Inverse photoemission of CO, H and H/CO adsorbed at Ni(100) surfaces

Reimer, W.; Fink, Th.; Küppers, J.

doi:10.1016/s0039-6028(87)80035-3

Cited by 18 publications

(1 citation statement)

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“…As a consequence a consistent picture of the unoccupied level structure and its dependence on the coverage and the structure of the adsorbate layer seems within our reach. Measurements on unoccupied electronic states in coadsorption systems [3,[13][14][15] are sparse and their interpretation remains rather vague up to now. They will therefore not be included in the present discussion.…”

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confidence: 99%

Unoccupied electronic states in adsorbate systems

Bertel

1991

Appl. Phys. A

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Abstract. Experimental work on unoccupied electronic states in adsorbate systems on metallic substrates is reviewed with emphasis on recent developments. The first part is devoted to molecular adsorbates. Weakly chemisorbed hydrocarbons are briefly discussed. An exhaustive inverse photoemission (IPE) study of the CO bond to the transition metals Ni, Pd, and Pt is presented. Adsorbed NO is taken as an example to demonstrate the persisting discrepancies in the interpretation of IPE spectra. Atomic adsorbates are discussed in the second part. The quantum well state model is applied to interpret the surface states in reconstructing and non-reconstructing adsorption systems of alkali metals and hydrogen. A recent controversy on the unoccupied electronic states of the Cu(110)/O p(2 × i) surface is critically reviewed. The quantum well state model is then compared to tight binding and local-density-functional calculations of the unoccupied bands and the deficiencies of the various approaches are pointed out. Finally, the relation between the surface state model and more chemically oriented models of surface bonding is briefly discussed.PACS: 73.20.Hb, 79.60.-i A knowledge of unoccupied electronic states in adsorbate systems is crucial to the understanding of the surface chemical bond. The unoccupied levels determine the reactivity of surfaces and surface species just as much as the occupied levels do. One example out of many is provided by the hydrogen dissociation on Ni, where the empty Ni d-levels as hole reservoirs and are indispensable for the reaction to proceed [-1]. Another motivation for the study of unoccupied states derives from the fact that widely used techniques, such as scanning tunneling microscopy (STM) or second harmonic generation (SHG) on surfaces are based on processes involving empty levels. A thorough understanding of these processes therefore requires a knowledge of the unoccupied electronic level structure. There are, however, only a limited number of techniques available for providing independent information on the empty states. Ultraviolet photoelectron spectroscopy (UPS), where the unoccupied levels are probed as final states in k-conserving dipole transitions, and secondary electron spectroscopy, which may also yield information on the kll resolved unoccupied density of states, only access empty states above the vacuum level. This is a serious restriction if one considers the fact that the chemical bond involves primarily the "frontier orbitals", i.e. the highest occupied and the lowest unoccupied adsorbate orbitals and substrate Bloch states, which in an adsorbate system are usually aUtogether located below the vacuum level. Similarly, the position of the empty states involved in STM and SHG is in general restricted to a narrow range extending a few eV above the Fermi level. Spectroscopy of the near-edge X-ray absorption fine structure (NEXAFS) is a valuable tool for probing final states just above the Fermi level, but the presence of a core hole in the final state commonly results in large ...

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confidence: 99%