Electron-stimulated desorption of cesium atoms from cesium layers adsorbed ongold-covered tungsten

Abstract

The electron-stimulated desorption (ESD) yields and energy distributions (ED) for neutralcesium atoms have been measured from cesium layers adsorbed on a gold-covered tungstensurface as a function of electron energy, gold film thickness, cesium coverage andsubstrate temperature. The measurements have been carried out using a time-of-flightmethod and surface ionization detector in the temperature range 160–300 K. Ameasurable ESD yield for Cs atoms is observed only after deposition of morethan one monolayer of gold and cesium on a tungsten surface at a temperatureT = 300 K, which is accompanied by the formation of a CsAu semiconductor film covered with a cesiumatom monolayer. The Cs atom ESD yield as a function of incident electron energy has aresonant character and consists of two peaks, the appearance of which depends on bothelectron energy and substrate temperature. The first peak has an appearance threshold atan electron energy of 57 eV and a substrate temperature of 300 K that is due to Au5p3/2 core level excitation in the substrate. The second peak appears at an electronenergy of 24 eV and a substrate temperature of 160 K. It is associatedwith a Cs 5s core level excitation in the Cs adsorbed layer. The Au5p3/2 level excitation corresponds to a single broad peak in the ED with amaximum at a kinetic energy of 0.45 eV at a substrate temperatureT = 300 K, which is split into two peaks with maxima at kinetic energies of 0.36 and 0.45 eV at asubstrate temperature of 160 K, associated with different Cs atom ESD channels. The Cs 5slevel excitation leads to an ED for Cs atoms with a maximum at a kinetic energy of ∼ 0.57 eV whichexists only at T < 240 K and low Cs concentrations. The mechanisms for all the Cs atom ESD channels areproposed and compared with the Na atom ESD channels in the Na–Au–W system.