RELATIONSHIPS BETWEEN ARRHENIUS ACTIVATION ENERGIES AND EXCITATION FUNCTIONS

Analytical expressions are derived which relate the Arrhenius activation energy to the energy dependent reaction cross section or excitation function. Results are presented for reactions which proceed with and without a threshold energy. It is shown that the activation energy for neutral-neutral reactions may display a strong temperature dependence, and that activation energies determined in thermal studies of ion-molecule reactions can commonly be expected to be either positive or negative, and strongly temperature dependent. Particular attention is given to recent rate measurements for the exchange reactions of atomic hydrogen with molecular hydrogen, and an attempt is made to derive excitation functions from these results. Three general formulations of the excitation function are used. These reflect the forms which have been suggested by experiments and by theoretical calculations. In particular one class of functions is found to be able to accurately reproduce the available data on the energy dependence of the reaction cross section for neutral-neutral reactions, and representative data for ion-impact induced fragmentation reactions. A second class of functions is introduced to include the special case of hard spheres which require a critical energy along the line of centers to react, while a final class adequately reproduces data for many ion-molecule processes.