Theories of the Dynamics of Inelastic and Reactive Processes at Surfaces

Abstract

The blossoming in the last fifteen years of the field of gas-phase chemical dynamics has greatly enhanced our understanding of how simple chemical reactions occur. Experiments have been approaching more closely the ideal of bringing together reactants that have been prepared in particular internal and translational states and measuring the cross sections for formation of individual final elastic, inelastic, and reactive channels (1). Theory has achieved, for a few simple systems, the goal of quantitative a priori prediction of the outcome of a molecular collision event (2-5). For more complicated systems, theory has pro­ duced valuable qualitative insights, such as how the location of a saddle-point on a potential energy hypersurface can influence the en­ ergy requirements for reaction and the energy disposal in products (6). Theory has also provided graphic representations, via classical trajecto­ ries (6) and quantum wave packets (7), of the actual unfolding of individual chemical encounters. Over the same period of time there has been a growing effort, both experimental and theoretical, directed toward elucidation of the detailed dynamics of gas-surface interactions (8-19). Accomplishments have been more modest than those of gas-phase investigations, but ex­ citing and important nonetheless. The objectives of these inquiries parallel those of gas-phase chemical dynamics. Quantitative information is desired about elementary processes, e.g. the accommodation of trans­ lational, vibrational, rotational, and electronic energy, sticking probabil­ ities, residence times, surface diffusion constants, etc. Qualitative in­ sights are desired in more complicated situations, e.g. how is the collision dynamics influenced by particular features of the gas-surface