Reorientational dynamics in supercooled m-tricresyl phosphate: Its relation to main and secondary relaxation—31P nuclear magnetic resonance study of relaxation, line shape, and stimulated echo

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Abstract
Supercooled m‐tricresyl phosphate has been studied by different nuclear magnetic resonance (NMR) methods using the 31P spin probe (180–400 K). Combining measurements of the spin‐lattice relaxation at two Larmor frequencies, of the line shape and of the stimulated echo decay, a reorientational correlation function F2(t) is probed over the entire supercooled regime. The analysis of the stimulated echo is described in detail. The long‐time behavior of F2(t) (α process) does not change its shape below 260 K and is approximated by a stretched exponential, however, some deviations are observed at short times. Above 260 K a decrease of the stretching of F2(T) is found. Reorientational correlation times τrot are reported in the range 10−11 s <τrotrot and the one of viscous flow separate only weakly upon supercooling, if at all, whereas translational diffusion and viscous flow clearly decouple below 270 K (∼1.26 Tg). These findings are taken as an indication of a change of transport mechanism in the supercooled liquid. For T≳Tg a change of spectral density is revealed by the relaxation experiments, and it is attributed to the short‐time behavior of F2(t), i.e., to the β process and to peculiarities of the α process itself. The β process has a small amplitude which decreases upon cooling.