A NEW BRILLOUIN-SCATTERING ANALYSIS OF HIGH-FREQUENCY RELAXATIONS IN LIQUIDS DEMONSTRATED AT THE HYPERSOUND RELAXATION OF PPG

We present a new method to analyse data obtained by Brillouin scatteri ng. Using two scattering geometries simultaneously and measuring separ ately the refractive index n(T) we can determine the logarithmic deriv ative partial derivative dlgM'(omega, T)/partial derivative lg omega = Q(M) and thus are able to characterize unambiguously the dominant fas t processes by high precision frequency shift measurements. For a suff icient narrow relaxation time distribution the imaginary part M''(omeg a, T) of the complex modulus M(omega, T) = M'(omega, T) + iM''(omega, T) can be derived. Analysing data of poly(propylene glycol) with M(w) = 1000 (PPG 1000) we find the fastest process to show Arrhenius behavi our. Using published data from impulsive surface scattering we can fin ally deconvolute a set of three different processes: two exhibiting Ar rhenius behaviour and one showing Vogel-Fulcher-Tammann behaviour. The se three processes describe the data to a high accuracy and are tentat ively assigned to molecular motions. At high frequencies the relaxatio n time distribution observed narrows to a single relaxation time. In t he light of these results it seems questionable to use linewidth data from Brillouin scattering experiments to determine the width of the re laxation process. From the present work we believe the experiments wit h surface acoustic waves to be the most effective since they can be co upled with Brillouin scattering experiments. Copyright (C) 1996 Elsevi er Science Ltd.