Ultrasonic studies of relaxation in dichloromethane and dibromomethane with high-resolution Bragg reflection method

A method of high-resolution Bragg reflection was utilized to measure ultrasonic velocity and absorption in liquid dichloromethane and dibromomethane near the room temperature, over the frequency range from 60 to 700 MHz. in both liquids, considerable velocity dispersion and decrease in a/f²were observed and described with single relaxation frequencies, 192 MHz for dichloromethane and 393 MHz for dibromoethane at 20° C. The observed relaxation strengths were roughly consistent with the theoretical values calculated from the hypothesis of the vibrational relaxation associated with all but the lowest mode. The hypersonic velocities were measured in dichloromethane with the technique of Brillouin scattering. No dispersion was observed between 700-MHz and gigahertz region. The second relaxation involved with the lowest mode was expected to be in the range higher than 10 GHz, and the volume viscosity was estimated to beη_v≥ 3η_s, the shear viscosity.