Time-of-flight scattering and recoiling spectrometry. I. Structure of the W(211) surface

Abstract

The technique of time-of-flight scattering and recoiling spectrometry (TOF-SARS) with detection of both neutrals and ions is presented and the scattering technique is applied to structural analysis of the clean W(211) surface. The recoiling technique is applied to structural analysis of oxygen and hydrogen on this surface in the two papers (II and III) immediately following this one (I). This series of three papers emphasizes the ability to obtain direct ``real-space'' information on the relative positions of atoms in the surface region based on simple classical concepts. In this first paper, both backscattering (BS) and forward-scattering (FS) from a pulsed 4-keV ${\mathrm{Ar}}^{+}$ primary-ion beam are monitored as a function of polar beam incident angle \ensuremath{\alpha}, polar beam exit angle \ensuremath{\beta}, surface azimuthal angle \ensuremath{\delta}, and scattering angle \ensuremath{\theta} for clean W(211). Plots of BS intensities in (\ensuremath{\alpha},\ensuremath{\delta}) space provide scattering structural contour maps and three-dimensional scattering structural plots of the clean W(211) surface.Measurements of BS intensities at \ensuremath{\theta}=163\ifmmode^\circ\else\textdegree\fi{} as a function of \ensuremath{\alpha} along crystal azimuths with known interatomic spacings in the first layer are used to obtain experimental points on the shadow cones. These experimental points allow calibration of the screening constants in the interatomic potentials used in trajectory simulations and calculations of the shadow cones as well as calibration of the surface vibrational amplitudes used for simulation of the BS flux distributions. The experimental data and calibrated computations are applied to determination of surface relaxation, both changes in first- to second-layer spacing and first- and second-layer registry, for the (211) structure. The results show that the first- to second-layer spacing is contracted by 0.12 A\r{} (9.3%) and that there is a lateral shift along the [1 1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}] direction such that the layer registry is shifted by 0.10 A\r{} (3.6%). The calibrated interatomic potentials are used to simulate blocking cones for FS measurements in the range 0\ifmmode^\circ\else\textdegree\fi{}<\ensuremath{\theta}<55\ifmmode^\circ\else\textdegree\fi{}. The sensitivity of TOF-SARS to the details of atomic structure, its ability to probe the first layer as well as subsurface layers, focusing and channeling effects that enhance BS and FS along certain azimuths, and the ability to extract surface structural information with only simple calculations are discussed.