1972}. 17. T. J. Chuang and K. B. Eisenthal, Chem. Phys. Lett;.!.!., 368 (1971). 18. American Petroleum Research Project 44, Serial Number 345 (1968). 19. See for example a. D. von der Linde, 0. Bernecker, and A. Laubereau, Opt. Comm. ~' 215 (1970). b. D. von der Linde, IEEE J. Q. E. Vol. QE8, 328 (1972). 20. K. F. Herzfeld, J. Chem. Phys. 47, 743 (1967). 21. See, for example a. K. F. Herzfeld and T. A. Litovitz, "Absorption and Dispersion of Ultrasonic Waves (Academic Press Inc., New York, 1959). b. K. F. Herzfeld, J. Chem. Phys. 36, 3305 (1961). c. G. M. Burnette and A. M. North, ed. "Transfer and Storage of Energy by Molecules" vol. 2 : Vibrational Energy (Wiley-Interscience, 1969), with references therein. 22. a. S. I. Chan, G. W. Feigenson, C. H. A. Seiter, Nature (London) 231, 110 (1971). b. G. H. Feigenson and S. I. Chan, J.A.C.S. 2-§_, 1312 (1974). 23. D. W. McCall and D. C. Douglass, Polymer!, 443 (1963). 24. J. E. Anderson and W. P. Slichter, J. Phys. Chem. 69, 3099 (1965). -11725. K. van Putte, Trans. Faraday Soc. 523 (1970); and J. Mag. Res.£, 216 (1970). -118Propos i tion l REPOPULATION TIME OF THYMINE AND URACIL Hauswirth and Daniels [l] reported the fluorescence, intersystem crossing, and calculated internal conversion rates of thymine and uracil. The fluorescence and intersystem crossing rates,'\,10 8 and 10 9 , respectively, are quite slow in comparison to that of the internal conversion, which is about 10 12 sec-l. Also, the quantum yield and chemical change are very low (¢fl "" 10 -4 and ¢chem = 10 -3 ) . It was thus believed that most of the energy undergoes internal . conversion. It is proposed here to study the internal conversion rate by measuring the rate of repopulation of the ground state from the first excited state. The experimental set-up can be derived from Rentzepis' work on azulene [2]. In addition, a pulse-selector [3] which selects only one pulse out of the whole pulse train is needed for a more accurate result. Two KDP crystals are employed in order to obtain a fourth harmonic generation at 26502 which will be used to excite the thymine and uracil to the first electronic excited state. As shown in the experimental diagram in Figure 1, a cut-off filter (F) is needed to filter out the 11 11 unwanted 0.53µ and 1.06µ lights. into the sample cell. The 0.265µ pulse will then be sent In order to measure the repopulation time, a dielectric mirror which reflects '\,75% at 0.265µ is placed on a translating micrometer which can move back and forth between the cell and the photodiode P2 . About 25% of the emerging light from the cell will to into the diode as a reference signal, whereas 75% of the light gets reflected back into the cell to be absorbed by the ground state LASER. I .L Frgure 1. kJ)9 l(1)p o.uµ, • o. 2.ltl5f4, 1.01,µ. ,., 0,2.i,,Sp., FILTH,S A Proposed Repopulatioti Time Experivnent. AND 4MPI.IFIC~ I PULSE UUCTOlt ......... ..1.1.114 I I (.0 >--" >--" -120- molecules. If we start out by placing the mirror right after the cell, the reflected or interrogating light will 11 see 11 molecules mostly in the excited state, and hence will go through unattenuated. This in- terrogating light will be reflected by a beam-splitter (BS) into a diode P3 . As we move the dielectric mirror backward a known distance, the reflected light will see more and more ground state molecules, and will be again absorbed by the solution. Hence, the change in the in- tensity at P3 as the function of time between the exciting and interrogating pulses will yield the repopulation rate of the molecules. The experiment is monitored by deflecting a small amount of 0.265µ light into diode P1 . References 1. W. Hauswirth and M. Daniels, Chem. Phys. Lett. 10, 140 (1971). 2. P. M. Rentzepis, Chem. Phys. Lett. 1, 717 (1969). 