Microcanonical Rates of Unimolecular Reactions Studied by Time-Resolved Photofragment Spectroscopy

Citation

Khundkar, Lutfur Rahman (1988) Microcanonical Rates of Unimolecular Reactions Studied by Time-Resolved Photofragment Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zfwr-rk30. https://resolver.caltech.edu/CaltechETD:etd-04042008-083918

Abstract

The use of modern technology in the study of classic problems in chemistry has afforded researchers the opportunity to understand the dynamics of reactions on a molecular level. This thesis represents one such instance, where the combination of pulsed lasers and molecular beams has made the study of ultrafast reactions of isolated molecules possible.

The optical technique is, in essence, a two pulse pump-probe method. One of the pulses resonantly excites the molecule to be studied, while the second one, delayed in time, measures the population in either the excited parent state or some quantum state of the product. The sample is cooled in a supersonic expansion and observed under collisionless conditions. Atomic and molecular species may be detected by laser-induced fluorescence (LIF) or resonantly-enhanced multiphoton ionization (REMPI), followed by mass-selective detection. The temporal evolution of the state being probed is mapped out as the delay between the pulses is varied. The resolution of this method is determined by the temporal widths of the two pulses, typically ~ 8 picoseconds in the studies presented here.

The technique, termed Time-Resolved Photofragment Spectroscopy (TRPS), has been used to study a number of unimolecular reactions under collisionless conditions. Microcanonical rates of the reaction NCNO → CN + NO have been measured for excitation energies near threshold. The rates are compared with predictions of various statistical theories, including a recently proposed variational method. State-selected studies of the production of iodine atoms in the UV photodissociation of 1,2—diiodotetrafluoroethane indicate the existence of a bound intermediate, the iodoperfluoroethyl radical. Energy dependent rates show that the amount of internal energy in the intermediate may be varied by changing excitation conditions, thereby affecting the rate of its decomposition. Time and frequency-resolved studies of the photodissociation of methyl iodide show evidence that perturbed species in a dissociating system (so-called transition-state species) may be observed by this technique.

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