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.