3. D. von der Linde, IEEE J. Quant. Elect. QE-8, 328 (1972). -121Proposition 2 INVESTIGATION OF THYMINE EXCIMER FORMATION IN AQUEOUS SOLUTION BY A PICOSECOND LASER By means of fluorescence study, excimer fonnations have been found in solutions of nucleic acids (for example, adenine-adenine) * , (adenine-thymine) * except (thymine-thymine) * [l ,2]. However, thymine dimer (TT) has been reported with very high quantum yield. The pre- cursor of dimerization was thought by the authors [1,2] to be the shortlived thymine excimer (TT) * . This is due to the argument that excimers may fonn without manifesting themselves by their fluorescence emission if another de-excitation occurs faster [l]. The fact that the fluores- cence yield of thymine is very small leads one to estimate the excited state lifetime of thymine to be in the order of io- 11 sec. One of the possibilities of the fluorescence quenching mechanism is the dimerization itself [2]. This may occur first with excimer fonnation (very fast process) and then the excimer going onto the dimer. It is proposed that thymine excimer fluorescence and lifetime be investigated by a mode-locked Nd-glass laser. Thymine in aqueous solution (both at room and low temperature) is first excited with a single picosecond 0.265µ pulse. The fluores- cence can then be observed with a spectrograph or spectrometer by means of a cs 2 light gate [3]. The proposed experimental set-up is shown in Figure l. In the experiment the two optical paths (1.06 path and 0.265µ path) are synchronized by means of one cs 2 optical shutter (lifetime~ -122- ., ~ e ·o )( uJ i Q) 0 s: -' ~It ·-E } tt Ii' 0 :n t.s;; 0 i.l • -d cu ~ C>- e Cl. <( 2 psec). -123Then a sample cell can be put in place. The excimer fluores- cence (excited by 0.265µ pulse) can then be searched with the help of a spectrograph setting around 0.3µ-0.5µ (the fluorescence spectrum of thymine is between 0.295µ-0.45µ, peaks at 0.33µ [4], but the excimer fluorescence is usually 5000-6000 cm-l red shifted). Once the fluores- cence is observed, the excimer fonnation time and lifetime can also be studied in the same manner (by delaying 1.06µ from the synchronized setting of the optical delay). The fluorescence intensity on the photo- graphic plate can be read by a microdensitometer as a function of time delay. It will be interesting to compare the lifetime of thymine excimers in liquid and frozen states. In the latter case, a low tempera- ture dewar with optical quartz windows is required. This experiment should bring a new light to the mechanism of thymine dimerization. References l. J. Eisinger, Photochem. Photobiol. ~' 597 (1968). 2. A. A. Lamola, ibid.,]_, 619 (1968). 3. M.A. Duguay and J. W. Hansen, Appl. Phys. Lett. J.i, 192 (1969). 4. W. Hauswirth and M. Daniels, Photochem. Photobiol. Jl.., 157 (1971). -124- Proposition 3 INVESTIGATION OF HYDRATED e-aq IN SOME HETEROCYCLIC COMPOUNDS IN AQUEOUS SOLUTIONS H. Joschek and L. I. Grosswiener [l] reported the studies on hydrated eaq ejected from aromatic compounds upon flash photolysis. However, there was an absence of e;q detection in certain compounds; namely pyridine, pyrimidine, benzoxazole, benzimidazole, benzothiazole, and purine. Since the authors were working with nanosecond flash photolysis, it is perceived that the eaq is formed but the lifetime is too short to be detected on such a time scale. The amount and . lifetimes of e;q formed in other aromatic compounds indicated that the structures and substituents of the ring compounds played important roles in the scavenging reactivity of e; . It is thus of interest to inves9 tigate the e; formation in the mentioned compounds on a picosecond 9 time scale, since it may yield some information on the scavenging reactivity, and toward a better understanding of mechanistic pathways