J. Günster

Study of the surface electronic structure of MgO bulk crystals and thin films

The electronic structures of the surfaces of MgO single crystals, oxidized Mg polycrystals and oxidized Mg films grown by molecular beam epitaxy on Si(100) surfaces were studied using several techniques. These include metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (He I)), and X-ray photoelectron spectroscopy (XPS). Spectra of oxidized Mg layers on Si(100) show additional features to those obtained for cleaved MgO crystals. These spectral features are attributed to dissociative adsorption of oxygen at bulk oxygen sites. Weak heating of the oxidized Mg layers removes these features and the electronic spectra for all three studied systems become similar. However, the experimental MIES and UPS spectra, both arising mainly from the ionization of the O 2p orbitals, have different structures. They are interpreted on the basis of ab initio Hartree-Fock and density functional calculations of the electronic structures of the ideal MgO(100) surface. It is shown, that the differences in the spectra can be understood by taking into account that UPS spectra reflect the density of electronic states within several surface layers, whereas MIES probes the surface states which are the most extended into the vacuum.

Atomic and electronic structure of the corundum (0001) surface : comparison with surface spectroscopies

Abstract The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 A, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.

Mg clusters on MgO surfaces : study of the nucleation mechanism with MIES and ab initio calculations

We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

CHARACTERIZATION OF COADSORBED MOLECULAR SPECIES IN A MULTILAYER SOLVENT ENVIRONMENT ON INSULATING SURFACES

By employing metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS, HeI) together with work function measurements and temperature-programmed desorption (TPD), the condensation of multilayer solvent systems, such as water and methanol on ultrathin Mo(100)-supported MgO films, has been investigated. In a first step, the techniques MIES/UPS and TPD are used to characterize the condensation of the pure solvent systems. Data collected in a coverage regime from submono- to multilayers at substrate temperatures between 100 and 500 K are surveyed in order to provide information about the buildup of the multilayer systems on an atomic level. Besides investigating the chemistry of pure phases, the present work provides insight into the chemistry of coadsorbed molecular species in a multilayer solvent environment. It is shown that thin films of amorphous water prepared at 100 K are a good candidate for studying aqueous multilayer chemistry under well-defined conditions, i.e. ultrahigh vacuum, by MIES. We are taking advantage of the unique properties of amorphous water films for investigating the interaction of water with coadsorbates, such as sodium and methanol. A deliberately chosen surface preparation technique in combination with the surface-sensitive electron-spectroscopic technique MIES enables the discrimination of various stages in the complex coadsorbate multilayer solvent reaction. Sodium has been chosen as a representative of a class of highly reactive and readily soluble species, and methanol as a surfactant.

The interaction of Al and O atoms on W(110) studied with metastable impact electron spectroscopy (MIES) and UPS

Al layers (< 2.5 adlayers thickness) are grown on W(110) under the “in situ” control of metastable impact electron spectroscopy (MIES) and photoelectron spectroscopy (UPS). The adlayer thickness was determined by combining LEED and work function measurements. The interaction of oxygen with the Al adlayers is studied. In order to analyze the adsorbed-oxygen/oxide transition, thin layers of alumina are grown by coadsorbing Al and oxygen on a 750 K hot W(110) surface. The resulting MIE and UP spectra are compared with those obtained during the oxygenation of the Al layers. Finally, the MIE spectra for Al/W, Al/W + O2 and alumina are simulated by taking into account information on the electronic structure available from previous studies.

The electronic structure of CsI adlayers on W(110)

Abstract The electronic structure of thin CsI adlayers deposited on W(110) at room temperature is studied by ion impact electron spectroscopy (IIES) with He + and He 2+ ions, metastable impact electron spectroscopy (MIES) with He ∗ atoms, ultraviolet photoelectron spectroscopy (UPS), work function measurements, and to some extent with electron energy loss spectroscopy (EELS). The transition from the eletronic structure of single adsorbed molecules to that of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap, as a function of the CsI exposure. The experimental results are consistent with the following conclusions: The first step in the layer growth is molecular adsorption of CsI perpendicular to the surface with the iodine end of the molecule pointing toward the substrate accompanied by decrease of the surface work function. The next step, starting with the occurrence of a pronounced work function minimum, is the formation of two-dimensional islands formed by CsI molecules. The results are consistent with lateral growth of the islands until the substrate is largely covered by CsI. The electronic structure of the adlayer cannot be distinguished anymore from that of CsI bulk material by means of electron spectroscopic techniques at exposures which are more than three times that of the occurrence of the work function minimum. The production of vacancies in the I − (5p) and Cs + (5p) orbitals under the influence of the ionizing radiation is studied as a function of the CsI exposure. Their relevance for the desorption process induced by electronic transitions (DIET) is discussed.

Electron emission from the interaction of He projectiles and UV photons with LiF on W (110)

The electronic structure of thin LiF adlayers deposited on W(110) at room temperature is studied by ion impact electron spectroscopy (IIES) with He + ions, metastable impact electron spectroscopy (MIES) with He * atoms,ultraviolet photoelectron spectroscopy (UPS), and to some extent with electron energy loss spectroscopy (EELS). The transition from the electronic structure of single adsorbed molecules to the emergence of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap, as a function of the LiF exposure. The first step in the layer growth is molecular absorption of LiF. The next step is the formation of islands formed by the LiF molecules

Iodine adsorption on clean and cesiated W(110) studied with UPS and metastable impact electron spectroscopy (MIES)

Abstract Photoemission spectra (UPS with HeI) and metastable impact electron spectroscopy (MIES with He∗(1s2s)) are reported for the adsorption of iodine alone, the coadsorption of iodine and cesium, and the adsorption of CsI on W(110) at room temperature. It is demonstrated that the I(5p) ionization is sensitively dependent on the chemical environment of the iodine adsorbate. For iodine in a strong external field, such as for adsorption on clean metals, the photoemission spectra reflect the Stark splitting of the (5p) orbital while the MIES spectra are due to the Auger capture process. When adsorbed near to cesium iodine feels a comparatively weak external field and the emission spectra both for UPS and MIES (due to the Auger deexcitation process) reflect the I(5p) finestructure splitting in the final state after the emission process.