Direct observation of slow molecular relaxation by high-resolution light scattering spectroscopy

We developed a novel dynamic light scattering system to observe elastic relaxation phenomena with hyper frequency resolution. The principle of the measurement is based on the theory, which describes the dynamic structure factor of fluid under the condition of the frequency dependent compressibility. The dynamic structure factor, which is usually composed of the Brillouin and Rayleigh triplet, is modulated and shows an additional central component that directly reflects the whole aspect of the relaxation. In the experiment, the output from a frequency-doubled cw-YAG laser was incident into the liquid sample and the power spectrum of the light scattered into the backward direction was analyzed by the optical beating spectroscopy technique. The sample is liquid acetic acid that is known to show a strong ultrasonic relaxation around 1 MHz due to the molecular association process. We could find in the observed spectrum, the central component introduced by the phenomenon, whose relaxation frequency and the strength can be obtained from the width and the intensity of the observed central peak, respectively. The results show very good agreement with those previously obtained by the conventional ultrasonic spectroscopy technique.