Nuclear Magnetic Resonance Studies of SolidifiedH2-D2Mixtures. II. Pulsed Techniques

Standard NMR pulse techniques are applied to the study of the ${\mathrm{H}}_2$ resonance at 9.3 Mc/sec in solid $n{\mathrm{H}}_2-n{\mathrm{D}}_2$ mixtures at 4.2 and 1.1°K. At 4.2°K the Bloch decay for $n{\mathrm{H}}_2$ exhibits an oscillatory behavior that resembles the beat structure in Ca${\mathrm{F}}_2$ observed by Lowe and Norberg. This beat structure is not observed for $n{\mathrm{H}}_2$ concentration below 0.25. At 1.1°K the splitting of the resonance line associated with the $\lambda$ anomaly is seen as a distinct beat pattern of the Bloch decay. In addition to these Bloch decays, spin echoes that persist for times long compared to the time required for the Bloch decay to vanish are observed to follow a $90°-\tau -\beta$ pulse sequence, $\tau$ being the time between pulses and $\beta$ the rotation produced by the second pulse. The decay of the echoes as a function of $2\tau$ is very nearly exponential and the associated time constant $T_F$ depends on the rotation $\beta$ in such a way that $T_E$ increases as $\beta$ decreases. It is shown that as a result of the latter effect the echoes have maximum amplitude for $\beta =\frac{1}{2}\pi$ when $\tau$ is short but $\beta <\frac{1}{2}\pi$ when $\tau$ is long. The essential features of the Bloch decays and echoes are adequately accounted for by a single-particle model similar to the one used by Solomon to explain the multiple quadrupole echoes in KI. The Hamiltonian describing the energy-level spacing of a given ortho ${\mathrm{H}}_2$ molecule (in the rotating frame) is taken as