Application of synchrotron radiation to investigation of the mechanism of increase in the yield of alkali metal ions in electron-stimulated desorption

Abstract

Core-level photoelectron spectroscopy with synchrotron radiation (hv = 140 eV) has been applied to study the variation in the Si+ charge state in silicon films deposited on the W(100) surface after thermal annealing of the substrate. The purpose of this study is to check the mechanism responsible for the sharp increase in the yield of Na+ ions in electron-stimulated desorption from a sodium layer adsorbed on the Si/W(100) surface after high-temperature annealing. The evolution of the W 4f7/2 and Si 2p photoelectron spectra and the valence band photoemission spectra is investigated for two silicon coverages (1 and 3 ML) on the W(100) surface in the temperature range 300<T<2200 K. It is shown that annealing of 1 ML Si on the W(100) surface results in the formation of a W-Si covalent bond, which can weaken the Si-Na bond and lead to an increase in the equilibrium distance X0 and, hence, to an increase in the yield of Na+ ions in electron-stimulated desorption. The variation in the photoelectron spectra under annealing of 3 ML Si differs from that observed after annealing of 1 ML Si in the direction of charge transfer, thus correlating with the opposite effect of annealing of 3 ML Si/W on the Na+ yield in electron-stimulated desorption.