R. Courths

Electronic study of SrTiO3(001) surfaces by photoemission

Abstract Defect rich and stoichiometric surfaces of SrTiO 3 (001) prepared by argon-ion bombardment and annealing in UHV or in oxygen atmosphere have been investigated by photoelectron spectroscopy, LEED and AES. Via core level shifts (Ti 3p level) surface Ti ions and Ti ions in different charge states have been identified and are set in correlation to characteristics defect-related gap states.

Dispersion of the oxygen-induced bands on Cu (110) - an angle-resolved ups study of the system p(2×1)O/Cu(110)

Abstract The two-dimensional electronic energy bands induced by adsorption of atomic oxygen on Cu(110) are measured with angle-resolved photoelectron spectroscopy (ARUPS). Three oxygen-derived bonding bands and two Cu - derived antibonding bands are found below the Fermi energy. The dispersions can be described by assuming nearest-neighbour σ and π CuO bonds along the [001] direction with oxygen in a long bridge site.

A photoemission investigation of the adsorption of potassium on perfect and defective TiO2(110) surfaces

Potassium adsorption on stoichiometric and defective rutile TiO 2 (110) surfaces at room temperature has been studied by means of angle-resolved photoemission spectroscopy (ARUPS and ARXPS) from the valence states and core levels. Potassium adsorption on the stoichiometric surface creates reduced Ti ions («Ti 3+ ») with lowered Ti 2p core-level binding energy and 3d-like band-gap states centered at 0.9 eV binding energy, arising from K-to-substrate charge transfer. The Ti 3+ 3d intensity versus coverage, which is a measure of the transferred charge, shows a behavior similar to that of the work function, with a very pronounced maximum at the coverage of the weak work-function minimum. In contrast, the Ti 3+ 2p signal, which is a measure of the number of reduced Ti ions, increases continuously with K coverage. The results are discussed in terms of a recently proposed « ionic-to-neutral transition » of the K-substrate bond. On the defective surface, K adsorption results in a decrease of the initial concentration of «Ti 3+ -V o » surface defects, probably caused by K-induced oxygen diffusion from the bulk to the surface

Bulk and surface Ti3d valence and defect states in SrTiO3(001) from resonant photoemission

Abstract Resonance photoemission from Ti3d states above the Ti3p absorption threshold has been used to extract regions of Ti3d-state hybridization in the 02p valence band of bulk SrTiO 3 and of the surface of SrTiO 3 (001). An enhanced covalent mixing in the surface is found. In addition it could be shown that localized band-gap states induced by oxygen vacancies have Ti3d-state orbital character. All results are consistent with theoretical predictions.

Surface-atom valence-band photoemission from Au(111) and Au(100)-(5 × 20)

Abstract We observe surface-resonance features at nearly equal initial state energies in photoelectron spectra taken at normal exit from reconstructed Au(111) and Au(100) surfaces. We propose a common origin in the outermost compressed and corrugated atomic layers. We demonstrate that our interpretation is fully consistent with experimental observations of several other authors.

On the band structure of silver and platinum from angle-resolved photoelectron spectroscopy (ARUPS) measurements

The general problems of an accurate a priori determination of electronic band structures are discussed. Data for the Γ ΛL line in Ag and Pt will be discussed as a case in point. Various methods have been employed to determine the electron wavevectorK of the direct transitions to the bands accessible to photon energieshv=16.9 eV and 21.2 eV: the disappearance and appearance angle method, the symmetry method, the triangulation method and the gap emission method. It is found that the experimentalE(K) points agree well within ±0.5 eV with a recently ab initio calculated conduction band structure. We further have measured Ag(111) normal emission AREDCs in the photon energy range 11 eV≦hv≦25 eV and have found that the intensity behaviour with photon energy of the direct transitions compares well with calculated bulk momentum matrix elements for Ag along Λ. A summary of the experimentalE(K) data for both the conduction and valence bands in Ag is given and compared with a recent local density calculation. Similar investigations for Pt are presented.

Photoelectron-spectroscopy investigation and electronic properties of LiNbO3 crystal surfaces

The electronic properties of Ar-ion and electron bombarded single-crystal surfaces of LiNbO3 have been investigated at room temperature by ultraviolet photoelectron spectroscopy (UPS) and x-ray excited photoelectron spectroscopy (XPS). In crystals reduced in this way, the loss of oxygen is accompanied with a loss of lithium and the creation of Nb4+ and Nb3+ ions. The relative concentrations of the various defects have been determined from their corresponding XPS core line spectra. When surface defects are produced an emission in the region of the bulk band gap appears. This gap emission is identified as arising from Nb 4d electrons due to the reduction of Nb5+ ions to Nb4+ and Nb3+ ions. The different behaviour of the gap emission upon surface treatment in the UPS and the XPS spectra is discussed.

Band structure of platinum from angle resolved photoemission experiments

Abstract Angle-resolved and angle-dispersed ultraviolet photoelectron spectroscopy (ARUPS) has been used to determine the experimental band structure of platinum at various points along the ΓX, ΓKX and ΓL directions in the Brillouin zone. Various methods have been employed to obtain the E(k) points, among them absolute methods which determine the energy and the momentum independently without any assumption about the final state dispersion. The experimental points are compared with a self-consistent relativistic band structure calculation for energies below and above the Fermi energy. Good agreement between theory and experiment is found.

Electronic structure investigation of Cu(001)-p(2 × 2) S using angle-resolved photoemission and inverse photoemission

Abstract Angle-resolved photoemission (ARPE) and inverse photoemission (ARIPE) are used to measure the dispersion of sulfur-induced electronic surface bands of the Cu(001)-p(2 × 2)S surface. Polarized synchrotron and rare-gas resonance radiation is used to identify the symmetry and the atomic origin of the three sulfur 3p-like bonding states between −4.9 and −5.4 eV below the Fermi energy, as well as of occupied antibonding Cu 3d-like states split off from the bulk d states to higher energy. The surface-band structure of the odd-symmetry states along the \ gD line of the Brillouin zone is interpreted in terms of a simple tight-binding LCAO picture, which takes into consideration only nearest-neighbour interactions between S 3p orbitals in the fourfold hollow adsorption site and neighbouring Cu3d orbitals in the surface plane. Two unoccupied S-induced states at +1.4 and +12.5 eV are detected. Using photoionization cross-sections, found in the literature, the latter is ascribed to S 3d. The work function (deduced from ARPE spectra) in dependence of the sulfur coverage is also given.

High-Resolution Normal Emission UV-Photoelectron Spectra and Band Structure of Cu

The energy distribution curves of photoelectrons emitted along the normal direction from Cu (111), (001) and (110) surfaces using unpolarized HeI (hv=21.2 eV) and HeII (hv=40.8 eV, 48.4 eV) radiation incident at 45° to the surface were measured. For the interpretation the band structure as calculated by Janak et al. and by the combined interpolation scheme was used. The high resolution spectra along the [111] and [001] directions show dominant features from direct transitions, indicating strong residualkz conservation, and density of states features, too. The ‘band gap’ emission along the [110] direction using HeI radiation can be explained with direct transitions into ‘evanescent’ states with nearly free electron bands. From the width of the peaks electron escape depths were calculated.