Adsorbate-induced effects on the H− ion collisions with Na/Ag(111) and K/Ag(111) surfaces

The H⁻ ion survival probabilities following on-top collisions with Na adsorbates deposited on Ag(111) at low coverage, are investigated for a wide range of exit angles from 20° to 90° measured from surface, and for various incident ion energies. A wave packet propagation approach is used in these calculations. The survival probabilities exhibit a series of well-defined peaks located at certain exit angles, that are indicative of avoided crossings between the various energy levels involved in the projectile/adsorbate/surface interaction. Both image states and the back-and-forth electronic motion between the ion projectile and the adsorbate/surface system contribute to the electronic population recaptured during the exit trajectory. For ion-surface collisions away from the on-top configuration, but in the close vicinity of adsorbates, a model is proposed to describe the variation of the H⁻ projectile's distance of closest approach to the adsorbate-covered Ag(111) surface in terms of the ion's impact point on surface, e.g., starting from the on-top collision with a single adsorbate and gradually moving away, towards the “clean” surface. The distance of closest approach is a key factor in calculating correctly the ion survival probabilities in the close region around the adsorbate, where the scattered ion fractions are affected the most. Results are shown for H⁻ in interaction with K/Ag(111).