Elastic and inelastic helium atom scattering at a cleaved mica sheet

A mica sheet has been cleaved in situ in a UHV beam scattering apparatus. The diffraction of the helium atoms shows sharp Bragg peaks. In the [110] and [110] directions of the hexagonal surface the intensities of the Bragg peaks are analysed in terms of a sinusoidal corrugation. With hard wall scattering calculations (Kirchhoff-Eikonal approximation) the corrugation is found to be 0.345 Angstrom (for incident wave vectors in the range of 5.7 to 8.8 Angstrom(-1)). The potassium cleavage plane is sandwiched by hexagonal sheets of SiO4 tetrahedra (height 0.5 Angstrom). Further results of the fit are the depth of an attractive well of 21.5 meV and a surface Debye temperature of 328 K. A series of time-of-flight measurements give for both directions an extrapolated Rayleigh wave energy maximum of 11.5 meV. Einstein modes around the zone center have nearly the same energy. The sound velocities are 3680 m/s for the Gamma M and 3410 m/s for the Gamma K direction. From drift spectra the mean step heights at the surface are extracted with a Fourier transform: 5.4 Angstrom with 45% contribution (1.45 Angstrom with 30%) and 2.5 Angstrom with 25%. Small broad and reproducible maxima between the Bragg peaks in the angular distribution are analysed for bound state. (BS) resonances (mean BS values: 15.7, 13.6, 8.7, 5.4, 4.7, 2.5, 1.7 meV). From the measured widths of the bound state features the night paths of the He atoms in the particular BS levels can be estimated. The path minimum has a value of about the same number as the most probable step height resulting from the Fourier analysis of the drift spectrum. All measured intensities are somewhat reduced by these steps compared with other single crystals as for example the alkali halides

THE SPECTROSCOPY OF SURFACE PHONONS BY INELASTIC ATOM SCATTERING

The recent great advance in the production of highly monochromatic atomic beams is opening new perspectives in surface physics, having made way to a full determination of the surface vibrational structure. After a short review of the earlier attempts to detect surface phonons from the angular distributions of scattered atoms, we report on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The state of the art in the theory of inelastic scattering processes is briefly illustrated in order to discuss the theoretical interpretation of TOF spectra. The one-phonon energy-loss spectra of He scattered from LiF (001) calculated for a hard corrugated surface model are found to be in good agreement with TOF spectra at all the incidence angles. Evidence is given that, in addition to Rayleigh waves, important contributions to the inelastic scattering come from the surface-projected density of bulk phonons. The possible observation of optical surface modes in KCl (001) is finally discussed