COMPRESSED CO ADSORPTION LAYERS ON THE (110) SURFACES OF NI, PD, AND PT - AN INVERSE-PHOTOEMISSION STUDY OF THE UNOCCUPIED ELECTRONIC BAND-STRUCTURE

The unoccupied band structure of a CO monolayer on Ni (110), Pd (110), and Pt (110), respectively, has been investigated by inverse photoemission. For Ni and Pd the CO-derived band structures exhibit only marginal differences. The observed bands are interpreted in terms of 2pi(x)- and 2pi(y)-derived CO-CO bonding and antibonding bands. The level ordering in the center of the surface Brillouin zone, as obtained from a polarization analysis, revealed a similar splitting for the 2pi(x) and the 2pi(y) levels at normal incidence. This indicates a CO-CO interaction of approximately the same magnitude in the x as well as they direction. Consequently, the CO molecules seem to be arranged in a nearly hexagonal structure at monolayer coverage which in turn is due to the CO-CO repulsion dominating over the CO-substrate interaction in the tightly packed monolayer. The observed level ordering of the 2pi(y) bands is consistent with such a geometry. On Pt(110), a distinctly different electronic structure indicates a different bonding of the CO to the metal substrate, namely a reduced 2pi backbonding and a more prominent role of the 5sigma dative bonding. This may also give rise to a slightly different adsorption geometry.