Wetting Phase Transition at the Surface of Liquid Ga-Bi alloys: An X-ray Reflectivity Study

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature $T_{mono}=222^{\circ}$ C. A Gibbs-adsorbed Bi monolayer resides at the surface at both regimes. However, a 30 {\AA} thick, Bi-rich wetting film intrudes between the Bi monolayer and the Ga-rich bulk for $T > T_{mono}$. The internal structure of the wetting film is determined with {\AA} resolution, showing a theoretically unexpected concentration gradient and a highly diffuse interface with the bulk phase.