o Inelastic He-atom scattering (k; =7 A ) was used to measure the dynamics of the RbBr(001) crystal surface (T, =115 Ki. Time-of-Aight spectra taken in two high-symmetry directions, (100) and ( 110),were analyzed to give the surface dispersion curves for the entire surface Brillouin zones I X and I M. The Rayleigh mode and its geometric "folded, " optical surface resonance (expected from the quasimonatomic behavior due to the nearly equal masses of the Rb and Br ions) were observed; the data also suggest the existence of another optical surface feature in the optical band.Although calculations had suggested that this crystal should have important surface relaxation eAects, the main one being a surface-localized optical branch lying above the optical band, no evidence of this was seen. Moreover, the calculated surface dispersion curves for the unrelaxed surface appear to fit the measurements slightly better than those for the relaxed surface, although neither predicts the optical resonance mentioned above. Finally, these observed diA'erences provide guidance on the use of the shell model for describing the physics of ionic insulator crystals.

A helium-atom surface-scattering instrument, employing a time-of-Aight technique to observe inelastically scattered He atoms, has been used to measure the surface-phonon dispersion curves of the KBr(001) surface. Data were collected in the two high-symmetry directions, ( 100) and (110), over the entire Brillouin zone for a target temperature of -115 K and incident He wave vector o k; =7 A . The results show important differences from some theoretical predictions of shell-model calculations that use parameters obtained by fitting bulk dispersion curves. In addition, the measured surface dispersion curves of KBr and of RbCl (reported previously) deviate from "mirrorsymmetry" behavior, unlike their bulk dispersion curves.

The crossing surface mode and an anomaly in the optical surface-phonon dispersion recently observed in KBr(001) by means of helium scattering are analyzed and interpreted on the basis of a Green s-function calculation of KBr(001) surface dynamics. The nature and origin of crossing modes in isobaric crystals like RbBr and nonisobaric crystals like KBr are reexamined in light of a general mechanism based on LA-LO hybridization in the (100) direction. Their dispersion is then accounted for in terms of zone-boundary bands, specificall TA2. LA-LO hybridization is also invoked to explain the anomaly observed in the optical surface mode.The first high-resolution He-atom-surface scattering experiments were carried out on the (001) surface of LiF. ' Since then a number of similar experiments have been carried out on other alkali halides, KC1, ' NaF, ' NaC1, RbCl, RbBr, KBr, and NaI, a sufficient number for the general features of surface-phonon dispersion common to all these materials to be discerned. In all of these

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High-resolution inelastic He atom scattering experiments, employing a time-of-flight (TOF) technique, have been carried out on NiO (001) in the (l00) and (Ito) high symmetry directions. The targets were prepared by cleaving single-crystal NiO in air and then baking in vacuum. Nearly all of the experiments reported here were carried out at a target temperature of 575 K, which is above the Neel point ( TN = 523 K), and with He atom wave vectors in the range of 7-9 A -I (25-45 meV). Because of these high temperatures, the peaks in the TOF spectra were found to be somewhat broader and the multiphonon background somewhat higher than in reported spectra from this instrument for alklai halide surfaces. Nonetheless, sufficient data were obtained to allow the determination of the Rayleigh wave almost to the Brillouin zone boundary. The results for this mode are found to be in good agreement with recently published, roomtemperature electron energy loss spectroscopy experiments. In contrast, there is a substantial difference between these data and the dispersion data reported for Ni (001) with chemisorbed o atoms [the O-e(2 X 2) structure]. Experiments that were tried at low temperatures were unsuccessful due to the rapid accumulation of surface contaminants.