Nuclear Magnetic Resonance in SolidH2andD2under High Pressure

Abstract

Nuclear magnetic resonance data are presented for solid ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ between 1.5 and 4.2\ifmmode^\circ\else\textdegree\fi{}K and at pressures up to 5500 atm. For solid hydrogen, the ortho-para conversion, the transition temperature ${T}_{\ensuremath{\lambda}}$ from the state of free rotation to that of a more ordered state, and the linewidth and second moment above ${T}_{\ensuremath{\lambda}}$ have been measured as a function of density $\ensuremath{\rho}$ and ortho concentration. The ortho-para conversion rate is approximately proportional to ${\ensuremath{\rho}}^{2.4}$ and the transition temperature ${T}_{\ensuremath{\lambda}}$ for a given ortho concentration is roughly proportional to ${\ensuremath{\rho}}^{2.0}$. These results are compared with other experimental evidence and with theoretical expectations. The second moment for both ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ (the latter with an ortho concentration of 0.67) is compared to that calculated for the dipole-dipole interaction in a rigid lattice having the structure and dimensions determined from x-ray and neutron data. The experimental second moments are found to be larger than the calculated ones, and the discrepancy is discussed.