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 ($\ensuremath{\lambda}\ensuremath{\sim}1.5$ \AA{}A), the angular dependence of the x-ray reflectivity was measured from grazing incidence (\ensuremath{\sim}0.0021 rad), where the reflectivity was greater than 0.96, to an incident angle of \ensuremath{\sim}0.05 rad, where the reflectivity was \ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$. A fit to the data by a theory with only one adjustable parameter obtains 3.2 \AA{}A for the root-mean-square roughness of the water surface.