Photoemission Studies of the Electronic Structure of Selected Metal Overlayers on TANTALUM(110) and Their Effect on Chemisorption

Photoemission spectroscopy and conventional surface science analytical techniques have been utilized to study the electronic structure of ultra-thin Ni, Pd, and Pt layers on Ta(110). Of great interest is the fact that the chemical properties of the overlayers differ from the bulk properties of either the substrate or the overlayer metal, and part of this work was designed to understand the origin of this difference. Photoemission studies showed that for initial coverage, each overlayer metal had its d-band moved below E(,F) and thus resembled a "noble" metal as opposed to a transition metal. A test of this observation was provided by extensive studies of the chemical reactivity of CO on Pd overlayers supported on Ta, where it was found that both incommensurate and commensurate Pd monolayers behaved like a 'noble' metal. Hence we suggest that our observation that CO does not chemisorb at room temperature on Pd monolayers does not depend on surface structure but instead is related to the electronic structure of the overlayer, namely the virtual absence of d-states near E(,F). Additional insights into the bonding between the substrate and the overlayer were provided by experiments where information could be obtained from core levels. The quality of the overlayer-substrate interface could be deduced from the nature of these levels and estimates of the bonding energies could be made from core level shifts. The combination of LEED, valence band photoemission, and core level spectroscopy allowed one to make significant progress in understanding how metal layers and adatoms bind to the substrate, change the geometrical and electronic structure of the surface and react with the environment. Further insight can be obtained by direct comparison of photoemission spectra with band structure calculations, as in the case of Pd on Nb. This has led to a fundamental understanding of how the d-bands change at an overlayer-substrate interface.