ELECTRONIC SPECTRAL DIFFUSION IN GLASSES - THE INFLUENCE OF COUPLING TO THE MEDIUM ON EXPERIMENTAL OBSERVABLES

The theory of electronic dephasing in low temperature glasses is exten ded to include the possibility that the strength of coupling of the ch romophore to the solvent medium depends on the nature of the bath dyna mical processes and the nature of the chromophore and, therefore, the chromophore-bath coupling can vary as a function of the rate of the dy namics of the medium. In the context of the sudden jump two-level syst em (TLS) model of low temperature glasses, this theory is used to reco ncile the apparent contradiction implied by differences observed in sp ectral diffusion data for cresyl violet and metal-porphyrins in deuter ated ethanol glass at 1.5 K. Previously, the coupling strength of a ch romophore to the TLS has been assumed to be independent of rate of the transition between TLS states. Within the context of this approximati on, spectral diffusion data yield, P-i(R), the intrinsic TLS fluctuati on rate distribution. With the inclusion of the rate dependent couplin g, C(R), it is shown that the spectral diffusion observables actually yield P-i(R)C(R). Therefore, the observed lack of spectral diffusion f or a particular chromophore over some range of times can imply C(R) is zero rather than the current interpretation that P-i(R) is zero. To i llustrate the importance of C(R), a hueristic model is analyzed. A flu ctuation rate distribution is introduced that consists of the sum of t hree log-normal functions each associated with a specific class of dyn amics 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 electr ic dipoles. By taking one of the types of TLS dynamics to only give ri se to electric dipole fluctuations, it is possible to fit all of the e xperimental data in deuterated ethanol with a single intrinsic distrib ution of TLS fluctuation rates. This analysis of previously reported d ata is supported by the presentation of new stimulated photon echo dat a on both cresyl violet and zinc meso-tetraphenyl porphine in deuterat ed ethanol. (C) 1996 American Institute of Physics.