CHARGE-TRANSFER IN HYPERTHERMAL ENERGY COLLISIONS OF LI-METAL-COVEREDCU(001) .1. DYNAMICS OF CHARGE-STATE FORMATION( WITH ALKALI)

We have measured the charge state fractions in the scattered flux vers us the work-function change induced by Cs adsorbates when hyperthermal energy Li+ ions are incident on Cs/Cu(001). As the work function phi decreases from its clean surface value (phi=4.59 eV), the positive ion fraction decreases from 0.64 to less than a few percent, while the ne gative ion fraction slowly increases from less than a few percent to 0 .14 at the lowest work function attained (phi=1.29 eV). Theoretical ca lculations based on a many-body solution to the time-dependent, multip le-state, Anderson-Newns model of resonant charge transfer qualitative ly reproduce the observed trends. Detailed examination of the calculat ions shows that the theoretically predicted dependence of the final ch arge state fractions on work function and the dynamics of the charge-t ransfer process are both strongly influenced by the wave function of t he ion-surface system when the incident ion is close to the surface an d also by the changing relationships between the relevant time and ene rgy scales as the scattered particle moves away from the surface.