Classical over-the-barrier model for neutralization of highly charged ions above thin dielectric films

摘要

We apply the classical over-the-barrier (COB) model to charge transfer between highly charged ions (HCIs) andntargets consisting of thin dielectric films on metals. Distances for the onset of classically allowed above-surfacenelectron capture are obtained as a function of HCI charge state, film thickness, film permittivity, and film-metalnelectron binding energies. The model describes the crossover between the previously existing COB model fornbulk metals and bulk dielectrics as the thickness of a dielectric film on a metal substrate increases throughnthree distinct regimes. For ultrathin films with low permittivity and positive electron affinity, over-the-barrierncharge transfer initiates from the metal electrons behind the film and critical distances are greater than thosenfrom bare metal targets. This result is consistent and compared with the recent observation of potential emissionnenhancement above thin C60 films on Au(111) with increasing film thickness [Bodewits, Hoekstra, Kowarik,nDobes, and Aumayr, Phys. Rev. A 84, 042901 (2011)].