Lifetime and energy of negative ion resonances of adsorbed molecules. A layer Korringa-Kohn-Rostoker approach

We present the first application of a new layer Korringa-Kohn-Rostoker technique for the calculation of the energy and lifetime of negative ion states in adsorbed molecules. This approach allows, for the first time, the proper treatment of elastic multiple scattering by both the surface barrier and the substrate and its effect upon the lifetime and energy of adsorbate negative ions. We present results for two model systems; the 2Hg negative ion resonance in N2 on Ag(111) and the 4Σ0 resonance in O2 on Pt(111) and Pt(100). For the low-lying 2Hg negative ion resonance in N2 on Ag(111), we demonstrate that the behavior of the lifetime and energy upon adsorption is dominated by the surface barrier; the substrate reflectivity has a negligible influence. In contrast, the lifetime and energy of the higher energy 4Σ0 resonance in O2 on Pt is dominated by the substrate reflectivity which causes the resonance lifetime to oscillate as the molecule approaches the surface. These findings are interpreted in terms of the interaction between the molecular resonant state and surface resonances.