Reorientational Correlation Functions of Simple Supercooled Liquids as Revealed by NMR Studies

31P, 1H and 2H NMR investigations on supercooled simple organic liquids are presented. Combining measurements of spin-lattice relaxation, spin-spin relaxation and stimulated echo decays the reorientational correlation function F2(t) of the second Legendre polynomial is probed over a wide temperature interval. Correlation times in the range of 10−11 – 1s are covered, and the scaling property of the correlation function F2(t) corresponding to the α-relaxation, i.e., the main relaxation in the supercooled liquid, is checked. The function F2(t) can be described equally well by a Cole-Davidson distribution of correlation times or by a Kohlrausch-Williams-Watts function (10−8 < τ < 1s). Near Tg in the highly viscous regime the correlation times become independent from the molecular size. At similar temperatures the bifurcation of a secondary process (β-process) is observed. Combining results from neutron scattering and NMR the complete F2(t) has to be described by a three-step function near the glass transition temperature Tg and by a two-step function at temperatures well above Tg. The transition at Tg is accompanied by the onset of non-exponential spin-lattice relaxation and by a discontinuous change of the temperature coefficient of the relaxation.