SOUND-WAVE PROPAGATION AND EXISTENCE OF A 2-STEP RELAXATION PROCESS IN A GLASS-FORMER MELT

The velocity and damping of longitudinal and transverse sound, waves o f a prototypical glass-forming melt, m-tricresylphosphate, are studied over a very wide frequency span by an ultrasonic and a light scatteri ng experiment (10(7) and 10(11) rad/s). In the Brillouin light scatter ing measurements, the central quasielastic line has been also analyzed . Unpublished data on velocity and damping (frequency approximate to 1 0(9) rad/s) are also included in the present analysis. A quantitative analysis of the data gives good agreement for individual spectra as we ll as for the general temperature trends. The analysis is based on the existence of fast (beta) and slow (alpha) relaxation processes in the liquid. A phenomenological description of a two step relaxation funct ion is proposed, and the phenomenological parameters [relaxation times , tau(alpha) and tau(beta), relative weight, (f) over tilde (T), etc.] of the two relaxation processes are derived. As a consequence of the two step relaxation process, we propose to revise the concept of ''inf inite frequency'' (c(infinity)) sound velocity, introducing two charac teristic sound velocities, one which applies if omega tau(beta)much gr eater than 1, c(infinity), and one, c(infinity alpha), which describes the elastic response of the liquid at frequency higher than 1/tau(alp ha) but smaller than 1/tau(beta). Within the framework of a two step r elaxation process, the meaning of ''nonergodicity'' parameter of mode- coupling theory and its relation with (f) over tilde (T) are also disc ussed. The weight (f) over tilde (T) is found to decrease monotonicall y from near 1 to near 0.4 between T-g and 1.5T(g). The comparison of o ur findings with the predictions of the mode-coupling theory shows qua litative accord in several important points.