ATOMIC-BEAM DIFFRACTION AND RESONANT SCATTERING STUDIES OF CLEAN RH(311) AND THE C(1X1)H PHASE

Diffraction studies on clean Rh(311) have been performed with low and high temperature He beams and with room temperature Ne beams. The corr ugation amplitude perpendicular to the close-packed rows as seen with He is similar to 0.30 Angstrom. The asymmetrical out-of-plane rainbow structure observed with He and Ne points to a more pronounced corrugat ion amplitude along the (100) microfacets and a much more smoothed amp litude along the (111) ones, in agreement with expectation but in cont rast with Ni(311), where the reverse situation was reported. Selective adsorption of He on clean Rh(311) and the c(1 x 1)H phase could be me asured with low temperature beams, allowing the determination of three and four bound-state levels, respectively. These energies were fitted with a two-parameter Morse potential, showing that the resonance leve ls as well as the potential depth of similar to 7.7 meV are essentiall y the same for Rh(311) as for Rh(110). The resonant scattering structu re measured for the c(1 x 1)H phase could be wen reproduced by close-c oupled channels calculations performed with a Morse potential, support ing the structure in which the lowest coverage ordered hydrogen phase on Rh(311) exhibits a coverage of one monolayer.