We present a systematic study of longitudinal nuclear relaxation times T1 in hcp D2 between 0.4 °K and the triple point, about 18.7 °K, at a frequency of 4.7 MHz. The concentration range of the para molecules (i.e., those with a rotational angular momentum J=1) was between C=0. 02 and C=0. 91. Following saturation, the recovery of the longitudinal magnetization was observed to be exponential as a function of time for all concentrations, and hence the relaxation time T1 was always well defined. In the region below 11 °K, where thermally activated diffusion is "frozen out," the main relaxation mechanisms are by means of the modulation of the intramolecular dipolar fields by the intermolecular electric quadrupole-quadrupole interaction, and by cross relaxation between the nuclei of molecules with J=0 (ortho D2) and J=1 (para D2). The agreement between the limiting high-temperature value T1 and that calculated from theory is good at high concentrations of (J=1) molecules, but only fair at low concentrations. For the latter region, systematic discrepancies between experiment and theory are discussed. The strong temperature dependence of T1 below 4°K is not well understood. In the region above about 12 °K, where diffusion narrows the linewidth, and for concentrations C above about 0.3, the relaxation time T1 is hardly affected by diffusion and shows only a small increase with T. However, T1 for low (J=1) concentrations shows a strong increase with T and a maximum near 17 °K. This behavior can be understood in terms of a theory by Bloom, which was applied to the similar case of solid HD with H2 impurities. At temperature above 13 °K, T1 becomes influenced by diffusion, and the data could be quantitatively fitted to Bloom's theory using two parameters with values close to those estimated theoretically.
ASJC Scopus subject areas
- Condensed Matter Physics