Impurity-Controlled Nuclear Relaxation

This work reports proton spin-lattice relaxation-time measurements performed as a function of temperature and magnetic field on a single crystal of paradibromobenzene, at liquid-helium temperatures and at room temperature. Strong evidence is displayed that at low temperature the proton spin-lattice relaxation is controlled by one kind of paramagnetic impurity. Both domains of diffusion-limited relaxation and domains of free diffusion relaxation are clearly exhibited, and the transition between these domains is consistent with a steep decrease of the spin diffusion coefficient $D$ at a distance $b_0$ from the impurity, the diffusion barrier. Order-of-magnitude calculations of $D$ and $b_0$ seem reasonable. The following information is derived as to the paramagnetic impurities: They probably have a Kramers degeneracy, in which case their spin-lattice relaxation in the neighborhood of 4°K is due to a Raman process. Their spin-lattice relaxation time $\tau$ at 4.2°K is comparable to 1.5×${10}^{-5\mathrm{}}$ sec and their concentration $N$ is of the order of ${10}^{-7\mathrm{}}$ impurities per molecule. At room temperature, a different relaxation mechanism is effective, which is likely to originate from the dipole-dipole coupling of the protons with the bromine nuclei.