Scattering of light by thermal ripplons on superfluid helium

It is well known that thermally excited waves on the free surface of a liquid can cause light to be diffusely scattered. In order to investigate the possible effects of the superfluid transition in liquid helium, we have measured the intensity of the surface scattering from the free surface of liquid helium as a function of temperature (1.3–2.1 K) and as a function of the scattering angle. The surface was illuminated vertically from above with a He-Ne laser. A measuring technique was devised which could not only distinguish among background scattering, scattering from the bulk liquid, and scattering from the surface, but could determine the ratio of surface to bulk scattering also. We have found that our results can be described by the theory of Mandelshtam, which is valid for classical liquids. According to this theory, the surface-scattered intensity is given by the ratio of temperature to surface tension. The angular dependence is determined by the characteristic wave vector dependence of the mean square amplitude of the fluctuations. The ratio between surface- and bulk-scattered intensity is calculated on the basis of the classical theory. The data indicate that in the frequency range around about 1 MHz no influence of the two-fluid nature of the superfluid is detectable.