Laser Studies of Gas Phase Chemical Reaction Dynamics

Abstract

This perspective is addressed primarily to active reaction dynamicists (i.e. chemical kineticists working on a reactant state to product state level of inquiry) to outline current capabilities of coherent light sources for the detailed study of chemical rate processes, to record past successes, and to identify future areas of important and likely progress. We assert that the following axioms, already well known to time­ independent spectroscopists, also apply to time-dependent measurements of evolving reactant-product systems: (a) any matter-radiation interaction can be studied better with a coherent, rather than an incoherent, light source, and (b) coherent light sources of nearly ultimate (i.e. uncertainty principle limited) frequency and time resolution are currently available or will soon be developed for the entire energy spectrum appropriate to chemical interactions. It is now timely and meaningful for every dynamicist to inquire whether existing experiments can be improved via laser devices and, indeed, whether totally new and highly refined experiments are feasible. We emphasize the state-to-state (i.e. microscopic) approach to chemical reaction dynamics; we wish to scrutinize chemical reactions on a global detailed level (1): at a given total system energy E, many reactant and product channels are open and many transition probabilities [i.e. the rich person's P-matrix (I)J must be con­ sidered. Of practical interest (2) to dynamicists (and others) are questions of energy requirements and energy disposal (1): e.g. (a) what is the optimal reactant internal state which promotes reaction at energy E? and (b) what is the distribution of product internal states for a particular reactant internal state at energy E?; as well as questions of energy redistribution: e.g. (c) are unimolecular reactants ergodic on the time scale of their fragmentation? and (d) what are the dominant collisional relaxation processes and time scales which degrade an initial nonequilibrium distribution? Laser methods aid the dynamicist in many ways: reactants can be selectively excited to particular internal states, product state distributions can be 1 Literature search completed in December 1974. 2 Camille and Henry Drcyfus Foundation Teacher-Scholar, Alfred P. Sloan Foundation Research Fellow. 3 Our own laser studies are supported by the Directorate of Chemical Sciences, US Air Force Office of Scientific Research under Grants AFOSR-73-2423 and AFOSR-74-2666.