Electron Transfer in Atom Surface Scattering

Surface production of H^- ions at low escape energies was investigated by backscattering H atoms from a variety of low work function metal and semiconductor surfaces. The H^- ions and electrons produced were self extracted from a planar diode surface conversion source. The surfaces investigated included cesiated polycrystalline Mo, cesiated Mo surfaces exposed to O_2, cesiated n and p type Si(100), thick films of (Ba/Sr/Ca) O produced by thermal decomposition of (Ba/Sr/Ca) CO_3, thick films of Cs_2O/Cs_2CO _3 produced by thermal decomposition of Cs_2CO_3 applied directly on the converter surface, and thin films of Cs_2O evaporated from Cs _2CO_3 heated to 963 K. The work functions of the oxide surfaces were measured during exposure to H and H_2 as functions of the surface temperature and the degree of thermal activation of the converter. The minimum work function obtained was 1.15 eV for the Cs_2 O converter. The yields of H^- ions and "exo-electrons" were measured as functions of the incident H atom temperature. The H^ - yields increased with decreasing surface work function and increasing H atom temperature. H atom conversion efficiencies of 1.6% were obtained from the Cs _2O converter at an atom temperature of 2700 K. For a Maxwellian distribution of incident atoms this corresponds to all atoms with energies greater than 1.45 eV being ionized upon reflection. The parallel energy distributions of H^- ions were measured as a function of the incident H atom temperature. The energy distributions were Maxwellian with a temperature equal to the temperature of the incident H atoms. This indicates that fast atoms are reflected in elastic collisions. A theoretical model based on a semi-classical rate equation of resonant electron transfer in atom surface scattering at low energies was developed. The results show that the basic limitation on the H^ - ion yield is trapping of slow atoms by the image force.