Positron annihilation studies of supercooled liquids. ortho-terphenyl

Abstract

Positron annihilation measurements are reported on ortho-terphenyl (o-TP) covering the temperature range 230–365 K. The lifetime data were analysed in terms of a three component fit; the two shortest lifetime components correspond to the decay of the free positron and the para-positronium (p-Ps) and the longest lifetime component corresponds to the annihilation of the ortho-positronium (o-Ps). The lifetime plot of the o-Ps against temperature exhibits two changes in slope, the first transition occurring at 240 K and the second at 304 K. The lower temperature is associated with the onset of motion in the glassy phase; the higher temperature is more obscure, but appears to correlate well with the onset of instability as predicted by a weak coupling mean-field treatment of the liquid state. Attempts to correlate the temperature variation of the o-Ps lifetime with the variation of surface tension, density and free-volume indicated that there is no simple explanation for the higher temperature transition feature. Calculations based on the “bubble” model and pick-off rate theory indicate that in the liquid phase and above the higher temperature transition the lifetime and intensity of annihilation of o-Ps correspond closely to those predicted for a simple liquid. In between the two transition temperatures, the variation is inexplicable in terms of a simple “bubble” model and leads to the conclusion that the void size distribution being sampled by the o-Ps is itself a function of the total free-volume available at the temperature of measurement. Comparison of the temperature variation of the self-diffusion coefficient and the o-Ps lifetime data indicate that both properties do not exhibit a simple linear variation with dependence on the free-volume in the supercooled liquid state. However, normalisation of the o-Ps data by the effective free-volume radius leads to an approximately linear correlation between these properties indicating a common basis to the molecular motion being sensed.