Pulsed NMR studies in solid H2. III. Spin-lattice relaxation times

We report a systematic study of the longitudinal relaxation timeT ₁ for solid H₂ samples with ortho concentrationX between 0.15 and 0.70 and for 0.1?T?2 K. It is over this temperature region that substantial orientational ordering takes place, which drastically affectsT ₁. The measurements were made using the solid echo technique at 9 and 27 MHz. By Fourier-transforming the echo decay, it was possible to investigate the recovery from saturation of the various parts of the NMR absorption line as a function of their distance ν?ν _L from the center at the Larmor frequency ν _L .The dependence ofT ₁ on ν?ν _L is particularly noticeable in the hcp disordered (para-orientational) phase at the lowest temperatures, where the NMR absorption is broad, and this observation is qualitatively explained. An attempt is made to understand the general features ofT ₁ in the cubic ordered phase and in the hcp para-orientational phase in terms of certain relaxation mechanisms. A comparison with previous work is presented.