Spectral diffusion of molecular electronic transitions in amorphous solids: Weak and strong two-level-system phonon coupling

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Abstract
The two-level-system (TLS) model of glasses is used with nonphenomenological TLS distribution functions [R. Jankowiak et al., J. Phys. Chem. 90, 3896 (1986)] to account for the time-dependent spectral-diffusion data for cresyl violet and zinc porphin in ethanol glasses at low temperatures [K. A. Littau and M. D. Fayer, Chem. Phys. Lett. 176, 551 (1991); H. C. Meijers and D. A. Wiersma, Phys. Rev. Lett. 68, 381 (1992)]. The two distributions of faster and slower TLS relaxation rates required to fit the data are characterized for both weak and strong TLS-phonon coupling. Comparison of the values obtained for the TLS parameters with those determined earlier for specific heat, thermal conductivity, pure dephasing, and spontaneous filling of nonphotochemical holes establishes that the faster and slower distributions are associated with intrinsic and extrinsic TLS. The pronounced effects of strong coupling on the TLS relaxation-rate distributions are discussed.