1,721,051 research outputs found
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
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
First-order orientational-disordering transition on the (111) surface of C60
No full textThe temperature dependences of the triplet exciton, of the vibrational modes, and of the low-energy diffraction pattern of the (111) surface of well-ordered C60 films indicate that this surface undergoes a first-order orientational order-disorder phase transition at 225±3 K, well below the temperature of the corresponding phase transition in the bulk of the same samples, which we find to be 260 K. The bulk phase transition is therefore initiated by the surface. To our knowledge, this is the first example of a surface orientational phase transition, a phenomenon that should occur on several other molecular crystal surface
Chemisorption and fragmentation of C60 on Pt(111) and Ni(110)
No full textThe interaction between C60 and the Pt(111) surface has been studied by high-resolution electron-energy-loss spectroscopy, photoemission and Auger spectroscopy, and low-energy electron diffraction from 100 to 1150 K. C60 forms strong covalent bonds with Pt with very small charge transfer (<0.8 electrons/molecule from C60 to the substrate), has a low surface mobility, and decomposes at about 1050 K, leaving a multidomain graphitic surface layer. C60 decomposes also on Ni(110) at about 760 K, forming a carbidic carbon layer, which transforms into graphitic carbon at higher temperatures. Both surfaces act as a catalyst for the fragmentation proces
Temperature and momentum dependence of the spectral function of the charge-density wave and of the normal α phase of Pb/Ge(111)
No full textAngle-resolved high-resolution photoemission spectra of the α phase of Pb/Ge(111)-(√3×√3)R30° have been measured below (100 K) and above (300 and 500 K) the transition to a charge-density-wave insulator state. The spectra strongly differ from those expected from the published local density approximation calculations showing a surface band without dispersion and correlation and fluctuation effects also at high temperature. A pseudogap is observed at 100 K
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