Ethanol Glass Dynamics: Logarithmic Line Broadening and Optically Induced Dephasing

Abstract

The TLS model has been quite successful in the description of the anomalous low-temperature properties of glasses. It also predicts that these properties are time dependent, which has indeed been shown to be the case. 5,6 This effect has been attributed to the existence of a broad distribution of rates in the glass at which the tunneling processes occur. The optical properties of a chromophore dissolved in a glass are dependent on dynamical processes in the direct vicinity of the chromophore. Therefore, these optical properties will also be time dependent. A number of optical techniques have been employed to explore the dynamics of the chromophores on different time scales and thus probe the distribution of rates of tunneling processes. Most notably hole burning, photon echo, and singlemolecule spectroscopy have been employed to probe glass dynamics on a range of time scales spanning up to 18 orders of magnitude. This allows for the investigation of the particular nature of the TLS’s. These so-called optical line-narrowing experiments provide a pivotal test for the adequacy of the TLS model in describing glass dynamics. However, the interpretation of chromophore dynamics in amorphous solids is frustrated by qualitative and quantitative discrepancies between the results rendered by different techniques. Thus, although separate results are in qualitative agreement with the TLS model, the quantitative discrepancies between results rendered by different techniques indicated the need for further refinement of the TLS model. Notwithstanding these problems, the concept of low-energy excitations being accountable for the properties of amorphous solids is well accepted. At the same time it is clear that the interpretation of experimental results has to be done with great caution. Indeed the optical dynamical properties can be influenced by various inconspicuous parameters in the experiments. Examples are the cooling history of the sample, the particular subset of chromophores selected in an experiment, 7,8 and also, as was recently pointed out, 9,10 laser fluence. In this paper we revisit the effect of laser fluence on the observed optical dynamics and confirm earlier conclusions regarding its importance in optical dephasing experiments. We further show that optically induced spectral diffusion, an effect unrelated to laser fluence, is of significant importance to the interpretation of all optical line-narrowing techniques. This finding points at the inadequacy of the currently used weakcoupling chromophore-TLS model for a complete description of glass dynamics.