On the Coverage-Dependent Adsorption Geometry of Benzene Adsorbed on Pd{111}: A Study by Fast XPS and NEXAFS

Fast XPS data obtained during benzene uptake on Pd{111} are consistent with those obtained during benzene desorption, and both are in accord with the corresponding NEXAFS results. At low coverage the molecule lies flat, is strongly adsorbed, and is characterized by a relatively low C 1s binding energy. At higher coverage, a second form of benzene is also present. This species is strongly tilted with respect to the metal surface. It is characterized by a relatively high C 1s binding energy and a narrower π* resonance: both attributes are in keeping with its observed weaker adsorption strength. The coverage-dependent change in adsorption geometry from flat to tilted benzene, originally suggested in order to account for the kinetics of acetylene trimerization on Pd{111}, is unequivocally demonstrated.