Infrared Photochemistry

Author: Bomse, David Scott

Year: 1981

Degree: Dissertation (Ph.D.)

Advisor: Beauchamp, Jesse L.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/p6k2-tm14

Abstract

Three types of photochemical processes are observed resulting from relatively low power (< 100 W cm-2) cw laser irradiation. Multiphoton excitation and dissociation of gas phase polyatomic ions is described in Chapters II, ill and V. Ions are produced, stored, trapped for irradiation and analyzed using techniques of ion cyclotron resonance (ICR) spectroscopy. The effect of isotopic substitution on infrared multiphoton excitation of (CH3)2Cl+, (CH3)2Cl+(CD3) and (CD3)2Cl+ is the topic of Chapter II. Infrared photodissociation spectra are used to probe ion structures. For example, the wavelength dependence of perfluorocyclopropane parent ion is identical to that of perfluoropropylene ion indicating isomerization of the former to the latter. Measurement of dissociation kinetics indicates 100% of the ions isomerize. Thermochemical data on the C3F6+/C3F6 system obtained by photoionization mass spectroscopy are discussed.

Low power multiphoton excitation guarantees that target molecules decompose along the lowest energy pathway. This result is used to single out the lowest energy process in a series of reactions for which activation energies are unknown. Infrared photochemistry of alcohol proton bound dimers is used to demonstrate the utility of this technique (Chapter V). Additional thermochemical data on these species are provided in Chapter IV.

Infrared excitation also leads to an increase of more than three orders of magnitude in a bimolecular reaction rate (Chapter VI). Gas phase ions are the reacting species. Excitation is very selective as only one ion in a complex chemical system is perturbed by the infrared radiation. The bimolecular process is competitive with multiphoton dissociation of the reacting ion.

Single photon infrared photodissociation is explored in a study of van der Waals molecules (Chapters VII and VIII). Clusters containing ethylene are formed in a molecular beam and detected mass spectrometrically. Photodissociation spectra show homogeneous lines dominated by lifetime broadening. Derivation of a lineshape equation is described. For each ethylene-containing cluster a band maximum, pre-dissociative lifetime and transition moment are determined. Vibrational mode selectivity is explored by studying C2D4 clusters. Results are compared with theoretical treatment of triatomic van der Waals molecule predissociation.

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