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.