Study of Nuclear-Magnetic-Resonance Line Shapes in SolidH2

Abstract

A detailed study of the NMR line shapes in solid ${\mathrm{H}}_2$ has been carried out. The ortho-${\mathrm{H}}_2$ concentration $c$ was between 0.09 and 0.95, and the temperature ranged from $T=0.5\mathrm{} \mathrm{to} 10°$K. In the hcp phase, a systematic compilation of the second and fourth moments as a function of $c$ and $T$ was made. In the cubic phase the splitting of the doublet structure was measured as a function of temperature. From an extrapolation of the measurements in the hcp phase to the limiting case of pure ortho-${\mathrm{H}}_2$, it is concluded that in the temperature region where thermal diffusion is unimportant, the second and fourth moments are independent of temperature, their values being, respectively, 80±2 k${\mathrm{Hz}}^2$ and (14.8±0.8)×${10}^3$ k${\mathrm{Hz}}^4$. The contribution to the second moment from intermolecular nuclear dipole interactions (this is the second moment extrapolated to $T=\infty$) is obtained as a function of orthoconcentration and is compared with theoretical predictions. Good agreement is found if one takes into account the correlation between nearest ${\mathrm{H}}_2$ neighbors, an effect that decreases the average value $〈R^{-3\mathrm{}}〉$, where $R$ is the distance between nearest neighbors. Experiments with $0.64<c<\mathrm{}0.74\mathrm{}$ indicate that, as the temperature is increased, the rotational ordering in the cubic phase disappears at almost the same temperature as does the last evidence of this phase. Extrapolation of this result to pure ortho-${\mathrm{H}}_2$ indicates that the rotational ordering disappears at about 3.0°K. No evidence of a transition to a rotationally ordered phase is observed at concentrations below $c=0.60\mathrm{}$. This would indicate that the critical ordering concentration is approximately $c_{\mathrm{crit}}=0.6\mathrm{}$, which is much higher than anticipated for theoretical models of cubic ${\mathrm{H}}_2$. For the concentration of $c=0.71\mathrm{}$, the cubic-to-hcp transition is followed by plotting the ratio of the intensity of the nearly Gaussian line (representing the signal from the hcp phase) to the total intensity. This ratio is compared with the results from x-ray measurements. The present data are discussed in connection with entropy results and the theory of rotational ordering in this phase. The doublet splitting in the cubic phase, extrapolated to $T=0\mathrm{}$, has been obtained as a function of orthoconcentration $c$ and has been found to decrease slightly with $c$, which is in qualitative agreement with theoretical expectations.