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

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 ${10}^{\mathrm{\ensuremath{-}}2}$ to ${10}^{13}$ Hz were analyzed using the mode-coupling theory. The coordinates of the susceptibility minimum, ${\mathrm{\ensuremath{\omega}}}_{\mathrm{min}}$ and ${\mathrm{\ensuremath{\chi}}}_{\mathrm{min}}$, as well as the position of the maximum, ${\mathrm{\ensuremath{\omega}}}_{\mathrm{max}}$ (\ensuremath{\alpha} peak), scale with temperature according to the mode-coupling theory, resulting in ${\mathit{T}}_{\mathit{c}}$=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 ${\mathrm{\ensuremath{\omega}}}_{\mathrm{min}}$ resulting from the mode-coupling-theory force fit were used. The width of the \ensuremath{\alpha} peak appears to increase with increasing temperatures for temperatures above ${\mathit{T}}_{\mathit{c}}$, 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 ${\mathit{T}}_{\mathit{c}}$ 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 \ensuremath{\beta} relaxation on both sides of ${\mathit{T}}_{\mathit{c}}$. Assuming a priori that the mode-coupling model is correct, it is possible to make the data compatible with the mode-coupling theory.