G. Rangelov

Thermally-induced evolution of codeposited Co-Si layers on Si(100) surfaces

Abstract The formation of ultrathin epitaxial layers of CoSi2 on Si(100) surfaces is studied by means of valence-band and core-level photoemission spectroscopy with synchrotron radiation, low-energy electron diffraction (LEED) and Auger electron spectroscopy. The CoSi2 films are prepared by the template method in which an amorphous layer with CoSi2 stoichiometry is coevaporated on top of a 2.6–3 monolayer thick Co overlayer at room temperature. The amorphous layer does not react to CoSi2. Annealing to 300°C leads to the crystallization of the layer demonstrated by the formation of a (✓2×✓2)R45° LEED pattern. This layer consists of disilicide in CaF2 structure with some contribution from another silicide phase. This surface is stable at least up to 460°C. After further annealing above 460°C the disilicide disappears, at least from the sub-surface layer, and the surface is covered by clean Si patches and patches of the second silicide phase. The new silicide phase is characterized by a (2✓2×✓2) R 45° LEED pattern and has a different electronic structure and Si 2p binding-energy shifts. A comparison with previous experimental results and existing theoretical electronic structure calculations suggests that the new silicide phase can be identified as adamantane disilicide.

The orientation of pyridine on Pd(110): An angle‐resolved photoemission study

The orientation of pyridine on Pd(110) has been studied by angle‐resolved UV photoelectron spectroscopy (ARUPS) with use of synchrotron radiation in conjunction with low‐energy electron diffraction (LEED) and work function measurements. The pyridine molecules in the saturation adlayer are arranged into an ordered c(4×2) LEED structure. The strong anisotropies of photoemission into the [001] and [110] azimuths suggest azimuthal orientation. Analysis of ARUPS in terms of polarization‐dependent symmetry‐derived selection rules provides strong support for a near‐parallel η6 configuration, and a surface bonding via the aromatic π electron system. Experiments utilizing photoemission into the mirror planes of the (110) surface allow to specify the azimuthal orientation of the molecules, viz., with the N atom aligned along the [001] direction. In combination with the ordered LEED pattern a structure model for the pyridine adlayer is presented.

The bonding of hydrogen on nickel studied by inverse photoemission

A k-resolved inverse photoemission study has been carried out on the low temperature phase of the Ni(110)/(1 × 2)H-adsorption system. The H adsorption results in a set of new surface states. Using a simple nearly-free-electron type model the surface state energies can be reproduced by adding an attractive potential on the Ni surface and simultaneously increasing the image plane distance from the surface. A bulk transition into the 4sp band observed on clean Ni(110) is modified by H adsorption indicating the formation of an sp-derived surface resonance. The measured dispersions are also borne out by the model calculation, although it does not take into account the half periodicity of the reconstructed surface.

Influence of the substrate on the band structure of alkali metal monolayers

Abstract The band structure of a monolayer of sodium on Ni(110) was studied experimentally by inverse photoemission and theoretically by a nearly free electron surface state model. We show that large gaps in the projected bulk band structure of the substrate can modify the electronic bands of the alkali metal overlayer considerably. Therefore the substrate band structure has to be taken into account explicitely for a realistic description of the band dispersion.

Symmetry reduction of benzene on Rh(111) by coadsorbed Na

Abstract Angle-resolved photoemission experiments using synchrotron radiation reveal a reduction from C 6v to C 3v,σv in the local symmetry of benzene on Rh(111) in the presence of coadsorbed Na. This is interpreted via a model of the coadsorbate layer in which the benzene molecules, adsorbed on-top in a pure benzene layer, occupy threefold hollow positions with a possible out-of-plane C-H bending distortion. The benzene energetics are influenced only weakly by coadsorbed Na.

The orientation of pyridine on Rh(111)

Abstract The orientation of pyridine adsorbed on Rh(111) at room temperature has been investigated as a function of surface coverage using angle resolved UV photoelectron spectroscopy (ARUPS), thermal desorption spectroscopy (TDS) and work function measurements. The ARUPS data exhibit significant differences in spectral behaviour at low and at high pyridine surface coverages, which are interpreted in terms of a transition from flat lying molecules at low coverage to inclined molecules at high coverage. The TDS results, which show some molecular pyridine desorption at T > room temperature, in conjunction with ARUPS spectra recorded as a function of temperature suggest the presence of intact pyridine molecules in the room temperature dosed adlayer.

EELS studies of oxygen and sodium adsorption and coadsorption on Ru(001)

Electronic excitations of sodium and of sodium coadsorbed with oxygen on Ru(001) have been investigated by electron energy loss spectroscopy in N(E) and d 2 n /d E 2 mode. For sodium adsorbed on Ru(001) a characteristic, sharp low-energy loss feature at 3–4 eV loss energy is observed, which shifts in energy as a function of Na coverage. This loss is discussed in terms of a one-electron excitation involving a Na(3s)−Ru(4d) interface state at low Na coverages, but is interpreted as a plasmon loss of the Na layer as the Na coverage approaches one monolayer. Both one-electron transition and plasmon excitation are strongly influenced and quenched by coadsorption of oxygen. The disappearance of the one-electron excitation is interpreted in terms of a modification of the interface state by coadsorbed oxygen, and the destruction of the Na plasmon resonance is associated with the loss of free-electron character in the Na layer as a result of oxidation.

Alkali metal oxides: Occupied, unoccupied and excited states

The occupied and unoccupied electronic states of NaO2 and KO2 have been investigated by UV photoemission spectroscopy and inverse photoemission spectroscopy, respectively. In addition single electron excitations from occupied into empty levels have been probed by x-ray absorption and electron energy loss spectroscopy. The experimental data are augmented by LCGTO-Xα model cluster calculations. A detailed assignment of initial and final states is given. The excitation cross sections and an inversion of the ligand field induced3d level splitting in NaO2 are discussed.

Growth of Ultrathin Si Films on fcc Co(100) Surfaces

Ultrathin Si layers were deposited at room temperature on fcc Co(100) films grown epitaxially on Cu(100). The surfaces were studied by low-energy electron diffraction (LEED), Auger electron, and photoelectron spectroscopy using synchrotron radiation. The growth mode was studied by Auger uptake curves. From the Si 2p core-level shifts the existence of a unique adsorption site is inferred at low coverages. Photoelectron forward-scattering experiments revealed no pronounced binding directions, whereas LEED patterns were observed at all coverages. This is compatible with the growth of an ordered layer of Si on top of the Co(100) substrate at low coverages followed by the growth of a disordered Si layer with holes.

Angular distributions of VUV photoelectrons from Cu(001)

Abstract A He resonance lamp and synchrotron radiation in the energy range from 15 to 60 eV were used to excite the valence band electrons on Cu(001) surfaces. The complete angular intensity distributions of the photoelectrons within 45° away from the surface normal were recorded with a two-dimensional display-type analyzer. The symmetry of the observed patterns permits an easy identification of the different character of the initial states as a function of energy. Additional information is obtained by exploiting the tunability and the polarization of the synchrotron radiation.