Interplay between emission of electrons and quasi-resonant ultrafast electron tunneling in cluster–surface collisions

Abstract

Low energy collisions between clusters and solid surfaces are studied both theoretically and experimentally. The relative electron emission yield γ( N) has been measured as a function of the cluster size and collision energy. Results for different cluster types and surfaces are presented. A marked size dependence of γ( N) is obtained for collisions between Pt − N , Pb − N and Pb + N clusters with a graphite (HOPG) target. Moreover, the form of γ( N) changes dramatically with the collision energy. In contrast, if aluminum, for instance, is used as target, γ( N) shows a monotonically decreasing behavior for increasing cluster size. We present a theoretical description of the collision process using a microscopic model. The time-dependent charge exchange between the clusters and the targets was studied. In order to describe the neutralization dynamics we calculated the nonadiabatic survival probability P s( N) of the charged clusters. For surfaces with narrow densities of states P s( N) exhibits damped Stückelberg oscillations as a function of N. For metallic surfaces P s( N) shows a monotonically decreasing behavior for increasing cluster size N. We found that P s( N) shows the same behavior as γ( N). Our results suggest that the electron emission reflects the femtosecond neutralization dynamics during the collision and that the size dependence of γ( N) can give information on the cluster–surface interaction potential.