Nuclear Spin-Lattice Relaxation in Hydrogen Gas

The proton spin-lattice relaxation time, T₁, for hydrogen gas has been measured, using pulse techniques, as a function of gas density, ρ, temperature, and orthohydrogen concentration. The maximum density investigated was of order 900 amagats and the temperature range extended from 20 to 400^∘{}K. The radial dependence of the anisotropic part of the intermolecular potential due to the interaction of two hydrogen molecules has been deduced from a comparison of the temperature dependence of T₁ρ in the dilute gas above 80^∘{}K with some calculations based on the theory of Oppenheim and Bloom. It is, approximately, an inverse thirteenth power of the intermolecular separation for the ortho-para interaction and an inverse sixth power for the ortho-ortho interaction. The results above 80^∘{}K are found to be sensitive to quantum-mechanical diffraction effects, in the manner suggested by Lipsicas and Bloom, and the temperature independence of T₁ρ for dilute normal hydrogen gas above 80^∘{}K is shown to result from the different radial dependence of the ortho-ortho and ortho-para interactions. The results for T₁ in the very dense gas appear to be consistent with the idea of a liquid-like assembly of molecules with predominantly short-range inter-molecular interactions. Calculations are not available at the present time for comparison with any of the results below about 80^∘{}K, which are expected to be strongly influenced by quantum-mechanical effects. Some qualitative discussion is given of the dilute and dense gas results in this temperature range. At very low orthohydrogen concentrations, an unusual density dependence of T₁ is observed in the temperature range 34-42^∘{}K. It appears that this effect is connected with the critical temperature phenomena, but it is not understood at present.