Fast relaxation processes in glasses as revealed by depolarized light scattering S.V. Adichtchev b , N.V. Surovtsev b , J. Wiedersich c , A. Brodin a , V.N. Novikov b, * , E.A. Ro ¨ ssler a a Experimentalphysik II, Universita ¨ t Bayreuth, D-95440 Bayreuth, Germany b IA & E, Russian Academy of Sciences, Laboratory of Physical Electronics, Prospect Koptyuga, 1, Novosibirsk 630090, Russia c Technische Universita ¨t Mu ¨ nchen, Lehrstuhl fu ¨ r Physik, Weihenstephan, Voettinger Strasse 40, 85350 Freising, Germany Received 12 October 2006; received in revised form 5 February 2007 Available online 9 April 2007 Abstract Applying tandem-Fabry-Perot interferometry together with a double monochromator, depolarized light scattering spectra were mea- sured in order to investigate the fast relaxation processes and vibrations in molecular, ionic and polymeric glasses in the 1–5000 GHz range covering temperatures from the glass transition temperature T g down to some 10 K. In addition to the boson peak, the spectra reveal quasi-elastic contributions that we attribute to (i) a nearly constant loss in the frequency range below ffi10 GHz and (ii) a power-law contribution with positive exponent a at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials as previously found for the light scattering spectra in silica. Following the Gil- roy–Phillips model the exponent a shows a master curve as a function of T/V 0 for the various glasses, where V 0 specifies the width of the exponential distribution of barriers g(V), i.e., g(V) / exp(V/V 0 ). The parameter V 0 is found to be T g /2 in most cases. The relative strength of the dynamics in asymmetric double well potentials and the nearly constant loss contribution is different in the glasses studied. Ó 2007 Elsevier B.V. All rights reserved. PACS: 7830.Ly; 7835.+c Keywords: Raman scattering; Rayleigh scattering; Glasses; Raman spectroscopy; Fluctuations; Structural relaxation 1. Introduction Over the last several decades, a large body of experimen- tal as well as theoretical work addressed the problem of the glass transition [1–6]. Understanding the dynamic suscepti- bility of simple glass formers as it evolves from high tem- peratures down to the glass transition temperature T g was the main goal. Here, significant progress was made. In contrast to that, the molecular dynamics persisting in the glass, i.e., below T g , is less investigated. Since the main relaxation (a-process) is frozen all persisting relaxational contributions are characterized by small amplitudes, and broad band spectra allowing for a systematic description down to cryogenic temperatures are rare. On the other hand, the anomalies of glasses at low temperatures attracted a lot of attention; however, most of the works focused on temperatures below say 4 K, and mainly inor- ganic network glasses were studied [7–10]. It is not clear whether such glasses exhibit the same relaxational patterns as molecular glasses. In some organic polymers the typical linear increase of the heat capacity with temperature was indeed identified [11]. Recently, a dielectric study covering the kHz range reported quite similar relaxation behavior in molecular glasses below say 30 K as found for silica [12]. Yet, further studies are needed, and the present 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.02.057 * Corresponding author. E-mail address: novikov@iae.nsk.su (V.N. Novikov). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 1491–1500