Light-scattering spectra of fast relaxation in glasses

Abstract

Light-scattering spectra of the glasses polystyrene ~PS!, polycarbonate ~PC!, and Ca0.4K0.6(NO3)1.4 ~CKN! are measured in the frequency interval 3 210000 GHz covering a broad temperature range. The low frequency wing of the fast relaxation spectrum is found to show a power-law behavior with an exponent a50.220.6. The exponent depends on system and temperature. No indication of a crossover to a white noise spectrum, as previously reported and discussed within mode-coupling analyses is found. It is shown that the Gilroy-Phillips model of thermally activated transitions in asymmetric double-well potentials well describes the power-law part of fast relaxation spectra in PS and CKN but fails in the case of PC. The distribution of barrier heights is extracted from the spectra. @S0163-1829~98!04645-1# Fast relaxation processes are a characteristic feature of the dynamics in glasses. They show up as a broad quasielastic contribution to neutron and light-scattering spectra, 1‐8 and also as a damping of sound waves as seen in experiments on ultrasonic attenuation or internal friction. 9‐11 Above the glass transition temperature Tg this contribution strongly increases, and here its investigation was stimulated by modecoupling theory ~MCT!, 12 which suggests that the fast process is a precursor of the structural relaxation ( a process!. Deep in the glass (T!T g ) thermally activated hopping in asymmetric double well potentials ~ADWP! is discussed. 13 These ADWP’s are believed to be essentially the same as those responsible for the tunneling states which govern the low-temperature anomalies of glasses. Assuming that ADWP’s determine the dynamics up to Tg , many authors have applied this approach in order to describe internal friction or light-scattering data above 10 K. 4,13,9‐11 As far as we know, no systematic investigations of the relaxation spectrum covering a broad frequency and temperature range have been reported for the glassy state ( T ,Tg). Most investigations used neutron or Raman spectroscopy, which have a low frequency limit around 100 GHz. The ADWP model of the fast relaxation below Tg has been tested by studying the temperature dependence of internal friction at a given frequency. Recent developments of tandem Fabry-Perot interferometers ~TFPI! opened up the possibility to measure light-scattering spectra in a broad frequency range down to ;0.3 GHz. This was actively applied during the last years in investigations of glass forming liquids above and around Tg and revealed a power-law spectral shape for the fast relaxation. 20,5,6