NMR relaxation measurements in solid H₂ at low ortho-H₂ concentrations

Nuclear transverse and longitudinal relaxation time measurements in solid hcp H₂ are presented for two frequencies, 5.3 and 29 MHz. The ortho molefraction X varied from 2×10^-3 to 0.1 and the temperature range extended from 0.4 K to near the triple point, ∼ 13.9 K. Over this range of X and T, the longitudinal time T₁is representative of the intramolecular relaxation processes that reflect the orientational fluctuations of the molecules. On the other hand, the rates from intermolecular dipolar coupling are calculated to be negligible. At concentrations X≲0.008, the transverse time T₂is dominated by the contributions from intramolecular relaxation processes, and is found to increase quite strongly with temperature. This new effect is ascribed to a coupling between molecular rotation and lattice vibrations, brought into evidence by the narrow width of the fluctuation spectral density at low X. For X>0.01 in the nondiffusion region, the NMR line shape is dominated by the signal from isolated ortho-H₂ molecules broadened via intermolecular dipolar interactions, and by the signal from isolated ortho pairs and triangles, etc., configurations. Above 9 K, however, the thermally activated diffusion produces an averaging out of these broadening effects, making it possible to determine the T₂from intramolecular nuclear spin interactions. For these mixtures, T₁also shows a temperature variation, but with a maximum near 7 K and a flat minimum at higher temperatures. The relaxation data are compared with previous experiments and some systematic discrepancies in the dependence on X are found. The frequency dependence of T₁extends to higher concentrations than one expects from the theory of Fujio, Hama, and Nakamura, where nuclear relaxation is treated in terms of the orientational fluctuations from intermolecular electric quadrupolar coupling and crystalline fields. In the appendix, results are presented of an earlier attempt in this laboratory to determine the crystalline field splitting in H₂from the temperature variation of the NMR line shape. An upper bound of |V_C/k_B|=0.028 K is obtained at zero pressure.