Unoccupied electronic states in adsorbate systems

Abstract

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 ...

CO Adsorption on GaPd—Unravelling the Chemical Bonding in Real Space

“…Contrasted to the clean surface, CO adsorption results in the depletion of the Pd 4d states between −2 and −4 eV, while the sDOS inflates between −4 and −5 eV as well as for energies around −6.9 eV, and three new and rather narrow peaks develop. By comparison to CO adsorption on Pd(100), the peaks at −11.5 and −8.5 eV can be attributed to 4σ and 5σ+1π states of the adsorbed CO molecule, respectively, while the peak observed at 3.3 eV above the Fermi level is assigned to the CO‐2π* molecular state. In agreement with these results, the He I UPS spectra recorded at 130 K and at different dosages of CO on the GaPd( ) surface (see Supporting Information, Figure S1) presents features that can be summarized as 1) a non‐uniform Pd 4d depletion and 2) the development of three states at around 6–7, 9–10, and 12 eV.…”

CO adsorption on the GaPd(1̄1̄1̄) surface: a comparative DFT study using different functionals

“…The Blyholder model is generally applied to describe CO adsorption on surfaces containing transition metal (TM) elements. [71][72][73][74][75][76][77][78][79][80][81] According to this model, there is (i) a CO-5s interaction with the empty TM states of the surface (molecule-to-surface transfer of electrons), that is compensated by (ii) a p-bonding interaction of the filled TM states of the surface with the CO-2p* antibonding states (surface-to-molecule back-donation). The analysis and discussion of the electronic structure of the CO molecule adsorbed at the GaPd(% 1% 1% 1) surface calculated with the HSE06 functional, in the light of the Blyholder model, is given below.…”

Decay of electronic excitations at metal surfaces

“…Angle-resolved photoemission spectroscopy was used to study QWS in alkali-metal overlayers on different metals [151,156,157], in silver overlayers on Fe(1 0 0) [152,158,159] and on V(1 0 0) [153,160,161]. Unoccupied QWS in overlayers have been studied with inverse photoemission [162][163][164][165][166][167][168][169][170][171] and with 2PPE spectroscopies [4,[172][173][174][175].…”