Sputtering of negative hydrogen ions by cesium bombardment

The production of negative hydrogen ions sputtered from a low work function converter surface has been investigated. Hydrogen and cesium admitted into the vacuum chamber are chemisorbed on a polycrystalline molybdenum target. H−, Mo−, and e− are sputtered from this cathode by Cs+ ions in the energy range 150–1000 eV. Angular and parallel energy distributions of H−, Mo−, and e− are measured as a function of hydrogen gas pressure, cesium coverage, and incident ion energy. For optimum coverage, the H− ion temperature varies from 0.65% and 0.35% of the incident Cs+ bombarding energy for Cs+ ion energies of 250 and 1000 eV, respectively. The secondary electrons have a temperature of 0.04% of the bombarding energy almost independent of Cs+ energy. The spreads increase with decreasing target coverage and are independent of surface roughness. The optimum H−, Mo−, and e− yields are also measured as a function of hydrogen pressure and incident Cs+ bombarding energy. The optimum H− ion yield is 0.41 at a Cs+ ion energy of 750 eV. By extrapolating the H− ion yield at low Cs+ bombarding energy, a Cs+ ion threshold energy of 120 eV may be estimated. This indicates a binding energy of hydrogen smaller than 3.6 eV.