The spectral density in simple organic glass formers: Comparison of dielectric and spin-lattice relaxation

The spin-lattice relaxation time T-1 of simple organic glass formers is ana lyzed by introducing a spectral density obtained from broadband dielectric susceptibility data chi "(omega). For this purpose chi "(omega) was measure d for several glass formers, that do not exhibit a Johari-type secondary re laxation process, covering a frequency range between 10(-2) Hz and 10(9) Hz at temperatures above and below the glass transition temperature T-g. We i ntroduce an analytical function to fit the shape of the main relaxation (al pha-process) above T-g, in particular taking into account high-frequency co ntributions in chi "(omega) commonly known as high-frequency wing. Below T- g the latter feature appears as a power law susceptibility chi "(omega)prop ortional to omega(-gamma), with gamma < 0.1 and a characteristic temperatur e dependence chi "(T)proportional to exp(T/const.), yielding almost 1/omega behavior in the spectral density. On the base of this complete description of chi "(omega), a quantitative comparison of dielectric and NMR spectrosc opy is possible, which is carried out in full detail for glycerol-d(3) (H-2 -NMR), yielding almost identical spectral densities at the Larmor frequency in both cases. In particular the temperature dependence of the high-freque ncy wing reappears in that of the spin lattice relaxation rate. In addition a semiquantitative analysis is given for trinaphthyl benzene (H-1-NMR) and tricresyl phosphate (P-31-NMR). (C) 1999 American Institute of Physics.