1,721,071 research outputs found
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
Growth kinetics of NiSi on (100) and (111) silicon
No full textThe interaction of Ni film deposited on Si has been studied. Growth kinetics of Nisi on (111) and (100) silicon single crystals are reported. Activation energies for NiSi are found to be 1.23 eV for (100) and 1.83 eV for (111)
On the growth kinetics and structure of the NiSi compound on silicon single crystals
No full textThe growth kinetics of NiSi has been evaluated by depositing a Ni film on and Si substrates. X-ray diffraction and MeV 4He+ backscattering have been used to identify the compounds formed and to measure their thicknesses. Carbon and oxygen impurities have been detected with Auger electron spectrometry with Ar sputtering. During thermal annealing Ni~Si is formed first, while NiSi appears only after the total exhaustion of the Ni film. It has been found that, on both Si substrates, the thickness of NiSi increases with the square root of time. Different growth rates have been observed for the two different Si orientations. The activation energy in the (300-370) °C temperature range is 1.83 eV for NiSi grown on and 1.23 eV for NiSi on Si. The two competing mechanisms of Ni diffusion through NiSi by grain boundary and by substitutional processes are correlated with the different microcrystalline structure of the NiSi silicide layers grown on different Si oriented substrates
Spin-Exchange Processes in Inelastic Electron Scattering from Metals
No full textSpin-flip electronic transitions have been recently observed in ferromagnetic metals by electron energy loss spectroscopy (EELS) with spin analysis. The strong energy dependence of the exchange excitation process involved allows the identification of these transitions also by ordinary EELS by taking the ratio of spectra obtained with low energy and high energy primary electrons. In rare-earth metals dipole-forbidden transitions between f levels with and without spin-flip are observed as the strongest features in the EELS spectra at low primary electron energies. In iron the spin-flip transitions (Stoner excitations) give mrise to an additional contribution at about 2 eV in the EELS spectra at low primary energies. The energy of the Stoner excitation is found to be constant up to the Curie temperature
CO dissociation and recombination reactions on Ni(100)
No full textThe kinetics of atomic carbon and oxygen buildup on a Ni(1OO) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 ≤ T ≤ 573 K) and at different CO partial pressures (3x10^-7 ≤ PCO ≤ 3x10^-6 mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and O recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes
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