Direct measurement of ultrasonic relaxation phenomena by hyper-resolution dynamic light scattering spectroscopy

We have developed a new system to study the ultrasonic relaxation phenomena with the hyper-resolution dynamic light scattering technique. Our new system is based on the theory, which describes the dynamic structure factor of fluid under the condition of frequency dependent compressibility of the material. The dynamic structure factor, which is usually referred to as the Brillouin and Rayleigh triplet, is modulated and expected to show a new central component that directly gives the whole aspect of the relaxation effect in the elasticity. As the result of the theoretical calculation, we found that this new relaxation peak can be observed by our light scattering system with sufficient sensitivity. In the experiment, a light beam from a frequency doubled cw-YAG laser was incident to the liquid sample and the power spectrum of the light scattered into the backward direction was analyzed by the optical beating spectroscopy technique originally developed by us. The sample is liquid acetic acid that is known to show a strong ultrasonic relaxation around 1 MHz due to the molecular association process. In the experimentally observed spectrum, we could find the central component introduced by the relaxation. The relaxation frequency and the strength can be obtained from the width and the intensity of the observed peak, respectively, and the result shows very good agreement with that previously obtained by the conventional ultrasonic spectroscopy technique. The newly developed method of the ultrasonic relaxation measurement would be a powerful tool to investigate the microscopic dynamics of various kinds of soft condensed materials.