Desorption from Metal Surfaces by Low-Energy Electrons

Abstract

The effect of low-energy (15–200-eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field-emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre-exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σvβ≥10—19 cm2. These results are interpreted in terms of transitions from the adsorbed ground state to repulsive portions of excited states, followed by de-exciting transitions which prevent desorption. Arguments are made to show that the excitation cross sections should be essentially ``normal,'' i.e., ∼10—16 to 10—17 cm2, and that the much smaller over-all cross sections observed are due to high transition probabilities to the ground state, estimated as 1014 to 1015 sec—1. A detailed calculation for the case of exponentially varying transition probabilities and repulsive upper states is presented and discussed, and the variations in cross section with binding mode made plausible. It is shown that low-energy electron impact constitutes a sensitive tool for studying chemisorption.