849 research outputs found
Novel Features of Potassium and Sodium Vibrations on Copper Surfaces Observed by High-Resolution Electron-Energy-Loss Spectroscopy
No full textHigh-resolution electron-energy-loss spectra of alkali metals adsorbed on Cu show three unexpected features: the adsorbate-substrate stretching frequency does not change with coverage, within 1 meV on Cu(111) and Cu(110), in agreement with the covalent picture of chemisorption; the spectra show an unusual overtone which is interpreted as a consequence of a strongly non linear relationship between the surface dipole moment and the adsorbate-substrate distance; the dynamical dipole moment is strongly coverage dependent.
Temperature dependence of the Ge(1 1 1) 3d threshold near the 1050 K surface order-disorder transition
No full textThe Ge(1 1 1)3d threshold has been studied near the 1050 K surface order-disorder transition by using Electron Energy Loss Spectroscopy (EELS). In the same temperature range in which the structural transition occurs, a sharp change in the lineshape is observed. The comparison with the EELS spectra of the 3d threshold measured on liquid and amorphous germanium favours the hypothesis of an amorphous-like surface layer
Exchange excited f-f transitions in the electron energy-loss-spectra of rare-earth metals
No full textDipole-forbidden f-f multiplet excitations are found to characterize the electron-energy-loss spectra of Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er when the primary-electron energy is close to the binding energy of the 4d electrons. In Sm and Gd the f-f transitions are also detected at lower primary energies. The identification of the sharp peaks observed in the spectra as f-f excitations comes from straightforward comparison with the absorption spectra of the trivalent ions. The excitation energies of the f-f transitions are found to depend very weakly on the chemical environment. The exchange nature of the f-f excitation process explains the strong decrease of the intensities of the f-f peaks when the primary energy is raised, and is consistent with the resonant enhancement observed for primary energies near the 4d-4f threshold. A similar resonance is also observed in the primary-energy dependence of the intensity of the plasmon peak
Luminescence line-shape analysis of the electron-hole plasma in direct-gap Ga-Al-As: Random-phase-approximation approach
No full textThe luminescence line shapes of an electron-hole plasma in Ga-Al-As have been calculated within the random-phase approximation (RPA). Different models have been tested for the spectral weight function. The fully self-consistent RPA is found to overestimate the weight of the plasmon sideband and to underestimate the energy-gap narrowing. An approach which neglects plasmon replicas instead yields luminescence spectra in excellent agreement with experiment over a wide range of plasma densities and temperatures. A further model is also introduced to estimate an upper bound for the weight of plasmon sidebands in the experimental spectra. We used both static and dynamical phonon screening. We found that the former screening correctly describes luminescence up to densities such that the plasmon energy is of the order of the longitudinal-optical-phonon energ
Image states on Pb/Ge(111) observed through electron scattering
No full textResonant inelastic electron scattering is a new method that allows to detect image resonances with an energy resolution that makes line-shape analysis possible at an energy far above the vacuum level. For a better understanding of image states on Pb/Ge(111) and of the scattering mechanism, the two symmetry directions of the surface Γ̄M̄ and Γ̄K̄, and the dependence of the image resonances on the surface phases, the sample doping and the Pb coverage have been investigated. The maximum intensity of the image resonances is found for coverages ≥1 ML
Thermoreflectance of Ag single crystals
No full textThermoreflectance measurements of single-crystal Ag have been performed at room temperature in order to solve some discrepancies in the position and attribution of optical gaps, which have arisen in the recent literature. An extension of a previous theoretical model allows a very detailed fit of the experimental data and elucidates completely the origin of optical structures
Temperature dependence of the interband absorption edge of copper
No full textThe optical absorption spectra of bulk copper measured by Pells and Shiga have been fitted by means of a model calculation in the interband edge region. The optical gap and the dipole matrix element of the L3-L'2(EF) transition have been determined as a function of the temperature and a comparison with similar results obtained for silver and gold is presented
f-f excitations by resonant electron-exchange collisions in rare-earth metals
No full textThe spin-forbidden f-f multiplet excitations are observed for the first time as the strongest features in the electron-energy-loss spectra of Gd, Dy, and Sm metals at low primary-electron energies. The exchange nature of the excitation process is argued, in analogy with that of spin-flip Stoner excitations in d-band ferromagnets. A resonance enhancement observed for primary-electron energies near the 4d-4f core threshold is found to be consistent with the proposed mechanism
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