LOOKING UP THE DOWN STAIRCASE - SURFACE RAMAN-SPECTROSCOPY AS A PROBE OF ADSORBATE ORIENTATION

Unenhanced, non-resonant Raman spectra of a H-terminated vicinal Si(111) surface, Si[6(111)-(1 ($) over bar 1 ($) over bar 2)], with dihydride-terminated bilayer steps have been measured. All four Si-H stretches (and a approximate to 4% ML kink-defect mode) can be observed if both the excitation and detection geometries are favorable. A method for extracting the orientation of adsorbates using polarized, angle-resolved Raman scattering is presented. Assuming the C-1 mode corresponds to a localized Si-H stretch, the step dihydride is found to be rotated 37 +/- 4 degrees away from the surface normal. This value is in agreement with recent ab initio cluster calculations.