Nuclear Spin-Lattice Relaxation in Hydrogen Gas

Abstract

The proton spin-lattice relaxation time, ${T}_{1}$, for hydrogen gas has been measured, using pulse techniques, as a function of gas density, $\ensuremath{\rho}$, temperature, and orthohydrogen concentration. The maximum density investigated was of order 900 amagats and the temperature range extended from 20 to 400\ifmmode^\circ\else\textdegree\fi{}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 $\frac{{T}_{1}}{\ensuremath{\rho}}$ in the dilute gas above 80\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}K are found to be sensitive to quantum-mechanical diffraction effects, in the manner suggested by Lipsicas and Bloom, and the temperature independence of $\frac{{T}_{1}}{\ensuremath{\rho}}$ for dilute normal hydrogen gas above 80\ifmmode^\circ\else\textdegree\fi{}K is shown to result from the different radial dependence of the ortho-ortho and ortho-para interactions. The results for ${T}_{1}$ 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\ifmmode^\circ\else\textdegree\fi{}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 ${T}_{1}$ is observed in the temperature range 34-42\ifmmode^\circ\else\textdegree\fi{}K. It appears that this effect is connected with the critical temperature phenomena, but it is not understood at present.