HE SCATTERING FROM SUBSTITUTIONALLY DISORDERED MIXED MONOLAYERS - EXPERIMENTAL AND THEORETICAL-STUDIES OF XE+KR ON PT(111)

The diffraction of thermal He atoms from mixed Xe + Kr monolayers on P t(111) was measured, and the results were compared with theoretical st udies of these systems. The results shed light on the structural prope rties of these disordered systems, and on their relation to the He dif fraction intensities. Experimentally, the specular (0,0), the (1,0), a nd the (2,0) Bragg peak intensities were measured for monolayers of di fferent Kr:Xe concentration ratios. The theoretical calculations inclu ded Monte Carlo simulations of the mixed disordered monolayers, and qu antum calculations in the Sudden approximation of the scattering inten sities from the simulated disordered structures. The following main re sults were obtained: (1) Both experiment and the Monte Carlo simulatio ns suggest that the mixed Xe + Kr monolayers are periodic for all Xe:K r concentration ratios, the lattice constant varies linearly with the Xe:Kr ratio. The domain size of the 2D crystals, from experiment and t heory, is found to be larger than 100 angstrom. (2) The Monte Carlo si mulations suggest that the Xe + Kr monolayers form an almost ideal sub stitutionally disordered lattice. (3) Using a semiempirical Debye-Wall er factor, reasonable agreement is found between the theoretical and t he measured diffraction intensities, thus supporting the calculated st ructural model for the disordered surface. (4) The theoretical scatter ing calculations show that in addition to the diffraction peaks, there are also intensity maxima at non-Bragg positions. These are entirely due to the lattice disorder, and are identified as a recently found ne w type of Rainbow effect that can furnish important information on dis ordered surfaces. The results demonstrate the power of He scattering a s a tool for exploring substitutionally disordered surfaces.