Development and Applications of Techniques in Laser Femtochemistry

Author: Pedersen, Søren

Year: 1996

Degree: Dissertation (Ph.D.)

Advisor: Zewail, Ahmed H.

Committee Member: Unknown, Unknown

Option: Physics

DOI: 10.7907/7q2y-1p50

Abstract

Femtosecond laser pulses provide the necessary time resolution to study fundamental dynamics of chemical reactions at the temporal scale of atomic resolution. New experimental techniques are introduced to the field of laser femtochemistry to generalize the range of accessible systems and further the understanding of molecular dynamics. In addition, theoretical models are developed that complement the experimental results and form the required basis for the interpretation of ultrafast measurements. The combination of multiphoton ionization with mass spectrometry and femtosecond lasers is introduced to isolate the elementary steps of reactions, differentiating parent and intermediate species and establishing the time scale for their dynamics. In two examples, using this approach, the direct real-time detection of diradical intermediates was made, and in another study the stepwise nature of a process initiated by the Norrish Type-I alfa-cleavage of acetone was revealed. This technique was also employed in the direct observation of vibrational motion in the transition state of a reaction (stilbene isomerization). The kinetic-energy time-of-flight (KETOF) method is developed to describe anisotropy and rotational alignment in the transition-state region and is applied to the dissociation of HgI2. An extension of the pump-probe scheme is made using the non-linear optical technique of femtosecond degenerate four-wave-mixing (DFWM) and application is made to the study of gas phase reaction dynamics of uni-(NaI) and bi-molecular (NaH2) systems. A detailed theoretical description of femtosecond transition-state dynamics is provided and illustrated via numerous examples. A kinetic model is developed, describing molecular response functions for ionization, fluorescence, depletion and absorption measurements. Effects of pulse width and saturation are discussed and comparison is made to classical and quantum mechanical models.

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