Proton Spin-Lattice Relaxation in Solid and Liquid Hydrogen Deuteride

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Abstract
Measurements of the proton spin-lattice relaxation time in solid and liquid HD containing H2 (and a smaller amount of D2) impurities have been made in the temperature range 1.3-21.9°K. The experiments in the solid were carried out at two radio frequencies, ∼10 and ∼40 MHz, on samples whose ortho-H2 concentration had been greatly reduced by ortho-para conversion for several weeks at 4.2°K. Below 8°K, T1 is strongly temperature-dependent, varying roughly as T−6.5 down to 4°K in the samples with lowest ortho-H2 concentration. Below 4°K, the temperature dependence decreases to about T−2 at 2°K. The magnetic field dependence of T1 between 0.5 and 10 kOe, at liquid-helium temperatures, is found to have the approximate form T1∝Hα, with α between 13 and 1. These low-temperature results are interpreted in terms of a fast cross-relaxation process between protons of different species, which gives a relaxation rate 1T1 proportional to the product of the ortho-H2 relaxation rate and the ortho-H2 concentration. Differences between our results and those of Hardy and Gaines are explained by a quenching in our samples of relaxation processes involving mutual interactions of ortho-H2 molecules, apparently as a result of crystal-field splittings of the rotational levels produced by the appreciable para-H2 (and perhaps D2) impurity concentrations in our samples. We were able to deduce the ortho-H2 concentrations of our samples (between 7 × 10−4 and 6 × 10−7), to determine the intrinsic relaxation time of the ortho-H2 impurity, which is approximately 16 msec at 4.2°K and 2.5 kOe, and to resolve some of the problems in Hardy and Gaines's interpretation. In the temperature region above ∼8°K for the solid substance, self-diffusion-induced relaxation becomes important for our range of sample ortho-H2 concentrations, and the results are in semiquantitative agreement with the predictions of Bloom's theory. In particular, near the melting point, T1 is independent of the ortho-H2 concentration, indicating dominance of an intrinsic HD relaxation mechanism, and T1 is proportional to H2. The activation energy for HD molecular self-diffusion was found to be approximately 190°K, which is considerably lower than the value of 302°K previously determined by Bloom. The rate of ortho-para conversion of H2 in solid HD was measured to be (0.68±0.05)%/h, in reasonable agreement with theoretical calculations by Urano and Motizuki. Measurements were also made on the rate of para-ortho relaxation at room temperature in a glass vessel, by monitoring the relaxation time at 4.2°K after various waiting times at room temperature. Proton T1 measurements in the liquid at 5.9 MHz indicate an increase of T1 with temperature. From these measurements, the relative relaxation rates due to translational and rotational processes are shown to vary from about 4:1 at the HD boiling point to 50:1 at the melting point, in agreement with Bloom's estimate.