Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

Abstract

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ∼30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ∼38 eV, which can be identified with the ionization energy of the W 5p3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ∼50 eV, and for Θ ≥ 0.5, another resonance peak appears at ∼80 eV. These peak positions correlate with the ionization energies of the W 5p1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ∼0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ∼0.5 eV and a narrow one at ∼0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs2+ ion; another channel, neutralization of the adsorbed Cs+ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.