Resonances in the photoemission cross section of a surface state: Results for Ag(111).

Abstract

We present a model to explain the resonances in the photoemission cross section of a surface state as a function of photon energy. The model is based on an extension of the direct-transition model for bulk states, and should be applicable to a surface state with a long decay length. Energy broadening of the final states and momentum broadening of the initial surface state are included in the consideration. Resonances in the surface-state photoemission cross section are expected to occur for photon energies corresponding to direct transitions from the surface state to final Bloch states. The cross section of a surface state in Ag(111) was measured over the photon energy range of 36--65 eV, and a resonance at 54 eV was observed. The data were compared with the theoretical results. The data for a similar surface state in Cu(111), studied previously by Louie et al. (Phys. Rev. Lett. 44, 549 (1980)) were reanalyzed in terms of the present model. The results for these two systems, Ag(111) and Cu(111), will be compared.