Revisiting impulsive stimulated thermal scattering in supercooled liquids: Relaxation of specific heat and thermal expansion

Abstract

A generalized physical model is introduced to describe the impulsive stimulated scattering (ISS) response of relaxing systems to photothermal excitation in a periodical grating geometry. The proposed approach starts from Debye and Havriliak-Negami expressions for both the frequencydependent heat capacity, C(ω), and thermal expansion coefficient, γ(ω). Simulations are carried out on glycerol to test and compare the developed models with the existing semi-empirical model [1]. Debye behavior of the specific heat capacity is shown to be compatible with a two-temperature scenario, in which, in addition to the classical, experimentally observable temperature that characterizes the distribution of the system over vibrational energy states, a second temperature characterizes the state of the configurational energy landscape. The models here developed have been applied for the interpretation of the experimental ISS signals of supercooled glycerol, illustrating simultaneous and separate assessment of C(ω) and γ(ω) up to sub-100 MHz from thermoelastic transients.

Time-resolved thermal lens investigation of glassy dynamics in supercooled liquids: Theory and experiments

“…Due to its time-varying and often spatially non-uniform character, the local thermal expansion response is unavoidably accompanied by the launching of acoustic waves, which carry information on the (relaxation behavior of the) elastic modulus. Besides, in many respects, the pulsed TL scheme is very similar to the one of the transient grating (TG), or impulsive stimulated thermal scattering (ISTS) [42,54]. The main difference between the two approaches lies in the geometry of the optical excitation pattern: while in TG the light pattern is spatially periodic and characterized by a single wavenumber, the Gaussian pattern used in a TL configuration results in a wide spectrum in the wavenumber domain.…”

Determination of Cooperativity Length in a Glass-Forming Polymer

“…23 Combinations of heat capacity and thermal conductivity are also available from other high-frequency methods like photopyroelectric, 24 thermal lens, 25 or impulsive stimulated thermal scattering. 26 Frequencies up to 10 8 Hz became accessible this way.…”

“…28,29 Therefore, the inherent problems of methods measuring effusivity, 28,38 like the 3ω-method, 21,22 transient grating techniques, 39 or other highfrequency thermal techniques, are not significant here. [24][25][26]40 The employed differential AC chip calorimeter (Figure 2) with pJ K −1 sensitivity with a small total heat capacity (addenda + sample) and the possibility to cool fast, not only high sensitivity is achieved but also AC measurements at relatively high frequencies are possible. 41 The calorimeter allows measurements in the frequency range 10 −2 rad s −1 to above 10 6 rad s −1 .…”

“…First studies on the relaxation process near the glass transition (α-relaxation) employed the so-called 3ω-technique to obtain the complex effusivity. , The accessible frequency range is between 2·10 –3 and 4·10 3 Hz . Combinations of heat capacity and thermal conductivity are also available from other high-frequency methods like photopyroelectric, thermal lens, or impulsive stimulated thermal scattering . Frequencies up to 10 8 Hz became accessible this way.…”

“…Such high cooling rates are accessible by fast scanning chip calorimeters. − By minimizing sample and addenda heat capacity (nJ K –1 ) and utilizing a gas as the cooling medium fast cooling can be realized . The thin-film limit for heat capacity determination is satisfied using thin (<μm) samples. , Therefore, the inherent problems of methods measuring effusivity, , like the 3ω-method, , transient grating techniques, or other high-frequency thermal techniques, are not significant here. − , …”