Classical models for electron capture by highly charged ions in the thin film regime

We present an extension of the classical over the barrier (COB) model[1] for highly charged ions (HCIs) that describes thin dielectric films on metal surfaces, bridging the bulk metal and bulk insulator regimes. Motivated by recent experiments [2,3], we detail the onset of charge transfer between a HCI and metal covered with a dielectric thin film. In this talk, capture distances as a function of C$_{60}$ film thickness on Au(111) will be presented. For ultrathin films, electron capture begins from filled levels in the metal and the C$_{60}$ film decreases the potential barrier for charge transfer and increases the critical distance compared to clean Au(111), increasing the time available for above-surface relaxation. ~This is consistent with the new observation of increasing HCI-induced electron emission yield as a function of film thickness [3]. As film thickness grows and reaches a critical value, the first captured electrons originate from the film at the distance expected for an insulator target.\\[4pt] [1] J. Burgd\"{o}rfer et al. Phys. Rev. A 44, 5674--5685 (1991) \\[0pt] [2] R.E. Lake et al. Phys. Rev. Lett. 107, 063202 (2011) \\[0pt] [3] E. Bodewits et al. Phys. Rev. A 84, 042901 (2011)