X-ray Scattering From Liquid Surfaces: Effects of Thermal Capillary Waves

As discussed in other articles in this issue of Synchrotron Radiation News, the availability of synchrotron radiation facilitated the development of X-ray reflectivity (XR) as an important technique for characterization of surface structure. Although the application of X-ray reflectivity to solids is relatively straightforward, as a consequence of the two-dimensional character of thermal fluctuations in the height of the liquid surface, the application to liquids is somewhat subtler. In this article we will discuss some of the fundamental differences between X-ray reflectivity from solid and liquid surfaces. We will also explain that, as a result of thermally excited capillary fluctuations of the height of the liquid surface, there is a limit to the largest incident angles a t which specular reflectivity from a liquid surface can be said to exist. Finally, we will demonstrate that the effect of thermal capillary waves can be understood quantitatively in terms of a rela-