F. Reinert

Electronic structure of 3d-transition-metal oxides : on-site Coulomb repulsion versus covalency

We have performed photoemission and inverse photoemission experiments on a series of 3d-transition-metal oxides with formal ionic configuration from to . The photoemission core-level spectra are analysed in terms of a simple cluster model leading to estimates for the charge-transfer energy , the Coulomb correlation energy , and the hybridization strength V. It is found that the ratio of the correlation energy to the hybridization energy significantly decreases from the late to the early transition metal oxides. This trend is attributed mostly to the increasing number of empty d states in the early transition metals which enhances the effective metal-ligand hybridization. We also compare the experimental valence band spectra with densities of states (DOS) from band-structure calculations. The rather good agreement between the theoretical DOS and the measured single-particle excitation spectra of the early 3d-transition-metal oxides as opposed to the failure of the one-electron description for most of the late transition metal oxides supports the results of the cluster model analysis.

Energetics of excited states in the conjugated polymer poly(3-hexylthiophene)

There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the general applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.

Strong hybridization in vanadium oxides: evidence from photoemission and absorption spectroscopy

We present x-ray photoemission spectra of the vanadium oxides , and , and their analysis in terms of a simple cluster model based on the Anderson impurity Hamiltonian. The electronic structure of these materials is characterized by a strong V 3d-O 2p hybridization energy which exceeds the energy scales related to on-site Coulomb correlation and metal-ligand charge transfer. This result is at variance with the usual Mott-Hubbard picture, but agrees with recent studies of other early 3d transition metal compounds. The V 3d ground-state occupations obtained by the cluster-model analysis are considerably higher than the values derived from the formal valencies. Covalency also affects the exchange splitting observed in the V 3s core-hole spectra. X-ray absorption measurements and resonant photoemission spectroscopy at the V 2p-3d threshold provide further evidence for a strong V 3d-O 2p coupling.

Role of Bulk and Surface Phonons in the Decay of Metal Surface States

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.

Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver

We demonstrate the application of orbital k-space tomography for the analysis of the bonding occurring at metal-organic interfaces. Using angle-resolved photoelectron spectroscopy, we probe the spatial structure of the highest occupied molecular orbital and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) and (111) surfaces and, in particular, the influence of the hybridization between the orbitals and the electronic states of the substrate. We are able to quantify and localize the substrate contribution to the LUMO and thus prove the metal-molecule hybrid character of this complex state.

Nanoelectron spectroscopy for chemical analysis : a novel energy filter for imaging x-ray photoemission spectroscopy

An ovel instrument for imaging ESCA is described. It is based on a tandem arrangement of two hemispherical energy analysers used as an imaging energy filter. The main spherical aberration (α 2 -term) of the analyser is corrected by the antisymmetry of the tandem configuration. The kinetic energy range useable for imaging extends up to 1.6 keV; this is compatible with Mg and Al Kα laboratory x-ray sources. First experiments on the chemical surface composition of a Cu0.98Bi0.02 polycrystal, a GaAs/AlGaAs heterostructure and Ag crystallites on Si(111) have been performed using synchrotron radiation. The results reveal an energy resolutio no f190 meV and a lateral resolution (edge resolution) of 120 nm. Besides elimination of the analyser’s spherical aberration, the tandem arrangement largely retains the time structure of the electron signal, unlike a singl eh emispherical analyser.

PHOTOEMISSION SPECTROSCOPY IN METALS : BAND STRUCTURE-FERMI SURFACE-SPECTRAL FUNCTION

Abstract Angular resolved photoelectron spectroscopy plays a key role in the study of the electronic structure of solids. We discuss recent methodical developments in its application to metallic systems. These include a new procedure for absolute E ( k ) band structure determination, which allows complete control of the three-dimensional wave-vector k , as well as a method for Fermi surface mapping based on measurements of the angular photoelectron intensity distribution. Going beyond a simple one-electron picture, we examine under which conditions the photoemission signal can be interpreted in terms of the electron removal spectrum of an interacting electron system and discuss an experimental test on a suitable Fermi liquid metal, which supports this many-body interpretation.

Electron and hole doping in NiO

We have investigated hole doped (by lithium) and electron-doped (by nickel metal) NiO with photoemission (PES), inverse photoemission (IPES) and low and high energy electron energy loss spectroscopy (EELS). Both types of doping create empty states approximately in the middle of the charge transfer gap of undoped NiO.

Surface and interface states on adsorbate covered noble metal surfaces

Abstract We present a systematic photoemission study of the influence of a one-monolayer rare gas coverage on the Shockley-type surface state on Ag(1xa01xa01) and Cu(1xa01xa01) surfaces. Depending on the rare gas––we used Ar, Kr and Xe as adsorbates––the surface state shows a characteristic shift towards the Fermi energy, whereas the spectral line-shape and the effective mass remain unchanged within the experimental uncertainties (energy resolution Δ E ≈3.5 meV).

Spin–orbit interaction in the photoemission spectra of noble metal surface states

By use of photoemission spectroscopy with high resolution in both energy and momentum, one is able to observe a considerable splitting of the Shockley-type surface state on the (111)-face of gold. This splitting exists over the complete two-dimensional Fermi surface of this surface state but disappears at normal emission. A comparison with fully relativistic density functional calculations proves that the splitting is caused by spin–orbit interaction, which—in combination with the lack of inversion symmetry at the surface—resolves the spin degeneracy of the surface state band. This paper gives a summary of recent experimental and theoretical investigations on the split Shockley state on Au(111). In addition, it will be demonstrated that the splitting is influenced by adsorbates, and it will be discussed why it has not been observed experimentally on other noble metals.