Slow Dielectric Relaxation of Supercooled Liqutos Investigated by Nonresonant Spectral Hole Burning
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ABSTRACT When supercooled propylene carbonate and glycerol are subjected to a large-amplitude, lowfrequency electric field, a spectral hole develops in their dielectric relaxation that is significantly narrower than their bulk response. This observation of nonresonant spectral hole burning establishes that the non-Debye response is due to a distribution of relaxation times. Refilling of the spectral hole occurs abruptly, indicative of a single recovery rate that corresponds to the peak in the distribution. The general shape of the spectral hole is preserved during recovery, indicating negligible interaction between the degrees of freedom that responded to the field. All relevant features in the behavior can be characterized by a model for independently relaxing domains that are selectively heated by the large oscillation, and which recover via connection to a common thermal bath, with no direct coupling between the domains.
INTRODUCTION One of the most prominent features in the response of many diverse materials is their slow nonexponential relaxation. Although slow mechanical creep was known to Robert Hooke more than 300 years ago,' and similar nonexponential relaxation has been found to occur in thousands of different measurements on hundreds of diverse substances,2 there is still no widely accepted explanation for the observed behavior. In fact, there is still considerable debate 3 6 about the fundamental source ofthe spectral broadening: is it intrinsic, where all regions of the sample exhibit similar nonexponential relaxation? or is it due to a heterogeneous distribution of local relaxation times? For supercooled liquids near their calorimetric glass transition temperature T. several studies',' have concluded that heterogeneity occurs on length scales of I to 5 nm, yet competing models proposed to describe the slow dynamics have often been based (at least in spirit) on opposite assumptions regarding the nature of the intrinsic local response. Such uncertainty punctuates the need for additional experimental evidence. Only in the past few years have techniques been developed that can characterize the intrinsic local response of supercooled liquids. These techniques generally utilize a local probe, and a spectral filter, to investigate selected constituents of the net response. A multi-dimensional NMR technique, developed by Schmidt-Rohr, Spiess and co-workers,9 uses nuclear magnetic moments as the local probe, and a spin-echo sequence as a low-pass filter for selecting slowly rotating molecules for subsequent investigation. Another technique, developed by Cicerone and Ediger,1 ° uses dilutely dispersed dye molecules as the local probe, and a photobleaching procedure as a low-pass filter for subsequent investigation of selected slowly rotating probe molecules. Recently" we introduced the technique of nonresonant spectral hole burning (NSHB), where a large-amplitude, low-frequency electric field is used to distinguish selected dielectric response for subsequent measurements. Two key advantages of NSHB are that th
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