Temporary negative-ion resonances in the NiO(100) high-resolution electron-energy-loss spectrum

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Abstract
The NiO(100) Fuchs-Kliewer phonon spectrum has been investigated by high-resolution electron-energy-loss spectroscopy (HREELS) at beam energies of 0.8–17 eV and the intensity distribution of the phonons relative to the dielectric properties of the NiO substrate has been considered. Over most of the range, the spectrum is well behaved, with the single-loss phonon energy at -70.5 meV and multiple excitations decreasing according to the predicted Poisson distribution. At 5.0-, 8.5-, and 13.5-eV primary beam energies, however, a substantial resonant enhancement in the phonon cross section is observed. The enhancement is accompanied by a shift from the Poisson intensity distribution to favor higher multiple excitations and an increased scattering away from the specular lobe. The energies correspond to maxima in the nickel oxide unfilled density of states and the resonances are assigned as temporary negative-ion states in which the electrons of the HREELS beam are trapped briefly into these levels.