Energy disposal and energy requirements for elementary chemical reactions

Abstract

The role of energy in promoting chemical change and the reverse problem of the energy distribution of the reaction products are of fundamental interest in the study of chemical dynamics. Since the earliest atomic flame experiments it has been known that exoergic elementary reactions often lead to products with high internal excitation. More recently, direct evidence from molecular and ion beam experiments indicates that, for reactions with an activation barrier, reagent internal energy is often more effective in promoting reaction than is translational energy. In addition, detailed product state distributions for exoergic reactions determined by chemiluminescence, chemical laser and molecular beam methods have frequently indicated substantial population inversion. Both phenomena (obviously related, through microreversibility) can be qualitatively understood from potential surface considerations (via classical trajectory analysis), but several problems remain. We address ourselves to the following questions: (a) optimal means of characterization of the product (or reactant) energy state distribution, (b)compaction of the voluminous body of data (or computer-simulation thereof) comprising such distributions, and (c) development of measures of the specificity of the energy release, of the selectivity of the energy consumption (and their mutual dependence). As a link between the reactive scattering and the non-equilibrium statistical mechanics we use the concept of the entropy(or information) of the product (or reactant) state distribution. The required measure of specificity(or of selectivity) is provided by the concept of the surprisal of a particular outcome. This approach leads to a means of dealing with molecular beam measurements of product angular and translational energy distributions.