Surface roughness of water measured by x-ray reflectivity

The roughness of the liquid-vapor interface for pure water was measured by a technique of x-ray reflectivity. With synchrotron radiation (λ∼1.5 A), the angular dependence of the x-ray reflectivity was measured from grazing incidence (∼0.0021 rad), where the reflectivity was greater than 0.96, to an incident angle of ∼0.05 rad, where the reflectivity was ∼7×10-8. A fit to the data by a theory with only one adjustable parameter obtains 3.2 A for the root-mean-square roughness of the water surface.