Adsorbate Effects in Electron Ejection by Rare-Gas Metastable Atoms

The effect of adsorbed-gas layers on the ejection of electrons by rare-gas metastable atoms has been investigated for some tungsten surfaces. These surfaces were the (111) and (110) planes of a tungsten single crystal and an essentially (100) oriented polycrystalline tungsten ribbon. With nitrogen as the adsorbate the ejected-electron yields were reduced by 42% on the polycrystalline ribbon and increased 1% on the (111) plane. The yield did not change for the (110) plane. With carbon monoxide as the adsorbate, the yield decreases were 63%, 51%, and 56% for the polycrystalline ribbon, the (100), and (111) planes, respectively. With hydrogen as the adsorbate the yield was decreased by 1% on the (110) plane and 4% on the (111) plane. In those cases where the ejected-electron yield was sharply reduced following adsorption, the electron energy spectrum revealed a marked diminution of the number of high-energy electrons and in the case of carbon monoxide on the (111) and (110) planes, essentially a “cutoff” in the energy spectrum. No correlation was found between the yield changes and the known work-function changes accompanying adsorption. The effectiveness of each adsorbate in reducing the ejected-electron yield was in the order CO>N2>H2. For CO it was found that the decrease in yield of ejected electrons was proportional to the number of CO atoms which had impinged on the surface. By this means the sticking coefficient for CO on the (111) and (110) planes was estimated to be unity. The results are interpreted with a model in which bonding electrons from the adsorbed atom participate in the ejection process, the degree of participation depending on the surface density of the adsorbed atoms as well as their size and position on the surface. The theory, applied to the data for CO, predicts that the CO bonding electrons lie 12.6 eV below the vacuum level. It is inferred also that nitrogen atoms adsorbed on the (111) plane lie in the surface, while on the (100) plane they protrude from the surface.