of some photochemical reactions. The absorption spectra [2] of the compounds in Figure l peak around 0.265µ which is the fourth harmonic of the Nd:glass laser. experimental set-up is shown in Figure 2. The The sample is excited by the 0.265µ single pulse, then probed with a continuum (0.4µ-1 .06µ) produced by focusing 1.06µ in liquid o2o. - absorbs around Since eaq 0.65µ, the formation times and lifetimes of e;q can be obtained from the change in absorption at 0.65µ as a function of delay time between -125- 0 0 ~Rl&>IN! f'J'.R.ltf lOINE W" a> N H BENZIMU>AZOLE PURINE (Q 8&N?OTHIAT.OLE &ENZ OXA%0L£ Figure 1 -126- ,t Q) .µ ~ cU E ·c 8. )( LJJ -127the exciting and probing pulses. filter peak at 0.65 , and In the figure, F4 is an interference E is the echelon [3]. The 0.65 pulse, after emerging from the cell, is detected by an image intensifier together with a camera. The synchronization with cs 2 shutter and the control of the experiment (i.e., monitoring the signals) can be done in the same manner as in Propositions 1 and 2. References 1. H. Joscheck and I. L. Grosswiener, J.A.C.S. 88, 3261 {1966). 2. a. F. Halverson, J. Chem. Phys. 11, 711 (1951). b. S. F. Mason, J. Chem. Soc. (1954) 2071. c. W. A. Boggust, J. Chem. Soc. (1949) 355. d. E. A. Steck, et al., J.A.C.S. 70, 3406 (1948). e. R. Passerini, J. Chem. Soc. {1954) 2256. 3. M. R. Topp, et al., J. Appl. Phys. 42, 3415 {1971). A glass echelon consists of a stack of glass slides, equally spaced. The light pulse is spread uniformly over the face of the echelon by by means of a thin diffusing plate, experiencing varying retardations, the interpoint delay being given by 2- sin 2e) 112 - cos e] t = (1)[(n C where n is the refractive index of the glass, e the angle of incidence and d the thickness per slide. In this way, the pulse is split up into many parts by the number of glass slides; one part gets to the sample after another with the time separation t given above. -128Proposition 4 ELECTRONIC RAMAN SPECTRA: URANIUM IN RAMAN TRANSITION OF TRIVALENT CaF 2 CRYSTAL Electronic Raman spectra have been reported for trivalent rare earth ions in crystals [1-3], and also for the normal spinels: Ag 2Mo0 4 , GcGe0 4 , and ZnA1 2o4 [4]. The theory and selection rules have been worked out [5]. Electronic Raman effect is a very useful approach to the study of low-lying electronic levels where absorption and fluorescence spectra are weak or forbidden, since the Raman transitions are allowed between states of the same parity. In this paper I would like to propose the electronic Raman study on u3+ doped CaF 2 crystal, for the following reasons: 1. u3+ doped in CaF 2 is a laser material. 2. The low lying electronic levels have been known by the method of absorption and emission spectra at liquid N2 and liquid helium temperature [6-11]. However, the lowest energy levels assigned vary from one paper to another; as for example, G. D. Boyd reported the highest Stark levels of 41 (ground state) to be 609 cm-l [7], R. Meredith 912 reported 464.8 cm-l [10], J. P. Wittke, 515 cm-l [9], V. F. Egorova observed 542 cm-l for crystal type I and 655 cm-l for crystal type II [11]. The electronic Raman spectra will provide another way of in- vestigating these low energy levels. 3. The u3+ site in CaF 2 has been reported to be in tetragonal [10,12] and trigonal [13] crystal fields. By looking at the spectral -129- shift and the symmetry properties of the scattering or polarizability tensor elements, the symmetry of the u3+ site may be confirmed. (The results strongly depend on the orientation of the crystal to the incident laser beam.) 4. u3+, being the analog of Nd 3+ (Actinide-lanthanide analog), seems to give confidence that good results are to be expected. Electronic Raman transitions have been observed from Nd 3+ in ytterium gallium garnet [3b]. 