Multicharged Ion Impact on Clean Au(111): Suppression of Kinetic Electron Emission in Glancing Angle Scattering

Abstract

We have measured the number statistics of emitted electrons in coincidence with the projectile scattering angle distribution for multiply charged slow Ar ions scattered off an Au(111) surface at grazing incidence. The comparison of the electron statistic spectra for different projectile exit and incidence angles permits a separation of potential and kinetic electron emission. A molecular dynamics simulation allows to associate the multiple electron emission with specific trajectory histories (planar and subsurface channeling, close collisions with target atoms at steps and imperfections). [S0031-9007(98)06964-6] PACS numbers: 79.20.Rf, 61.85. + p, 79.60.Bm The interaction of slow highly charged ions (HCI) with clean surfaces reveals new phenomena like the transient formation of exotic “hollow atoms” [1 ‐5] and the recently discovered effect of “potential sputtering” [6]. Above the surface the formation and evolution of hollow atoms can be described theoretically by the classical over-the-barrier (COB) model [1,7]. The image energy gain and the distance of hollow atom formation predicted by this model could be verified in several independent experiments [8‐ 10]. Other signatures of hollow atom formation and decay like potential electron emission (PE) [11] are often masked by significant contributions from below surface interaction processes, in particular, at higher projectile energies where kinetic emission phenomena become dominant [12]. So far, most such results could therefore be used only as indirect proof for the COB model. We have developed an experimental approach which enables us to distinguish between electron emission induced above (or at) the surface from below surface emission. This information can, in turn, provide distinction between kinetic and potential emission. In this work we correlate the electrons emitted during grazing incidence scattering of multiply charged argon ions at a clean metal surface with specific projectile trajectories characterized by the resulting scattering angle, to achieve a separation of above and below surface emission. A comparison with electron emission data for normal incidence with the same projectile, charge state, and target species having equal velocity normal to the surface allows the direct identification of potential and kinetic electron emission