Zero-THz absorption profiles in glassy solutions. High frequency γ process and its characterisation

Abstract

It is established that in addition to the well-documented α and β losses in viscous fluids, there exists a third (γ) process at THz frequencies (far infrared, or submillimetre) which is necessary for the overall time-correlation function of the dipole moment to be well-behaved at short times. The β and γ peaks of the zero-THz loss profile survive in the glassy state as remnants of the microwave and Poley absorptions observable in the liquid at ambient temperature. In order to probe the glassy environment in non-dipolar solvents such as decalin, the behaviour of the following solutes is monitored: dichloromethane (10 % v/v); dibromomethane (3 % v/v); tetrahydrofuran (10 % v/v); toluene (20 % v/v); chloroform (10 % v/v); fluoro-, chloro-, and bromo-benzene (10 % v/v). Some of these solutes are used as probes also into the weakly dipolar glassy environment of o-terphenyl and (10 % pyridine + 90 % toluene). The zero-THz absorption profiles of the solutes display a variety of behaviour above and below the glass transition temperature. For example, CH₂Cl₂+ decalin has a very pronounced β process in the viscous liquid which is absent from CH₂Br₂+ decalin. The β process gradually disappears across the halogenobenzene + decalin series from fluoro- to bromo-.

The behaviour of the β and γ peaks for asymmetric, planar and intermediate solutes (e.g., CH₂Cl₂, halogenobenzene and tetrahydrofuran) in dilute solution in glassy decalin, for example, confirm that they are different expressions of the same overall dynamical evolution. For CH₂Cl₂+ decalin solid solutions the β peak moves very rapidly to high frequencies with increasing temperature. The associated Arrhenius activation enthalpy (ΔH_β) is thus extremely high compared with those of the larger molecules studied in the glass by Johari et al. and by Williams et al. There is a correspondingly dramatic shift in the THz(γ) peak to higher frequencies from its value in the liquid solution at 298 K to that in the glass at 107 K. In tetrahydrofuran + decalin glass ΔH_β is low and the shift in the γ peak much smaller. The same is true for the halogenobenzene + decalin glasses where present data for the β process are in excellent agreement with those of Johari et al., where available.

A crude but tractable model capable of reproducing both β and γ processes of the glassy condition is that of itinerant libration.