Depolarized-light-scattering study of orthoterphenyl and comparison with the mode-coupling model

Authors:

• W. Steffen,
• Adam Patkowski,
• H. Gläser,
• G. Meier,
• E.W. Fischer

Abstract

The dynamics of the molecular glass-forming liquid orthoterphenyl above the glass-transition temperature was studied combining several experimental techniques: depolarized Raman, depolarized Rayleigh-Brillouin light scattering, and photon correlation spectroscopy in the temperature range from 250 to 440 K. The combined spectra covering a frequency range from 10-2 to 1013 Hz were analyzed using the mode-coupling theory. The coordinates of the susceptibility minimum, ωmin and χmin, as well as the position of the maximum, ωmax (α peak), scale with temperature according to the mode-coupling theory, resulting in Tc=290 K. The construction of the predicted master curve in the vicinity of the minimum of the rescaled susceptibility was possible in a narrow frequency range only if the values of ωmin resulting from the mode-coupling-theory force fit were used. The width of the α peak appears to increase with increasing temperatures for temperatures above Tc, although when the effects of fast processes on the high-frequency wing are included, the corrected width appears to decrease instead approaching a Debye relaxation shape at high temperatures. Below Tc it was not possible to fit objectively the data using the mode-coupling theory; thus it was impossible to corroborate the divergence of the scaling time of the mode-coupling β relaxation on both sides of Tc. Assuming a priori that the mode-coupling model is correct, it is possible to make the data compatible with the mode-coupling theory. © 1994 The American Physical Society.