Irreversible energy transfer in ionization of physisorbed adsorbates: II. Inelastic energy spectra of temporarily ionized adatoms

Abstract

We have studied inelastic processes following core ionization of a physisorbed adsorbate by an external probe (electron impact or photon absorption), which typically occur in the intermediate, temporary ionized adatom states of electron (ESD) or photon (PSD) stimulated desorption and XPS of adsorbates. In the ionization process the adsorbate makes a diabatic or Franck-Condon transition from the neutral state potential V (0) (r) to the ionic potential V (+) (r) in which the charge of the moving ion is cou the charge density fluctuations of the substrate. This suddenly switched on dynamical interaction gives rise to inelastic energy transfer from the probe and the ionized adsorbate to the substrate degrees of freedom (heat bath). In order to qualitatively assess such dissipative processes we have calculated the characteristic energy loss spectrum of the adatom, the spectrum of energy absorbed by the heat bath and the characteristic energy spectrum of the whole system. From these spectral functions and their characteristics (e.g. the Debye-Waller factor, the shape of the inelastic side-band, mean energy loss, etc.) we can obtain information on the dynamics of each constituent of the system in the presence of dissipative coupling between the ionized adatom and the substrate heat bath. This enables us to discuss the role which inelastic effects may play in recently studied cases of stimulated desorption of neutral Ar atoms from metal surfaces.