Anomalous ortho-para conversion of solid hydrogen in constrained geometries

Abstract

Using cw NMR techniques, we have measured the ortho-para conversion of solid hydrogen constrained to the interior of the molecular cages of zeolite. The conversion observed in the constrained geometry is very different from that of bulk solid hydrogen. Two distinct conversion rates were observed for short and long times. An apparently bimolecular conversion rate of 0.43% ${\mathrm{h}}^{\mathrm{-}1}$ (one-fourth of the bulk value) dominates during the first 500 h, and the rate then increases to 2.2% ${\mathrm{h}}^{\mathrm{-}1}$. The initial slow rate is explained in terms of a reduced number of nearest neighbors and possible wall effects, and the fast rate is attributed to the formation of small ortho-${\mathrm{H}}_2$ Rclusters at later times. Surface effects due to magnetic impurities do not appear to determine the conversion rate in the samples studied.