Quantum diffusion and tunneling in the solid hydrogens: a short review (Review Article)

Abstract

The first observations 30 years ago of molecular clustering via quantum diffusion in solid H₂ in nuclear magnetic resonance (NMR) and pressure experiments are described. A previous review on quantum diffusion and tunneling up to 1986 is updated to reflect progress in the last ten years. In particular I review the effect of clustering on heat capacity in solid D₂ and on the thermal conductivity in solid H₂. The configurational relaxation time observed in D₂ at 1.75K is found to scale with that in H₂ at the same temperature in terms of the predictions of ortho-para resonant conversion-induced hopping. The relaxation times measured simultaneously on the same sample in NMR and conductivity experiments reflect different configurational processes. The theory of molecular hopping and pairing in solid H₂ is outlined and compared with experiments. The tunneling frequency of HD in solid H₂ deduced from transverse and longitudinal NMR relaxation experiments between 0.03 and 14K is discussed. The hopping of ortho-H₂, invoked as a mechanism in the enhanced ortho-para conversion in presence of O₂ impurities, is mentioned. Finally, recent tunneling results for H, D and H₂- anions in solid H₂ during the studies of low temperature reaction dynamics are also briefly described. In the Appendix, the hopping frequency determination in HD from NMR transverse relaxation time measurements is reviewed.