Anomalous NMR Line Shapes in Solid Ordered Ortho-Hydrogen and Para-Deuterium

Some highly anomalous features have been observed in the low-temperature NMR spectrum of high-purity (> 99% $J=1\mathrm{}$), solid para-deuterium. The intensity of the low-frequency side of the Pake doublet is enhanced and that of the high-frequency side reduced, sometimes becoming negative, corresponding to an emission of power. The anomaly is particularly pronounced and of the opposite sign for the $I=2\mathrm{}$ resonance of the $J=0\mathrm{}$ impurity molecules which exhibit enhancements as large as 50. The effect, which has also been observed in high-purity ortho-hydrogen but with smaller enhancements, is ascribed to a pumping process resulting from ortho-para conversion. In the ordered phase, conversion takes place from only one of the $J=1\mathrm{}$ substates since the excited states, the librons, are not thermally populated. This results in a disturbance of the nuclear-spin populations away from thermal equilibrium, and the observed intensity depends on a balance between the ortho-para pumping mechanism and the normal spin-lattice relaxation processes. The results are compared to the theory of ortho-para conversion given in a companion paper. Measurements of the temperature dependence of the spin-lattice relaxation time $T_1$ in ordered para-${\mathrm{D}}_2$ are also presented. At high temperatures there is a fair agreement with theory based on the inelastic scattering of librons, but at lower temperatures $T_1$ is much shorter than predicted and depends strongly on the $J=0\mathrm{}$ impurity concentration. The two Pake doublets predicted by Harris for the $J=0\mathrm{}$ ${\mathrm{D}}_2$ molecules have been resolved and an accurate determination made of the widths. Finally, some measurements of the order parameter via the width of the $J=1\mathrm{}$ Pake doublet are presented and compared to current theories and previous experiments.