Transition to fixed final states in two-photon photoemission from adsorbate-metal systems

A simple theory is presented to study the final state transition in the two -photon photoemission (2PPE) processes of the adsorbate on a metal surface, in the light of recent experimental observation for benzene/Cu(1 1 1) syst em. The 2PPE spectrum is calculated for a model system assuming that an uno ccupied adsorbate level above the vacuum, populated by the two-photon excit ation, couples to a continuum of free photoelectrons. An analysis of the 2P PE spectra as a function of the incident photon energy reveals the spectral features including the possibility of Fano-type modification and resonance enhancement of the final state transition in the 2PPE processes. Attempt i s also made to explain why the d-band emission of the substrate is enhanced in the presence of adsorbates. (C) 2001 Elsevier Science B.V. All rights r eserved.