Low frequency Raman spectroscopy of supercooled fragile liquids analyzed with schematic mode coupling models

Abstract

Recent experimental studies of the glass transition of molecular liquids have exploited light scattering techniques in order to support the dynamical model proposed by the mode coupling theory. In the framework of the dipole-induced-dipole (DID) formalism and the Stephen’s approximation, we have checked this theory with several memory functions in the microscopic region, where phononlike excitations dominate, i.e., in the frequency window of 5–130 cm−1 accessible by a classical Raman spectrometer. The fitting procedure compares the experimental susceptibility spectra of one of the simplest fragile molecular liquid, m-toluidine, to the theoretical ones and estimates, in each case, the T dependence of the different control parameters as well as the crossing point of the transition line of type B. The agreement observed for spectra from a temperatures above the melting point down to the glass transition temperature Tg suggests, on the one hand, that information about the dynamical behavior of the supercooled liquid are contained in this frequency region and, on the other hand, that vibrational contributions are incorporated in this formalism, independently of the form of the relaxation kernel. Finally, the two-peak shape in the microscopic range of the susceptibility spectra is related to the relaxation of a linear combination of the Fourier components of the two density correlators.

Structure and relaxation in liquid and amorphous selenium

“…Below T c , a third regime appears, the so-called ␤ regime, which is first seen as a shoulder and saturates at a finite value below T g . This nonergodicity has been seen in many experimental measurements [3][4][5][6] and computer simulations [7][8][9][10] on different types of materials ranging from the fragile polymeric glasses to strong glasses, such as SiO 2 . In this paper we want to go a step further.…”

Multiple-scattering effects on smooth neutron-scattering spectra

“…In cases where the asymptotic regime is not reached numeric solutions of schematic mode-coupling models have been used to fit experimental data [25][26][27][28]. In a most recent example data from incoherent neutron scattering [22], depolarized light scattering [22,29] and dielectric spectroscopy [30] on glass-forming propylene carbonate have been analysed first in terms of scaling [22] and then by integration of the F 12 -Sjögren model, where the different observables were all governed by one and the same density correlator φ 0 (t) [31].…”

Semischematic model for the center-of-mass dynamics in supercooled molecular liquids

“…The lack of theoretical predictions for the behavior of the center of mass (COM) and angular degrees of freedom in molecular liquids has been compensated by the use of schematic models, where the spectrum of different q-vectors and orientations is condensed into one or two representative correlators. The exact solution of these schematic models and the comparison with experiments [11,15] and simulations [16] provide compelling evidence that even molecular liquids can be described in a MCT framework. A full and detailed study assessing the ability of MCT to predict not only the general features of supercooled molecular liquids, but also the q-dependence of the COM correlators is highly desirable.…”

“…It has now become apparent that the ideal modecoupling theory (MCT) for structural relaxation [2,8] contains the essential ingredients for the description of the molecular dynamics in supercooled states, down to a cross-over temperature T c , where "hopping" processes become dominant [9,10]. Although the MCT was originally developed for simple (atomic) liquids, it appears to be able to rationalize in a coherent way experimental [3,11,12] and numerical simulation results [6,13,14] also for molecular liquids, i.e. for molecules with anisotropic interaction potentials.…”