Relaxation effects and thermal vibrations in a Pt(111) surface measured by medium energy ion scattering

Medium energy ion scattering, in conjunction with channeling and blocking has been applied to search for differences in the planar spacing of the first two layers of a Pt(111) crystal surface as compared to the spacing in the bulk. We find a small expansion of 1.5 ± 1%. The measurements have yielded a value for the transverse rms thermal vibration amplitude of a surface atom relative to its neighbour along the [001] direction and likewise for the [110] axis. The results are giving evidence for an enhanced amplitude for the surface atom perpendicular to the surface. A strong blocking effect has been observed along the [110] axis, which may be due to correlation effects, although the role of surface defects cannot be ruled out.