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 st rongly adsorbed, and is characterized by a relatively low C Is binding ener gy. At higher coverage, a second form of benzene is also present. This spec ies is strongly tilted with respect to the metal surface. It is characteriz ed by a relatively high C Is binding energy and a narrower pi* resonance: b oth attributes are in keeping with its observed weaker adsorption strength. The coverage-dependent change in adsorption geometry from flat to tilted b enzene, originally suggested in order to account for the kinetics of acetyl ene trimerization on Pd{111}, is unequivocally demonstrated.