Electron stimulated desorption study of the MgO(100)–D2O system

Abstract

A study of the MgO(100)–D2O system at 300 K by electron stimulated desorption (ESD) over the energy range up to 600 eV is presented. Auger electron spectroscopy is used as a complementary technique. ESD of the nearly perfect clean surface shows O+ desorbed ions as the main desorbed species with small amounts of H+ and F+ contaminants. The adsorption of D2O produces a dramatic decrease of the O+ intensity in the ESD spectrum and new signals corresponding to D+ and OD+ ions are observed. There is a first energy threshold at about 30 eV for D+ and H+ desorbed ions and a second one at 55 eV for O+ desorbed ions. The desorption of these ions corresponds to a primary excitation of the O 2s (24 eV) and Mg 2p (55 eV) core levels, respectively. The ion kinetic energy distribution of the desorbed O+ ion from the clean surface has two peaks at energies of 6 and 9 eV. The ion kinetic energy distribution of the D+ desorbed ions also shows a bimodal structure with most probable kinetic energies of 3 and 6.5 eV at 150 eV incident electron energy. The ion kinetic energy distribution of the H+ desorbed ions from the H contamination correlates quite well with that of the D+ desorbed ions both in shape and in energy. The most probable kinetic energy of the OD+ is about 3 eV. From these experiments, we propose an adsorption model with the D2O dissociated as OD+D. The D species bonds to O surface ions to form OsurfD radicals, and the OD species bonds to Mg surface ions. Since the experiments were performed at D2O constant pressure there is a large increase of the D+ intensity due to ESD of the D from OD species. The final equilibrium of the adsorbed species is a consequence of the D2O adsorption and ESD desorption rates.