Abstraction of D on Ag(100) and Ag(111) surfaces by gaseous H atoms -: The role of electron-hole excitations in hot atom reactions and the transition to Eley-Rideal kinetics

H and D were adsorbed on Ag(100) and Ag(111) surfaces and characterized by thermal desorption spectroscopy. On Ag(100) surfaces hydrogen desorption from surface sites at 150 K and subsurface sites around 110 K was observed. Similarly, desorption of deuterium desorption around 150 and 110 K indicated surface and subsurface bound D. On Ag(111) surfaces only adsorption related desorption near 160 K was monitored. Abstraction of adsorbed D by gaseous H on Ag(100) is affected by reconstruction of the surface and the HD kinetics exhibits a H fluence (coverage) dependant cross-section. On Ag(111) adsorbed D is abstracted by gaseous H with a HD kinetics strictly according to the Eley-Rideal phenomenology. These features are close analogues of those observed on Cu(100) and Cu(111) surfaces. A comparison of the abstraction kinetics on various transition metals suggests that sticking of hot atoms, the reacting species on the surface, is controlled by electron-hole excitations. By this effect, the HD kinetics in abstraction on noble d-metals like Cu and Ag with a small density of states at the Fermi level and a small probability for e-h excitation exhibit Eley-Rideal phenomenology. Due to the small Ag-D bond energy on Ag(111) the attraction between incoming H and adsorbed D causes an increase of the abstraction cross-section at low D coverage, as was recently predicted by theory and verified by experiments on graphite surfaces. (C) 2003 Elsevier B.V. All rights reserved.