Electron stimulated desorption from CO and O adsorbed on clean and Cu covered W(110) surfaces

Abstract

Electron stimulated desorption from CO adsorbed on Cu n W(110) with n = 0, 1, and 2 was carried out in order to compare cross sections and thresholds for CO, CO +, and O + desorption. It was found that the thresholds for all three products were virtually independent of the presence or absence of Cu, indicating very similar mechanisms; however cross sections for n = 1, 2 were appreciably higher than on clean W(110). The threshold for CO production is attributed to creation of a 5σ hole; absence of 2π b DOS for n ⩾ 1 rules out a 2 π b → 2 π a transition as the mechanism. CO + production could correspond to creation of 5σ −2. CO + formation near threshold may occur by creation of a 5σ −2 state. Arguments are presented to show that formation of 4σ −1 also corresponds to the CO + threshold and may lead to CO + desorption. O + production from adsorbed CO corresponds to formation of various 2 hole states. Comparison of yields of neutral CO from n = 0 and n ⩾ 1 also made it possible to obtain desorption and β-CO conversion cross sections for CO adsorbed on bare W(110). The ESD behavior of O/Cu n /W(110) layers for n = 0, 1–4 was also investigated. Only O +could be detected in all cases and there was no change in O coverage after prolonged electron bombardment. It is argued that O + comes from minority states corresponding to O adsorption on edge or corner metal atoms at steps and other imperfections. The thresholds are consistent with initial creation of a 2s hole on O, followed by filling of this hole by a metal electron and ejection of an O2p Auger electron. The resulting M +O + state leads to desorption of energetic O +. This process restricted to O bound to metal atoms in sufficiently poor contact with the rest of the substrate to give M + and O + the required lifetimes. None of the processes investigated showed openings of new channels at energies corresponding to creation of holes in substrate metal atoms.