Structural investigation of Rb adsorption on Al(111) using normal incidence standing x-ray wavefield absorption triangulation

Abstract

The adsorption site, and adsorbate-substrate bondlength, has been investigated for the adsorption system Al(111)/Rb, using normal incidence standing x-ray wavefield absorption (NISXW), over a range of coverages from 0.12 to 0.33 ML including the ordered (2×2) and (√3×√3)R30° phases. By measuring both (111) and (1̄11) Bragg reflections in NISXW we obtain Rb–Al layer spacings in two real space directions and can therefore fully determine the adsorption structure by triangulation. For all phases we find adsorption in the atop sites, with a coverage-independent bondlength corresponding to an effective radius for the adsorbed Rb of 1.70±0.10 Å, significantly closer to the ionic (1.48 Å) than the metallic (2.43 Å) value. The NISXW results also show that the adsorbed Rb atoms have large vibrational amplitudes parallel to the surface which may be associated with the atop site adsorption.