Electronic spectral diffusion in glasses: The influence of coupling to the medium on experimental observables

Abstract

The theory of electronic dephasing in low temperature glasses is extended to include the possibility that the strength of coupling of the chromophore to the solvent medium depends on the nature of the bath dynamical processes and the nature of the chromophore and, therefore, the chromophore-bath coupling can vary as a function of the rate of the dynamics of the medium. In the context of the sudden jump two-level system (TLS) model of low temperature glasses, this theory is used to reconcile the apparent contradiction implied by differences observed in spectral diffusion data for cresyl violet and metal-porphyrins in deuterated ethanol glass at 1.5 K. Previously, the coupling strength of a chromophore to the TLS has been assumed to be independent of rate of the transition between TLS states. Within the context of this approximation, spectral diffusion data yield, Pi(R), the intrinsic TLS fluctuation rate distribution. With the inclusion of the rate dependent coupling, C(R), it is shown that the spectral diffusion observables actually yield Pi(R)C(R). Therefore, the observed lack of spectral diffusion for a particular chromophore over some range of times can imply C(R) is zero rather than the current interpretation that Pi(R) is zero. To illustrate the importance of C(R), a hueristic model is analyzed. A fluctuation rate distribution is introduced that consists of the sum of three log-normal functions each associated with a specific class of dynamics occurring over three overlapping ranges of rates. The uncharged and nonpolar metal porphyrins is taken to couple to TLS strain dipoles, while the charged and polar cresyl violet also couples to TLS electric dipoles. By taking one of the types of TLS dynamics to only give rise to electric dipole fluctuations, it is possible to fit all of the experimental data in deuterated ethanol with a single intrinsic distribution of TLS fluctuation rates. This analysis of previously reported data is supported by the presentation of new stimulated photon echo data on both cresyl violet and zinc meso-tetraphenyl porphine in deuterated ethanol.