HINDERED ROTATION OF MOLECULAR ADSORBATES - APPLICATION TO PHOTOIONIZATION

Hindered rotation, i.e., the libration and precession of a molecule adsorbed on a surface is demonstrated to play a role in the photoionization of adsorbates. Here we develop a theoretical model including hindered rotation and unravel the qualitative influence of this nuclear mode on photoionization spectra. We consider two typical examples. First, for physisorbed species, the effect of hindered rotation is significant placing the adsorbate at the midway between a model situation where the molecule is freely moving in the gas phase and a molecule frozen in space. Second, for the chemisorbed species the influence of hindered rotation shows up as a measurable modification of the angular distribution of electrons and our model is able to account for the experimentally observed temperature effects on the photoionization spectra. Finally, we give a recipe connecting the angle (mean angle in the quantum-mechanical sense) between the molecular axis and the surface normal and the photoionization cross section. Using experimental data this recipe will allow a better estimation of the position of the molecule on the surface.