TEMPORARY NEGATIVE-ION RESONANCES IN THE NIO(100) HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTRUM

The NiO(100) Fuchs-Kliewer phonon spectrum has been investigated by hi gh-resolution electron-energy-loss spectroscopy (HREELS) at beam energ ies of 0.8-17 eV and the intensity distribution of the phonons relativ e to the dielectric properties of the NiO substrate has been considere d. Over most of the range, the spectrum is well behaved, with the sing le-loss phonon energy at -70.5 meV and multiple excitations decreasing according to the predicted Poisson distribution. At 5.0-, 8.5-, and 1 3.5-eV primary beam energies, however, a substantial resonant enhancem ent in the phonon cross section is observed. The enhancement is accomp anied by a shift from the Poisson intensity distribution to favor high er multiple excitations and an increased scattering away from the spec ular lobe. The energies correspond to maxima in the nickel oxide unfil led density of states and the resonances are assigned as temporary neg ative-ion states in which the electrons of the HREELS beam are trapped briefly into these levels.