Surface phonon modes of the RbBr(001) crystal surface by inelastic He-atom scattering

Abstract

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.

Lattice dynamics of ultrathin layers of KBr grown epitaxially on RbCl(001)

“…For ordered surfaces these measurements principally show the intensity of coherent elastic scattering from the surface ͑specular and Bragg dif-fraction͒ along with smaller selective adsorption features. 8,9 With the chopper in the in-beam chopper configuration, 7s pulses produced by the chopper are used in a time-of- flight ͑TOF͒ technique to provide an energy analysis of the intensity for any selected angle in the angular distribution.…”

“…[8][9][10] In terms of the incident and final wave vectors k i and k f , and for the 90°c onfiguration between the incident and scattered angles i and f , K i ϭk i sin i and K f ϭk f sin f . For the fcc crystals RbCl and KBr,…”

“…One should note that the peaks at negative energies correspond to creation of single phonons and, conversely, peaks at positive energies correspond to annihilation of single phonons. [8][9][10] The broad base or ''foot'' upon which the single-phonon peaks are sitting is due primarily to multiphonon scattering. 33 The relatively small diffuse elastic peak shown here, comparable to single-phonon peaks, is indicative of a relatively defect free film.…”

“…This problem arises mainly because electron scattering requires a treatment that takes into account multiple scattering collisions as well as penetration into the bulk, 2,7 In recent studies structural data and layer-by-layer monitoring of the growth process have also been forthcoming using neutral helium-atom scattering ͑HAS͒. [8][9][10][11][12][13] Although not as widely used as RHEED, HAS has two distinct advantages. These are ͑1͒ the He atoms do not penetrate the surface and ͑2͒ scattering conditions can be realized where single-phonon collisions dominate.…”

Thin epitaxial AgBr(001) films grown on NaCl: Growth, surface structure, and phonon dynamics

“…This mode was also observed for the ͑001͒ surfaces of some alkali halides, such as NaF, 53 KCl, 53 NaCl, 49 and KBr. 57 For the two former crystals the ion mass difference is rather small, for NaF Ϫ18% and for KCl Ϫ9%, whereas for the latter two, the mass difference is already very significant: for NaCl Ϫ35% and KBr Ϫ51%. The intensities of the C modes were observed to be appreciable for the crystals with a small ion-mass ratios, while for crystals with a large ion-mass ratios the intensities of the C modes were found to be quite small.…”