Evolution of Negative Ion Resonances at Surfaces: Effect of Environment and Orientation

Abstract

The influence of the environment and orientation on the evolution of negative ion resonances at surfaces following low energy (0-15 eV) electron attachment to adsorbed and condensed molecules is studied by means of electron stimulated desorption (ESD) of negative ions. In the low energy regime ESD of anions usually proceeds via dissociative electron attachment (DEA) to an individual molecule at or near the surface. Three prototypical cases are discussed in the light of the corresponding gas phase process. (i) ESD of F− from a multilayer film of NF3 shows desorption features which are considerably narrower compared to the corresponding gas phase DEA analogues. This is a mirror of the particular dynamics of the unimolecular reaction in relation to the energy constraints operative in ion desorption. In addition to the strong F− desorption it is demonstrated by applying surface vibrational spectroscopy that (resonant) dissociative electron attachment at low energies (0–5 eV) is responsible for chemical changes within the NF3 film, viz. formation of NF2 radicals and N2F4 molecules. (ii) In the ESD of Cl− from submonolayer Cl2 adsorbed on rare gas surfaces a strong dependence of both the signal intensity and shape of the desorption resonance on the angle of incidence of the electron beam with respect to the surface normal is observed. This behavior points towards a preferential orientation of the molecules perpendicular to the surface. (iii) The absolute DEA cross-section can appreciably be enhanced by the medium due to an increase of the autodetachment lifetime of the negative ion resonance. This is demonstrated in ClCN for an electronically excited resonance.