Nonresonant Spectral Hole Burning in the Slow Dielectric Response of Supercooled Liquids

Abstract

Large-amplitude, low-frequency electric fields can be used to burn spectral holes in the dielectric response of supercooled propylene carbonate and glycerol. This ability to selectively modify the dielectric response establishes that the non-Debye behavior results from a distribution of relaxation times. Refilling of the spectral hole was consistent with a single recovery time that coincided with the peak in the distribution. Moreover, refilling occurred without significant broadening. which indicates negligible direct exchange between the degrees of freedom that responded to the field. Nonresonant spectral hole burning facilitates direct investigation of the intrinsic response of systems that exhibit nonexponential relaxation.W h e n subjected to a n external perturbation, lnany materials, including polynlers (1), protelns ( 2 ) , and supercooled liquids ( 3 ) , exhibit nonesponent~al relaxation. T h e response of thousands of different sul~stances (4) has been character~zed by the Kohlrausch-Will~a~ms-Watts (KWW) stretched exponential, exp[-(t/r)@], where 9 < 1, t is time, and 7 IS a character~stic relaxation t~m e .It has long been Jehated (5-8) whether t h~s dvnam~cal complexity is intrinsic, \ \~t h all repions of the salnole exhihitine a silllilar nor-Llebve response, or ~vhether it is the result of heterogenelty, r i t h localized degrees of freeilom relaxing exponentially hut w~t h a distribution of relaxat~on times that y~elds the net lxhavior. Several itudies (9-1 1 ) of supercooled liquids near t h e~r calori~netrlc glass transition tenlperature T,, have concludeii that heterogeneity occurs on length scales of 1 to 5 nm. Recent lnagnetlc resonance measurements 11 2 ) h a w s h o~v n that these domains are not static, but that their d~stinct relaxation rate. persist long enough to cause the broadened response. Such dvnalnic heterogeneity, n h~c h evolves as the saml?le responds, has not previousl\-been observed directly in the slonr d~electric rebponse.Spectral hole burning (SHB) map be the no st direct t e c h n i q~~e for investijiating the constituents in the net response of a macroscopic sample. O h s e r~. a t~o n of S H B g e rerally requires a heterogeneously broadened spectnlm, a strong external sigllal that can modify selected parts of the spectrum, and a resulting "hole" that is persistent enough to allo\v subsequent measurement of the modified response. A n in-portant advantage of the technic~ue is that the responding de-B Schiener and A Loid;, lnst~tut fur Festkorperpi/sk,

Polymer-specific effects of bulk relaxation and stringlike correlated motion in the dynamics of a supercooled polymer melt

“…To test this idea experimentally several techniques, such as multidimensional NMR, 8,9 optical bleaching, 10 nonresonant spectral hole burning 11 or solvation dynamics, 12 have been applied to a variety of glass formers ͑for reviews see Refs. 4 -6͒.…”

Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol

“…These are, the Glarum's defect diffusion model and its modifications, 39,40 models for a dynamic heterogeneity, 12,41,42 and those that require a multiplicity of single relaxation times. [43][44][45] A somewhat unrelated mechanism that can account generally for both a single relaxation time and a distribution of relaxation times was proposed by Anderson and Ullman. 46 These arise from two conditions of the relative rates of fluctuations of the dipole with respect to the potential energy contour of its surroundings.…”

“…This created a difficulty in understanding how the orientational diffusion of a relatively small number of molecules that contribute only ϳ2. 45 to the permittivity of 1-propanol can be seen as responsible for its supercooled liquid's viscosity and structural relaxation.…”

Mode-coupling theory for multiple-point and multiple-time correlation functions

“…Furthermore, at later times, a region of the fluid which previously appeared crystalline may exhibit fluid-like properties. Such heterogeneous behavior is characteristic for structural glasses and super-cooled complex fluids [4][5][6][7][8][9]. To describe this behavior, it is natural to examine how the local density of the liquid is correlated over various spatial domains [10,11], or, when one is more interested in the different time scales of slow global changes of structure and the local decay of correlations, multiple time correlation functions [12].…”