Charge transfer in hyperthermal energy collisions of Li+ with alkali-metal-covered Cu(001) .1. Dynamics of charge state formation

We have measured the charge state fractions in the scattered flux versus 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 negative ion fraction slowly increases from less than a few percent to 0.14 at the lowest work function attained (phi=1.29 eV). Theoretical calculations based on a many-body solution to the time-dependent, multiple-state, Anderson-Newns model of resonant charge transfer qualitatively reproduce the observed trends. Detailed examination of the calculations shows that the theoretically predicted dependence of the final charge state fractions on work function and the dynamics of the charge-transfer process are both strongly influenced by the wave function of the ion-surface system when the incident ion is close to the surface and also by the changing relationships between the relevant time and energy scales as the scattered particle moves away from the surface.