Microscopic structure of the wetting film at the surface of liquid Ga-Bi alloys

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature T{sub mono}=222 degree sign C . A Gibbs-adsorbed Bi monolayer resides at the surface in both regimes. However, a 30 Angstrom thick, Bi-rich wetting film intrudes between the Bi monolayer and the Ga-rich bulk for T>T{sub mono} . The wetting film's internal structure, not hitherto measured, is determined with Angstrom resolution, showing a concentration gradient not predicted by theory and a highly diffuse interface with the bulk phase. (c) 2000 The American Physical Society.