EXCITED-STATE FORMATION IN LOW-ENERGY LI+-SURFACE COLLISIONS

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 DELTAPHI. 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 DELTAPHI almost-equal-to -1.7 eV. For 400 eV Li+, the peak in the photon yield is shifted to a larger work function change (DELTAPHI almost-equal-to -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 DELTAPHI), 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 DELTAPHI can partly be explained by competition from negative ion formation.