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Nuclear spin relaxation in gaseous H₂, HD and D₂ Hardy, Walter Newbold

Abstract

The longitudinal and transverse nuclear relaxation times, T₁ and T₂, have been measured in normal H₂ gas at 77.5°K in the pressure range 0.05 to 2 atmospheres. In this region T₁ goes through a minimum, and T₂ deviates significantly from a linear dependence on the density. Comparison of the experimental data with existing theory establishes the following results for the J=1 state of orthohydrogen: i. autocorrelation functions of the molecular angular momentum operators are exponential or nearly so, ii. the ratio of the correlation times , Ʈ₁, Ʈ₂, which are associated with operators of the form J₊, and J²₊ respectively, lies within the limits 0.6 ≤ Ʈ₁ / Ʈ₂ ≤ 1, iii. the splitting of the molecular Zeeman levels cannot be neglected as in the original Schwinger theory. T₁ for the proton and deuteron in HD gas and for the deuterons in normal D₂ gas was measured as a function of temperature and pressure in the range 20 to 373°K and 0 to 8 atmospheres. To within experimental error the dependence of T₁ on the density p is linear. In HD below 65°K, when only the J=0 and J=1 states of the molecule are appreciably populated, the temperature dependence of T₁/p is identical for both proton and deuteron, leading to a value of Ʈ₁/Ʈ₂ = 1,07/± 15% for the J=1 state of HD. Above 100°K, T₁/p for the proton is inversely proportional to the temperature, whereas for the deuteron T₁/p is almost temperature independent. The experimental results are interpreted as evidence that in HD gas the process of molecular reorientation is dominated by the anisotropic intermolecular force arising from the separation of the centres of mass and charge of the molecule. In D₂ gas two relaxation times were found, one associated with the S=1 spin state of paradeuterium and the other associated with the S=2 spin state of orthodeuterium. At 40°K (T₁/p)s=₂ appears to go through a minimum; the analogous quantity in H₂ measured by previous workers also goes through a minimum, but at 80°K. This is consistent with interpreting the minimum as a quantum mechanical diffraction effect. The J=2 component of (T₁/p)s=₂ however, does not go through a minimum, which suggests that the intermolecular interactions are significantly different for the J=1 and J=2 states of the molecule.

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