5. The sample, u3+ doped in CaF 2 crystal, can be easily obtained. Theory The intensity of any Raman transition is given by the scattering tensor [14] (1 ) where (MA)nr is the matrix element of the electric dipole vector and the summation goes over all excited states in the system. k and n are initial and final states for Raman states, respectively. v is the frequency of the incident light beam. state Yv is the damping constant of r . 0. S. Mortensen and J. A. Koningstein worked out the scattering tensors for electric Raman transitions in their papers [5a,5c] as follows. -130- The total wavefunction can be written by means of the zeroth order Born-Oppenheimer approximation 'l' ~ tjJ(r,Q) where cp(Q) (2) r and Q denote the electronic and nuclear coordinate, respectively. The first order in the nuclear displacement, tjJ{r,Q), can be expanded as (3) where ~ is the electronic Hamiltonian that depends parametrically on Q and the zero point for the nonnal coordinates fixed at the equilibrium position of the nuclei. Substitute (3) in (1), leading to the sum of scattering tensor )dynamic (aAµ ) ku;nw = (aAµ )static+(~ ku;nw Aµ ku;nw where {a and . {M) {M) )static= l I A nr µ rk, <cp I~ ><cp let>>+ c.t. Aµ ku;nw h r,v vrv;kµ - v- 1yr w v v u -131- + = hl l <¢ I Q I¢> Q w µ where The steµ; in (4) and (5) are carried out by the use of the closure theorem in the space made up of the vibrational wave functions. If the static part of the scattering tensor is dominant, the transition will be independent of vibronic coupling and leads to the pure electronic effect. the vibronic effect. On the other hand, the dynamic part leads to The major factor that determines the scattering tensor is the frequency of the region of the exciting radiation. ttte transitions in the region are comparatively weak, then and the static term predominates. If h~k << l If one or more first order transi- tion in the region of the frequency of the incident beam is relatively strong, then the dynamic part predominates. For experimental reasons, the chosen and appropriate freguencies of incident radiation are those outside any strongly absorbing frequency range of the scattering system. So the vi~ronic effect can be neglect~d. -132The main difference between the electronic and vibrational Raman effects is that in electronic effect the antisyrrmetric part (static in this case), aAµ; aµA , often dominates over the syrrmetric part, and vice versa in the vibrational case. Thus, in the electronic effect, the polarizability of the system need not be expanded in terms of a particular normal coordinate (equation (3)). Selection Rules. These are the same in electronic effect as i- vibrational effect, i.e., the transition is allowed if the product of the irreducible representation of the ground and terminal excited states of a species occurs in which one or more, or a combination of components of the scattering tensor, belongs. Sf The selection rules for o4h and o3d are given on page 133. The electronic Raman transitions originate mainly from the crystal field splitting of (4,5f)n levels. The (4f)n-l(5d) levels and the charge transfer states are of too high energy [15]. Energy Levels of u3+ in CaF 2 Crystal. Trivalent uranium has an electronic configuration consisting of closed shell and three electrons in the unfilled (Sf) shell - (5f) 3 . By Hund s rule, the lowest term of 1 an f 3 configuration is 41, with J = 9/2 state lying lowest since the uranium multiplet is regular. The number of Stark splittings depends on the symmetry of the site of u3+ in CaF 2. Each splitting level is doubly degenerate (Kramer s doublet). 1 The absorption and emission (fluorescence) spectra with fairly high resolution at low temperature have been reported by R. E. Meredith [10]. He interpreted the spectra as arising from ions in the tetragonal sites. In such a case the ground 4r912 level splits -133- ( r..11), tr&, ,.