Temperature-jump NMR: Molecular twisting at the phase transition inpara-terphenyl

Abstract

Molecular twisting in a polycrystal of para-terphenyl at and below its 193.3-K phase transition has been studied by a new NMR technique. The first two pulses of a three-pulse sequence are applied to the proton-decoupled ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ spins above $T_c$ and store information about the high-temperature orientation of the molecules in the form of $z$ magnetization. The sample temperature is then reduced below $T_c$ in a few seconds. The final pulse of the sequence is applied, forming a stimulated echo. The amplitude of this echo is decreased by molecular reorientation or twisting during the temperature decrease. By studying the echo amplitude and its Fourier transforms, we have confirmed the picture of twisting put forward from scattering experiments. The temperature-jump experiment is just one of a broader class of experiments in which the sample condition is changed during the pulse sequence. These experiments are applicable to amorphous as well as polycrystalline samples and promise to extend the kinds of information available from NMR.