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.