Time-resolved studies of molecular reaction dynamics and development of experimental methodology

Abstract

The six research topics presented in the following chapters are concerned with several diverse problems of molecular reaction dynamics in isolated gas-phase environments. The scope of the studies ranges from performing direct measurements of bond-breakage on electronically dissociative potential energy surfaces, to monitoring the time-course of a restricted geometry bimolecular reaction. The common experimental method used in all of the studies has been a variant of pump-probe time-resolved spectroscopy. The underlying theme of the endeavors has been threefold: 1) To gain a better understanding of the role of intramolecular dynamics that precede or are commensurate with the reaction dynamics; 2) To begin to appreciatiate the observable manifestations of specific features of the reactive potential energy surface; and 3) To utilize the specific temporal behavior to elucidate quantitative information for the said potential surface. The studies of molecular dissociation on repulsive electronic surfaces has lead to a quantification of the timescale for primary steps in reaction processes. Moreover, transform limited temporal/spectral studies have begun to focus on specific long-range reaction fragment interactions in a state-specific manner. The latter endeavor has identified a mechanism for the reaction-fragment(s) interaction in the near-asymptotic product region. Predissociative reaction and intramolecular dynamical behavior has been studied on ground potential energy surfaces. Overtone excitation of the OH-stretch mode of hydrogen peroxide enables molecular ground state excitation and state-specific detection of the OH reaction product. These investigations point out the potential of this picosecond pump-probe method for directly elucidating the intramolecular energy redistribution process and the possibility for direct investigation of the long-range tail region of the free-radical recombination potential surface. The investigation of a spatially oriented bimolecular reaction has conclusively shown that the IH-OCO reaction proceeds by way of the [HOCO] reaction complex species. The close proximity of the van der Waals bound reactants produces unique multi-body interactions not found in the gas phase but which may arise in condensed phases. Moreover, these investigations have obtained evidence for a unique reaction reasonance (which is analogous to a shape-resonance) feature. The presence of such a resonance in the reaction entrance channel region affects the temporal behavior and yield of product formation.