Computer simulation of ion images as an aid to indexing small crystallites

The magnitude of the sticking coefficient S of H₂ on polycrystalline platinum and its variation with temperature and coverage are compared with the exchange and equilibration f H₂ and D₂ under similar experimental conditions. The sticking coefficient measurements indicate the presence of at least two adsorbed states, a weakly bound precursor state and a more strongly bound state. The high temperature equilibration reaction is limited by the rate of adsorption into the precursor state. The low temperature exchange rate is limited by desorption from the strongly bound state and a model involving desorption from an immobile activated complex gives reasonable agreement with experiment. The heats of adsorption were determined at the coverage of interest. The low temperature equilibration reaction probably proceeds via mixing of H₂ and D₂ in the precursor state.