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

✍ Scribed by S. Washburn; R. Schweizer; H. Meyer

• Book ID: 104626568
• Publisher: Springer US
• Year: 1980
• Tongue: English
• Weight: 885 KB
• Volume: 40
• Category: Article
• ISSN: 0022-2291
• DOI: 10.1007/bf00115990

✦ Synopsis

The time-dependent effects in the NMR spectrum of ortho-Hz impurities in two single crystals of Hz are investigated over the temperature range between 0.02 and 3 K and for an ortho-Hz 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 resui'ts are discussed in the light of the theory of hopping impurities (resonant ortho-para conversion) and possible coherent tunneling effects at low temperamres. Finally we briefly report some studies of the longitudinal relaxation time for the o-H2 pair NMR lines.