New light scattering technique based on phase time derivative correlation function

By using a heterodyne two-cross-beam configuration, we develop a new light scattering technique to study critical fluctuations. This optical arrangement gives the possibility to measure the critical dependence of both the intensity linewidth and the correlation function of the phase time derivative, in SF₆ near its gas-liquid critical point. We showed both experimentally and theoretically that the critical behaviour of the phase time derivative correlation function is determined by the thermodiffusive mode critical dependence, similar to the intensity linewidth. A good agreement between the experimental results and the theory is found.