On the vibrational and orientational probing of surface-adsorbates by direct and resonance photoemission

Vibrationally resolved direct photoemission spectra of physisorbed molecules separated from the substrate by spacer layers, show remarkable splittings of vibronic progressions. The corresponding resonant photoemission spectra have recently confirmed these splittings and also revealed their strong dependence on the polarization of the exciting radiation. We apply a self-consistent mirror image model on the physisorbed nitrogen and oxygen molecules to explore the origin of these shifts and their dependence on the orientation of the adsorbate molecule with respect to the surface. The symmetry character and orientational probing of autoionization is analyzed for the resonant cases by a decomposition of the interacting matrix elements.