Evidence for Vibrational Excitation of the Adlayer in Exoergic Processes at Metal Surfaces: H-atom Abstraction and Recombination and Adsorption-stimulated Desorption of CO

Abstract

Abstract The theoretical model presented in the previous paper predicts the possibility of vibrational excitation of the adlayer in exoergic process at metal surfaces to an extent determined by the interplay of reaction rates and energy dissipation into the metal. In the present paper this model will be employed for studying the following systems: a)The abstraction of Ds adspecies by Hgas and the accompanying H-atom recombination and b) the adsorption-stimulated desorption of COs in the presence of adsorbing COgas. Proper reduction of literature data provides the evidence for the existence of vibrational excitation of the H-Me and of the CO-Me adlayers, witnessed by desorption rates that are orders of magnitude larger than those expected for systems in Boltzmann equilibrium. Application of the model to Ds+Hgas relates the vibrational excitation of the adlayer and the corresponding non-Boltzmann desorption rates to the parameter Z/K and to the flux of adsorbing species. Rate coefficients K for vibrational relaxation of the H-Me bondare in the range 1013-1012 s−1 and decrease with increasing surface coverage σ. The analysis of COs desorption in the presence of adsorbing COgas confirms the dependence of desorption rates on Z/K, the coverage dependence of the rate coefficients K for energy relaxation of the CO-Me bond and brings out the predicted influence of gas pressure on the over-population of the vibrational levels of adsorbed CO. The decrease of K observed in both systems is discussed in terms of energy relaxation processes involving electron-hole pair excitation at the metal surface and it should be linked to the decrease of the surface electron density caused by the adsorbates.