Electron-stimulated desorption of alkali metal and barium atoms from an oxidized tungsten surface

Abstract

Electron-stimulated desorption (ESD) of Li, Na, K, Cs, and Ba atoms has been studied from metal layers adsorbed on an oxygen monolayer-covered tungsten and on tungsten oxide surfaces. The ESD yields and energy distributions for metal atoms have been determined as a function of the incident electron energy and the adparticle concentration at various surface temperatures. The ESD threshold for alkali metal atoms is close to the oxygen 2s level ionization energy of 25 eV. At T = 300 K the ESD energy distributions for alkali metal atoms consist of one peak. No ESD yield of Ba atoms has been detected at T = 300 K. At T = 77 K, the ESD energy distributions for Li, Cs and Ba atoms consist of several peaks. The Li, Cs, and Ba ESD yield dependencies on the incident electron energy reveal secondary thresholds corresponding to the inner shell ionization energies of adsorbed metals. The ESD of Li and Ba atoms arises from physisorbed and chemisorbed states, while that of Cs atoms arises only from a chemisorbed state in the position between two neighboring oxygen ions. It is shown that there are several ESD mechanisms for Li, Cs, and Ba atoms from layers adsorbed on oxidized tungsten.