Positron annihilation lifetime response and relaxation dynamics in glycerol

Positron annihilation lifetime spectroscopy (PALS) data on glycerol are related to the results of broadband dielectric studies from the literature. Specifically, the temperature variation of the ortho-positronium (o-Ps) lifetime, tau(3), and of the relative intensity, 13, from 100 K up to 300 K have been compared with the spectral features of the alpha and beta relaxation from the decomposed dielectric loss spectra. First, it is found that in the tau(3) VS. temperature plot, in addition to the universal characteristic PALS temperatures: T-g(PALS) and T-b2(L) = 1.53T(g)(PALS) where large discontinuities in slope are observed and which are associated with the glass-liquid transition and the onset of a quasi-plateau level, respectively, a discontinuity in the slope occurs at T-b1(L) = 1.28T(g)(PALS). Further, it is found that tau(3) at T-b1(L) is approximately the same as the mean secondary beta relaxation time, tau(3)(T-b1(L)) approximate to tau(beta)(T-b1(L)). Next, it is suggested that the decrease in I-3 vs. T plot as temperature increases above T-g(PALS) can be related to the beta and alpha relaxations qualitatively. These findings are consistent with the notion that free volume redistribution in liquid glycerol is due to the local beta process as well as the high-frequency part of cooperative a process causing a partial elimination of smaller holes from the PALS spectrum as temperature increases. An alternative or perhaps additional explanation is that as temperature increases, some of the small, detectable holes break up into holes too small for the PALS measurement. (c) 2005 Elsevier B.V. All rights reserved.