Dynamics of specific heat and other relaxation processes in supercooled liquids by impulsive stimulated scattering

Abstract

Laser based impulsive stimulated scattering or transient grating excitation in a heterodyne diffraction scheme is a powerful method to extract information about different relaxing properties from different signal contributions. Longitudinal acoustic waves are detected simultaneously with thermal expansion and thermal diffusion. Careful fitting of the time-domain density response at different temperatures makes it possible to obtain the various relaxing physical parameters, and to construct Arrhenius plots for the respective relaxation processes. In this work we focus on the influence of the specific heat capacity C on the slower part of the density response function Sρ(t), and, inversely, on the possibility to extract from experimental Sρ(t) data the relaxation behaviour C(ω). The specific heat capacity is relevant for both the initially rising part of the impulsive stimulated scattering signal (together with the time and frequency dependent thermal expansion γ(t)), and for the thermal diffusion dominated decrease of the signal at later times after the excitation. By simulating Sρ(t) data in different scenarios, we address the feasibility of unravelling the impulse response functions C(t) and γ(t) (and via Fourier transform also C(ω) and γ(ω)) by careful fitting of the signal. This approach offers a unique possibility to extend the 100 kHz bandwidth of current dynamic calorimetric techniques determining C(ω) (photopyroelectric spectroscopy) to the sub-GHz range.