Nuclear magnetism of solid hydrogen at reduced ortho concentration

The NMR of 200-ml STP of pure ${\mathrm{H}}_2$, frozen in a cavity resonating at 310 MHz, was studied as the sample aged for 400 h while cooled by a dilution refrigerator. The frequency separation between points of maximum and minimum slope of the NMR line decreased linearly with time for $24<t<\mathrm{}240\mathrm{}$ h at the rate of 180 Hz/h. At the highest ortho concentrations and the lowest temperatures, the line shape corresponded closely to the doublet expected for interacting randomly oriented dipole pairs broadened by intermolecular dipolar interactions. However, as the ortho concentration decreased below 0.5 the line became more square than this model would predict. At several times during the run, the temperature was raised and again lowered. At the lowest temperature, the line changed with time through shapes very similar to those found with increasing temperature at an earlier time. Evidence from the integrated intensity of the line suggests that the ortho-para conversion during the run increased relative to the rate expected for a random lattice with interacting pairs. Studies of recovery from selective saturation showed that cross relaxation within the NMR line creates a spin temperature in about 5s.