HeI photoelectron spectroscopy of small molecules adsorbed on metal surfaces

Multilayer condensation of CBr4 on Pt causes the UPS bands to broaden by some 0.8 eV and to shift by 1 eV towards lower ionization energy. These observations are related to dielectric polarization in the adlayer. Larger shifts, observed in chemisorption, likewise can be explained from image charge screening of the support. For α-CO/Pt a two-band spectrum is observed, whereas for β-CO/Pt only one band is observable. This is ascribed to either the effect of heavy-metal 5d orbital admixture in the CO 2pσ-derived band, or the formation of bridge-bonded CO.Demuth and Eastman's results for C6H6/Ni(111) have been reproduced for polycrystalline nickel. Similar measurements on Pt, Cu and Cu/Ni, however, are severely hampered by the overlap in the spectrum of metal-d and carbon-2p derived bands. Annealing induces a change in the UPS spectrum of 0.33 Cu/0.67 Ni, probably due to enrichment of the surface in copper.