NMR studies on single crystals of H₂. III. Dynamic effects

The time-dependent effects in the NMR spectrum of ortho-H₂ impurities in two single crystals of H₂ are investigated over the temperature range between 0.02 and 3 K and for an ortho-H₂ mole fraction X between 0.1% and 2%. The disappearance of the signal from isolated o-H₂ impurities, after cooling the crystals to a given temperature, and the simultaneous increase of the signal from isolated o-H₂ pairs indicates a clustering process. The characteristic times from both spectra are comparable and pass over a flat maximum near T = 0.3 K. The clustering process then accelerates drastically as T is decreased below 0.1 K. The characteristic times are only weakly dependent on the concentration X. Furthermore, the subsequent decay of the o-H₂ pair signals with time suggests a slow diffusion of pairs and formation of larger clusters. It is found that the period associated with the decay of in-plane pairs (having their axis parallel to the basal plane) is considerably shorter than that for the out-of-plane pairs. The results are discussed in the light of the theory of hopping impurities (resonant ortho-para conversion) and possible coherent tunneling effects at low temperatures. Finally we briefly report some studies of the longitudinal relaxation time for the o-H₂ pair NMR lines.