Spectroscopic evidence of a wetting and prewetting transition in liquid K-KCl mixtures

Spectroscopic ellipsometry and simultaneous reflectivity measurements of liquid KxKCl1-x solutions clearly exhibit a first order wetting transition in metal-rich melts. At the sample/substrate interface, salt-rich wetting films of mesoscopic thickness ( approximately 100 nm) are observed at and off of coexistence. They are uniquely characterized by the liquid F-center absorption band. However, crossing the prewetting line towards metal-rich concentrations, the F bands disappear. From the observation of the liquid F-center band, it is concluded that a strong undercooling of the wetting films of about 200 K may occur in binary metallic fluids, which is demonstrated here for the first time.