Rotation Diffusion and Ortho-Para Conversion
In all solids near the melting temperature diffusion occurs due to thermally activated interchanges of neighboring particles. In ionic crystals, for example, this diffusion is responsible for the electrical conductivity of these crystals at high temperatures. In solid H2 and D2 the diffusion of J = 1 impurities just below the melting temperature has been observed through its effect on the NMR spectra.1, 2 This molecular motion decreases exponentially with decreasing temperature and is essentially frozen out a few degress below the melting point. The diffusion of J = 1 impurities in solid para-H2 has, however, been observed to persist down to temperatures on the order of 1 K.3 Since this low-temperature diffusion must be due to nonthermal effects of a quantum nature, it has been called quantum diffusion. In contrast to the hopping processes responsible for the diffusion, the diffusion itself is entirely classical, and we therefore prefer to use the term rotation diffusion which also agrees more closely with the fact that, in this process, the molecules do not interchange positions. The mechanism responsible for the diffusion is the conversion of ortho-para into para-ortho pairs and is due to the magnetic dipole-dipole interaction between the nuclear spins in neighboring molecules.4 This same interaction also gives rise to ordinary ortho-para conversion which changes the over-all ortho-para concentration ratio.
KeywordsRotation Diffusion Jump Frequency Solid Hydrogen Quantum Diffusion Energy Mismatch
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