The electronic structure of Rhodium: Angle-resolved studies of photoelectron and secondary electron emission

We report electron spectroscopic studies of the Rh(111) surface, with the aim to obtain bulk band-structure information. We have measured normal photoemision using tunable synchrotron radiation in the range of photon energies between 11 eV and 55 eV, and angle-dependent photoemission along the LUX and LKL azimuths using the He resonance lines (=21.2 eV, 40.8 eV). To complement these data, we studied angleresolved secondary electron emission after excitation with electrons and photons. We derive parts of the one-electron energy dispersionE(k) along L, and determine the energies of several bulk critical points (in eV):E(> ₇₊/₈₊)=–2.75±0.10,E(> ₈₊=–0.85±0.10,E(> _7–=16.1±0.5,E(> _6–/> _8–)=20.5±0.5,E(X ₇₊)=–5.0±0.1,E(L ₆₊)=–5.6±0.5,E(L ₆₊/L ₄₊₊₅₊)=–2.65±0.10,E(L ₆₊)=9.0±0.5 eV. Our results are compared to several available band structure calculations.     

Promotors,poisons and surfactants: Electronic effects of surface doping on metals

The interaction of adsorbates with metal surfaces is discussed. It is shown that the evanescent charge density produced by occupied sp derived surface states yields a considerable contribution to the Pauli repulsion experienced by adsorbate particles with a closed-shell electronic structure, e.g. rare-gases or molecules such as H₂ or N₂. For rare-gases this results in a reduction of the binding energy in the presence of occupied surface states, for molecules this gives rise to an additional contribution to the dissociation barrier. Suitable surface dopants are able to depopulate surface states and thereby to reduce the dissociation barrier. Such dopants can substantially promote catalytic reactions in which the dissociation from the gas phase or a physisorbed precursor is the rate limiting step. In contrast to closed-shell systems the bonding interaction for metal adsorbates on metal substrates is enhanced by occupied surface states. This leads to an extra diffusion barrier at steps, because the surface state amplitude drops to zero at upper step edges. The additional step-edge barrier, which is a kinetic hindrance for layer-by-layer growth, can be reduced by surface dopants depopulating the corresponding surface state. Such dopants promote layer-by-layer growth and act therefore as surfactants. It is concluded that the effect of promoters in catalysis and of surfactants in metal epitaxy is in part due to the same basic mechanism, namely the depopulation of surface states.     

Adsorbate-site determination by surface-state spectroscopy?

Electronic surface states can interact with adsorbate orbitals only if they have the same symmetry with respect to the adsorption site. Therefore, the interaction between a surface state and a particular adsorbate orbital provides information about the symmetry properties of the adsorption site. This is discussed for the adsorption of hydrogen and ethylene on Ni(110).     

Unoccupied electronic states in adsorbate systems

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, Pb, 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(2×1) 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.     

A Raman spectroscopic study of the low-frequency vibrations of o-dimethoxybenzene and its methyl deuterated analogues

The low-frequency (100–400 cm⁻¹) Raman spectra of liquid (at 300 K) and solid (at 130 K) veratrole (o-dimethoxybenzene), and its methyl deuterated analogues, have been measured. The methyl and methoxyl torsional transitions have been identified, and the corresponding rotational barriers have been determined. The interpretation of the spectra points to a conformationally mixed situation for solid veratrole, in which both planar and non-planar conformers may co-exist.     

Amplitude-detuning characteristic of a CO2 laser with biharmonic loss modulation

The nonlinear response of a CO₂ laser to biharmonic loss modulation was theoretically investigated for tuning of the radiation frequency within the limits of the amplification band. It is shown that the value and form of the amplitude-detuning characteristic (and, consequently, the temporal and energy parameters of the laser radiation) can be controlled within sufficiently wide limits by changing the modulation frequency and the phase shift of oscillations. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 43–47, January–February, 1999.     

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-X 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.     

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.