Correlation between glass transition induced

Sekkat, Z., Pretre, P., Knoesen, A., Volksen, W, Lee, V. Y, Miller, R. D., Wood, J., and Knoll, W. (1998). Correlation between polymier architecture and sub-glass-transition-tem-perature light-induced molecular movement in azo-polyimide polymers influence on linear and second- and third-oixier nonlinear optical processes. /. Opt. Soc. B 15, 401-413. 

The considerations above apply to fast dynamic processes in the sense that the amplitude of the modulation of the resonance frequency Amq (induced by modulation of the local field) multiplied with the correlation time Tc is much smaller than unity. This Redfield regime [27] is usually attained in solutions with low viscosity [2], but may also apply to small-amplitude libration in solids [28]. For slower reorientation in solutions with high viscosity or in soft matter above the glass transition temperature (slow tumbling), spectral lineshapes are directly influenced by exchange between different orientations of the molecule (Section 4.1). Relaxation times in solids outside the Redfield regime carmot be predicted from first principles except for a few crystalline systems with very simple structure and few defects [29]. In such systems, qualitative or semi-quantitative analysis of relaxation data can still provide some information on dynamics. 

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