Transition to ®xed ®nal states in two-photon photoemission from adsorbate±metal systems

A simple theory is presented to study the ®nal 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) system. The 2PPE spectrum is calculated for a model system assuming that an unoccupied adsorbate level above the vacuum, populated by the two-photon excitation, couples to a continuum of free photoelectrons. An analysis of the 2PPE spectra as a function of the incident photon energy reveals the spectral features including the possibility of Fano-type modi®cation and resonance enhancement of the ®nal state transition in the 2PPE processes. Attempt is also made to explain why the d-band emission of the substrate is enhanced in the presence of adsorbates. # 2001 Elsevier Science B.V. All rights reserved.