•.l, (r. .r. >, rr., r.. l (~.ri>, ('fa,T.) ~ (r.,lll, <~,Tl> )(X-l;iY (t:,r4 ),( ~.Ta) lt~-1,i" ( -r, ,ls),( ti.Ti), ( r.,r,>,<r.,Ti> (S,c,S~) ,r..r&) S2 , iz, xx-~y ,"-~•~" (Z>'+><Z.,ldZ+Z.i;i) .. . . <r..r"),<r,,r,> S~, zz., XX+~y, (S",S~), (Z1'+x-z.,~z.+Z~) ( r.•~' ~·1 ) x>'-~'j, x~•~", (Sic,S~l, (~x+,cz, yz+z.~) r.-+A 1 , r;--.42., r;-&,, r_.-&1 , r 5-E r"-+ Ey1 r1 - E,,2. u;,r,)' ( r.. n) ~x.' xx ... ~~ (T.', T'.,) Sz (f.T'1), (~ls) (S.,S,>,t zx txz.~ yz+zy >, l XIC-y~, xy-t!:f"') (t;r,) ALL PORARIZ.ATIONS ALL-OWEJ> ("'4" ,.:, ALL (1;• Ti,•.. ) CS,oSi), ( %)(-tXz, ~z+z~ l, ( xx-~y, xy + ~x > (r. ... ,1'5,1,) Sz , Z'Z. , XX+~~ .,. . T1-+ A ' P0LAltl'Z.ATIONS ALLOWED r-. A,_' r-. E '4--+ E t1. , r ---. E~z. -134into 5 doublets and the 4 11112 levels split into 6 doublets implying that six absorption transitions should be observed in the near infrared. The diagram of the energy levels is shown on page135. However, J. P. Wittke [9] reported only 3 doublets in the 41 4 The diagram is shown on page 912 levels and 4 doublets in 11112 135. The author had to use the cubic crystal field to interpret the data despite the fact that the distortion in the symmetry occurs when u3+ substitutes ca 2+ in the crystai. V. F. Egora looked at the absorption spectrum and the stimulated emission spectrum of u3+ :CaF2 crystal and observed two different: results, depending on whether u3+ is in crystal type l or type II or hybrid (mixture of both) (see page 135). A. Yariv reported the observation of trigonal s6 system for u3+:CaF 2 about 111 direction and five Kramer doublets were expected for 411112 level. Experimental Set-up. Either a He-Ne laser or an argon laser is appropriate for the exciting source. The laser beam is focused on a single crystal which is mounted on top of a cold finger of a low temperature research dewar. A Jarrell-Ash monochromator is used. Some appropriate filters may also be needed to cut unwanted lights or emissions (for example, Rayleigh scattering}. A high sensitivity diode is used as the detector for the laser from the monochromator which is then fed into the digital voltmeter and the sample signal displayed on the recorder. It is better to work at low temperature since the electronic levels at room temperature are diffused and broad. Hence, the Raman -135- { &I.I'-'-'" e,~-·) ♦ 51'1, . \ ,u;11.t sw1-1tg,r tw..·•, ♦ 4'1S.1 '\ 41, ♦$ 44.11.t 4S'I!; 4450. , ♦ rfi'O 4-'S ♦ -1 "4'14 "' !"' ~-' ,n+.1' ....,. ,u ·'\, Spt'-'ti""i M ~r_d r r, 0 0 8i i'W. '"r'll q..,M 'r¼ ~ t>tAXHtl ~-h..vel oU~r~ttf ~ ~i~ ~ t,tM•Ui~ ,o us• io"'s .w- C#.f,_ • C Wrn-k.t., t-t. cJ..) ~f>(~ ( MtA..lC.U.t&.v MAA. ~ ~ ) ...." "" •"'-"' "" ~~11 ,.o, ----o '"" K 0 Lwd. " ~ o-\, u'~ ;,., C#.Fi, • f.- ~ ~fK I b- wr~ 1f><-~ ( V.F. ~ , ',t,. A.t,.) ti 1 -136- spectra may not be observed. Two sets of Raman lines are expected, one from 4r912 level, the other from 4 r 1112 level. However, the phonon band from the crystal will also be observed. References 1. J. T. Hougon and S. Singh; Phys. Rev. Lett. lQ_, 406 (1963). 2. J.J.H. Chau, J.C.P. 44, 1708 (1966). 3. a. J. A. Koningstein, J. Opt. Soc. Am. 56, 1402 (1966). b. J. A. Koningstein, J.C.P. 46, 2811 (1967). c. J. A. Koningstein, Appl. Spect. 22, 438 (1968). 4. J. A. Koningstein, et al . , J.C.P. ~' 354 ( 1972). 5. a. 0. S. Mortensen, J. A. Koni ngstei n, J~C.P. 48, 3971 ( 1968). b. 0. S. Mortensen, J. A. Koni ngstei n, Chem. Phys. Lett. l, 409 (1967). c. J. A. Koningstein, Phys. Rev. 174, 477 (1968); 169, 74 (1968). d. G. C. Tobisz, Chem. Phys. Lett. ]1_, 203 {1971). e. J. A. Koningstein and 0. S. Mortensen, J. Opt. Soc. Am. 58, 1208 (1968). f. A. Kiel and S.P.S. Porto, J. Mol. Spect. 