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

摘要

The spin-lattice relaxation time T_1 of simple organic glass formers is analyzed by introducing a spectral density obtained from broadband dielectric susceptibility data X"(#omega#). For this purpose X"(#omega#) was measured for several glass formers, that do not exhibit a Johari-type secondary relaxation 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 introduce an analytical function to fit the shape of the main relaxation (#alpha#-process) above T_g, in particular taking into account high-frequency contributions in X"(#omega#) commonly known as high-frequency wing. Below T_g the latter feature appears as a power law susceptibility X"(#omega#)~(propor. to) #omega#~(-#gamma#), with #gamma# < 0.1 and a characteristic temperature dependence X"(T)~(propor. to) exp(T/const.), yielding almost 1/#omega# behavior in the spectral density. On the base of this complete description of X"(#omega#), a quantitative comparison of dielectric and NMR spectroscopy is possible, which is carried out in full detail for glycerol-d_3 (~2H-NMR), yielding almost identical spectral densities at the Larmor frequency in both cases. In particular the temperature dependence of the high-frequency wing reappears in that of the spin lattice relaxation rate. In addition a semiquentitative analysis is given for trinaphthyl benzene (~1H-NMR) and tricresyl phosphate (~(31)P-NMR).