Image effects on direct and inverse photoemission of CO adsorbed on NiAl(110)

The aim of this investigation is to elucidate the nature of the direct and inverse photoemission behaviour of CO adsorbed on NiAl(110). ARUPS reveals that the CO-5 sigma -derived ionization energy on NiAl(110) is surprisingly large in view of the moderate adsorption energy. A dynamical image theory explains the large 5 sigma ionization energy. The general shape of the theoretical electron-hole excitation spectrum is in accordance with the experimental direct and inverse photoemission spectra. The splitting of the 2 pi -derived features in the inverse photoemission spectrum is explained by the theory as well. CO adsorption on NiAl(110) is studied by means of a model Hamiltonian which includes electron correlation effects in the environment of the adsorbed molecule and image effects as a coupling of the adsorbate electrons to the surface plasmons. The coupling constants for the adsorbate-plasmon interaction are calculated from first principles. The charge density describing the image screening by the surface plasmons is calculated self-consistently. The electron-hole excitation energies are obtained as differences between the self-consistent (negative and positive) ionized states and the ground state.