Core-level processes in the electron-stimulated desorption of CO from the W(110) surface

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Abstract
Franchy and Menzel recently reported a significant increase in the electron-stimulated desorption (ESD) yield of O+ ions from CO adsorbed on the (100) surface of W at 120 K when the incident-electron energy exceeded that necessary to excite the oxygen 1s core level. Disintegration of the adsorption complex which becomes multiply charged by Auger decay of the core hole was offered as an explanation. In the present work we have investigated this effect in detail for adsorption of CO at 80 K on the W(110) surface. In agreement with Franchy and Menzel, we observed an increased O+ ESD yield for electron energies above the O 1s threshold for saturation coverages of CO adsorbed at 80 K. In addition, we find that the O+ yield in this region is strongly dependent on coverage and post-adsorption thermal annealing. We present data which indicate that, in fact, the magnitude of the O+ yield for electron energies much greater than the threshold appears to be rather insensitive to the CO binding site and follows closely the total CO coverage. In contrast, it is found that the O+ yield from excitation processes which have their thresholds at low energies, i.e., less than 100 eV, is strongly dependent upon the chemical state of the adsorbed CO and is greatly suppressed for coverages above about 0.5 monolayer. The O+-ion kinetic-energy distribution for excitation energies less than the O 1s threshold is found to peak at about 6 eV and cut off rapidly for energies less than approximately 3 eV, whereas for the high-energy desorption, i.e., for electron energies much greater than the O 1s threshold, the distribution extends to considerably lower ion kinetic energies. Coadsorption of C12 O16 and C12 O18 shows an isotope effect in ESD with yield ratio values of O+16 to O+18 equal to 1.6±0.21 and C12 O+16 to C12 O+18 equal to 1.27±0.027, which are found to be essentially independent of excitation...