Neutralization and negative ion conversion of low-energy proton scattered from Ar, Kr, and Xe condensed on the Pt(111) surface

Abstract

Low-energy H+ ions (E0=100 eV) are scattered from Ar, Kr, and Xe layers condensed on Pt(111) to investigate the mechanism of neutralization and negative ion conversion of H+. The surface peak of H+ scattered from Xe is absent in a submonolayer coverage regime and increases in intensity as the multilayer grows. The H+ ion survives neutralization on Ar at any coverage. The Xe 5p orbital has covalency with the valence-band orbitals of Pt(111) through which the H+ 1s hole is delocalized, whereas Ar is basically physisorbed on Pt so that the H+ 1s hole (or the Ar+ 3p hole) is localized during the ion scattering time (around 5 femtoseconds). For the thick rare-gas solids, the H+ yield from Xe is almost two orders of magnitude as small as that from Ar since the H+ 1s hole is not perfectly localized in the former due to the hybridization between the Xe 5p orbitals. The H− yield from the solid Xe (Ar) surface is highly increased (decreased) relative to that from the Pt(111) surface. The H− ions are formed during a close encounter with a target atom and the existence of the band gap quenches the decay channels of H− on the outgoing trajectory from the surface. The experimental results of neutralization, electron-hole excitation, and negative ion conversion of H+ are elucidated consistently on the basis of the formation of the quasimolecule and the localization/delocalization of the valence holes in it.