H− ion fractions scattered from Ag(111) and polycrystalline Ag

Abstract

Experimental H− ion fractions measured after H− scattering from Ag(111) and polycrystalline Ag at 1 keV incident energy in specular conditions revealed a larger ion fraction on H−/Ag(111) compared to polycrystalline Ag for exit angles less than 100 measured from surface, while a smaller ion fraction has been obtained on H−/Ag(111) compared to polycrystalline Ag for exit angles larger than 100. The H− ion fractions on Ag(111) are calculated with the Wave Packet Propagation method adapted to ion-surface interactions. The calculated ion fractions nicely reproduce the experimental results. A detailed study based on the analysis of the projected density of states for various ion-surface distances, the ion's interaction time with the surface, and the ion's distance of closest approach to surface, explains the behaviour of ion fractions on Ag(111) and polycrystalline Ag at small and large exit angles. At small angles, the effect of a larger distance of closest approach dominate over the ion's interaction time with the surface. This translates into less ion decay on Ag(111), because the states correlated asymptotically with H− and the surface state do not fully go through their avoided crossing and are mostly located inside the band gap. At larger angles, the ion is getting closer to the surface and the two states fully go through the avoided crossing. The state correlated asymptotically with H− moves into the image states energy region, while the state correlated asymptotically with the surface state approaches the bottom of the band gap. The electron loss is significant, and translates into a reduced ion survival on Ag(111).