32, 458 (1969). 6. P. P. Sorokin and M. J. Stevenson, Phys. Rev. Lett. i, 557 (1960). 7. G. D. Boyd, et al., ibid§_, 269 (1962). 8. L. N. Galkin and P. P. Feofilov, Soviet Phys. Doklady £, 255 (1958); Opt. Spectrometry]_, 492 (1959). 9. J.P. Wittke, et al., Proc. IRE~, 56 (1963). 10. R. Meredith and J. A. Menney, J.C.P. 39, 3127 (1962). 11 . V. F. Egorova, et al . , Opt. Spectrometry 20, 491 ( 1966). 12. B. Bleaney, et al., Proc. Phys. Soc. 698, 858 (1956). 13. S.P.S. Porto and A. Yariv, J. Appl. Phys. 33, 1620 {1962). -13714. G. Placzek, in Handbuch der Radiologie, Bd. VI, Teil II (Leipzig, 1934), page 205. 15. H. M. Crosswhite, et al., M.C.P. 43, 2047 (1965). -138- Proposition 5 ORGANIC PHOTOCHEMISTRY: STUDIES ON MECHANISMS OF PHOTODECONJUGATION OF CYCLOALKYLIDENE ESTERS The study on ring size and confonnational effects on photodeconjugation of cycloalkylidene esters [l] has shown that I (CH2)n \ H, I /CH 2co 2Et cC•C) C=CHCO 2-Et I (CH 2 ) n n =6 n =5 slow n=5 n =4 smooth n =4 n=3 slow n=3 7 I . n=2 The mechanism was studied by putting one methyl group in the five and six membered ring systems. In the five-membered ring, the reaction underwen~ as expected from the stability of the free radicals that were formed in the intennediate state, However, for the six-membered ring system, the reaction underwent in the opposite direction. In fact, there was only one product fanned (or enough to be detected by conventional methods): C.H! OCH,.C01£t.. X -139- The cis-trans fonns of this compound have been identified by H. Hauth, et al .[2] Ha Ha Ha He. 0 He. 0 OEt ~OEt trar1s CIS- cfavored. form) As for the cis-fonn, the available H for abstraction seems to be the equatorial one, whereas in the trans-fonn, there are both the axial and equatorial ones. This seems to lead to the interpretation of the selectivity in tenns of chemical demand for this reaction, I which dictates the removal of the hydrogen which has its cr bond parallel to the TT-orbital of conjugated system (i.e., the axial hydrogen). These results also implicate a concerted sigmatropic 1,5 hydrogen shift--an antarafacial process, of hydrogen to oxygen atom [3], and the transition of TT-TT * excited state [4]. However, in order to have a better understanding of the mechanisms of this reaction (of the six-membered ring), the following tests are proposed: l. Studies on the confonnationally rigid systems, by putting one more methyl group to the monosubstituted ring system. favored form should be The more -140- In this way, we can identify whether the axial hydrogen is the stereochemical requirement for the electronically excited molecule, since there are only equatorial hydrogens available. 2. In order to identify the type of transition, the dependence of the rate of product formation on the wavelength variation should be determined [5]. If the dependence does occur, that implies it is a transition. Consequently, this will lead to the interpretation TT-TT* that it is not the equatorial hydrogen that is abstracted. 3. The flash photolysis technique to identify whether it is a singlet or triplet excited species that is involved in the mechanism. The irradiations may be conducted in about 1% solution in hexane, using a Hanovia lamp and vycor filter (as done in Ref. [l]). Different filters are required in the wavelength variation experiment. Products are identified by the NMR technique, and reaction progress by vapor phase chromatography. References l. M. J. Jorgenson and S. Patumtevapibal, Tetrahedron Lett. 489-492 (1970). 2. H. Harth, et al., Helv. Chim. Acta. 48, 1087 (1965). 3. R. Hoffman and R. B. Woodward, Acc. Chem. Res. l, 17 (1968), and references therein. -141- 4. N. J. Turro and D. S. Weiss, J.A.C.S. 90, 2185 (1968). 5. K. 0gura and T. Matsuura, Tetrahedron 26, 449 (1970).