Excited state formation in low energy Li+–surface collisions

Abstract

The formation of multiple final electronic states in the scattering of ions from surfaces has been used to probe the dynamics of the atom-surface charge transfer process. We present measurements of the formation of the Li(2p) state when scattering 100 and 400 eV Li+ off Cs-covered Cu(001) as a function of the Cs-induced work function change ΔΦ. For an initial energy of 100 eV, the Li(2p→2s) photon emission shows an initial increase as the work function decreases and a maximum at ΔΦ≊−1.7 eV. For 400 eV Li+, the peak in the photon yield is shifted to a larger work function change (ΔΦ≊−2.0 eV) and has an approximately one order of magnitude higher intensity. The charge state fractions of Li+, Li0, and Li− in the scattered flux have been measured as functions of work function changes for a beam energy of 400 eV. At the lowest work functions (i.e., largest ΔΦ), negative ions start to appear. The data are qualitatively reproduced using a model of nonadiabatic, resonant charge transfer, which includes spin, electron–hole pairs, multiple final charge states, and excited neutral atoms. The maxima in the Li(2p) formation versus ΔΦ can partly be explained by competition from negative ion formation.