Transition state theory: A reaction dynamics tool applied to gas-surface reactions

Abstract

Recombinative desorption of molecules from a metal surface is a fundamental step in heterogeneous catalytic reactions. Understanding this elementary mechanism can bring precious information on both the dynamics and the kinetics of gas-surface reactions. The aim of this work was to combine classical trajectory calculations and transition state theory based approaches to study the dynamics of molecular associative desorption. We were particularly interested in the description of state distributions in the products of associative molecular desorption. For late barrier processes such as H 2/Pt(111), energy transfers between vibrational, rotational and translational motions of the departing molecule are too weak to alter its state distributions. Accordingly, transition state theory gives a straightforward description of final state distributions. On the opposite, for early barrier processes, such as H2/Cu(111), strong energy transfers occur in the exit channel. Therefore we must apply the so-called "statistico-dynamical approach". This method is partly based upon transition state theory and takes into account energy transfers which occur between rotational and translational motions en route to the gas phase. Therefore, the statistico-dynamical approach gives a description of rotational state distributions of desorbed molecules. For both processes under investigation, statistical methods were found to be in very good agreement with both classical trajectory calculations and experimental results.