Surface-phonon dispersion curves of KBr(001) via helium-atom scattering: Comparison with calculations

Abstract

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.

Surface dynamics of epitaxially grown potassium bromide overlayers on a sodium chloride substrate

“…18 The first and the last of these panels are included for comparison. The TOF spectra for the overlayer results in Figure 3 were measured under different experimental signal-to-noise conditions as implied by the specular intensity vs thickness curve in Figure 2.…”

Investigation of epitaxial growth via high-resolution helium atom scattering: potassium bromide onto rubidium chloride (001)

“…For this instrument the angle between the incoming beam and the detector axis is fixed at 90°. 31 The He atoms which scatter from the surface such that the sum of the incident and scattering angles 0j + 0f 90°( measured with respect to the surface normal) then pass through four stages of differential pumping to the detector chamber where their flux is measured by a quadrupoie mass spectrometer (QMS) operated in a pulsecounting mode. Not shown in Figure 1 is a "sump" chamber which follows the detector.…”

Surface lattice dynamics of KCl(001): A high-resolution helium atom scattering study

“…7,8,15 Briefly, the instrument consists of three main sections: ͑i͒ a beam source chamber, ͑ii͒ a scattering chamber, and ͑iii͒ time-of-flight ͑TOF͒ and detector chambers. A nearly monoenergetic ͑⌬E / E Ϸ 2%͒ helium atom beam is produced by expanding the helium gas at ϳ25-30 bar pressure from a nominal 30 m nozzle and then passing it through a 1 mm skimmer.…”

“…A similar-appearing optical branch, called a "crossing mode," was seen in measurements and in some calculations for the nonisobaric KBr͑001͒ surface. 8,9 For KCl͑001͒, both theoretical treatments predict the existence of such a branch.…”

“…7 Despite very good agreement, in general, features not apparent in the theoretical work were found in a few of these experiments. 8,9 Foldy and co-workers have pointed out the "extended symmetry" that occurs in the bulk phonon dynamics of isobaric alkali halide crystals such as KCl. 10,11 These authors noted the near degeneracy of certain bulk phonon dispersion branches at the Brillouin zone boundaries, which results from the similarity of the ionic masses and the absence of distinct molecular units in these materials.…”