1 M ay 2 00 7 Experimental X - Ray Studies of Liquid Surfaces

Over the past two decades synchrotron facilities dedicated to the generation of xrays for study of atoms, molecules and condensed matter have proliferated in Europe, Asia and the United States. As a result of the special properties of x-rays produced by these machines, there has been an enormous growth in the experimental possibilities. In this work we will demonstrate how these special properties can be used to carry out hitherto impossible measurements of the Ångström level structure of the free surfaces of liquids. A large part of this thesis is devoted to surface-induced atomic or molecular layering phenomenon in liquids. While the atomic structure of metallic and dielectric liquids is similar in the bulk, the microscopic atomic structure at the liquid-vapor interface of dielectric liquids is believed to be fundamentally different from that of a metal: metallic liquids exhibit an oscillatory density profile extending over a few atomic lengths into the bulk, as opposed to a more simple monotonic density profile that is thought to be characteristic of dielectric liquids formed from atoms or small molecules. Unfortunately, this oscillatory density profile is partially obscured by the surface roughness generated by thermally excited capillary waves. As a result, extraction of the microscopic form of this local density profile requires the special experimental and theoretical methods that are the subject of this thesis. It will be shown that the layering is present in all metallic liquids studied so far. Although there have been x-ray reflectivity studies of other liquids, the only measurement to probe the surface structure for a non-metallic liquid made of small molecules is the one reported here on the surface of water. In view of the fact that the surface tension for liquid K is not very different from that of water these measurements demonstrate