Adsorption of Molecules on Metal Surfaces Studied by High Resolution Core Level Photoemission

Sammanfattning: Various aspects of the adsorption of molecules on metal surfaces have been studied making use of different properties of high resolution core level photoemission. A thorough investigation has been performed of the ordering and the site occupancy of CO overlayers on Rh(111). CO has been shown to adsorb in on-top and three-fold-hollow sites. The distribution of the molecules between these two sites is shown to depend sensitively on the temperature for certain coverages. Order-disorder phase transitions for the (2x2) and (4x4) structures have been studied. Investigations of CO coadsorbed with K or O have shown the formation of different overlayers and a strong influence on the CO adsorption sites. The adsorption of CO on laterally inhomogeneous surfaces has been investigated. CO adsorbing on 0.5 ML Pd on Rh(111) has been found to diffuse between the two dimensional Pd islands and the uncovered Rh(111) patches between the islands. We have argued that the molecules preferably occupy the domains with the highest adsorption energies which can vary for different densities of CO and other adsorbates such as O on the domains. An important application of two-domain surfaces is the detection of molecules adsorbed in a laterally mobile state with short lifetimes. We could show the existence of such a mobile state for CO molecules impinging on a one atomic layer thick Pd film on the Mo(110) surface at room temperature. We have shown that the excitation of C-H vibrations in the photoemission process induces a splitting in C 1s components for chemisorbed molecules such as C2H3 and C2H4 adsorbed on Rh(111). The characteristic appearance of the vibrational fine structure can be used to identify molecules with C-H bonds. This has made it possible to identify C2H3 as a reaction product of C2H2+H on Pd(111). It has been demonstrated how core level binding energies and the photoelectron diffraction behaviour can provide fingerprints for molecules adsorbing in particular adsorption sites or domains on a surface. Quantitative information on the relative occupation of different adsorption sites can be extracted from the peak intensities.