Proton-spin thermometry at low fields in liquid crystals

The relaxation rate, resulting from the order-director fluctuation, of the dipolar energy of benzene proton-spin pairs in a liquid crystal was calculated. For this mechanism the ratio $\frac{T_1}{T_{1D}}$ is 3. Proton-spin-lattice relaxation times, $T_1$, $T_{1X}$, and $T_{1D}$, as well as the transverse and longitudinal magnetization, were measured in the liquid crystals para-octyloxybenzoic acid and para-azoxyanisole (PAA). Since the spins on one liquid-crystal molecule are isolated from spins on other molecules, each molecule represents an independent small ensemble. Although the macroscopic thermodynamics applies to a large sample of these small ensembles, the spin-temperature concept should only be applied to such systems with care. In both liquid crystals, the spin-relaxation processes were analyzed using the concept of spin temperature. It is shown that in PAA the relaxation of Zeeman energy in the rotating frame is influenced by the proton-to-nitrogen relaxation.