Local adsorbate-induced effects on dynamical charge transfer in ion-surface interactions

We have measured positive-ion survival probabilities for ${\mathrm{K}}^{+}$ scattered from clean and alkali-metal-adsorbate-covered Cu(100) and Cu(110). As expected from most models of resonant charge transfer, we find that very small coverages of alkali-metal adsorbates on the surface can dramatically decrease the ion survival probability. However, the dependence of the ion survival probability on adsorbate coverage is qualitatively different from that expected on the basis of the adsorbate-induced work-function change. We find that a treatment of the charge transfer based on the spinless Newns-Anderson Hamiltonian explains the data only if the local electrostatic potential of the adsorbate overlayer is included in the calculations.