Ortho-para conversion of hydrogen at high pressures

Ortho-para conversion rates in solid H-2 measured as a function of pressure up to 58 GPa are examined theoretically. Analyses of the data provide information on the relative role of diffusion versus intrinsic dependences of the conversion rate on ortho concentration. A theory of the conversion has been developed using a closed-form representation of the conversion promoting nuclear magnetic interaction H-ss expanded in spherical harmonics. The mechanisms considered include double conversion, excitations in the J=1 and J=2 manifolds as conversion energy sinks, and a possibility of intermediate states from which the conversion energy is dissipated via the strong electrical quadrupole-quadrupole (EQQ) interaction. Conversion rates were evaluated for a total of 12 new channels; the two other channels considered previously for moderate pressures have been reconsidered to account for factors that influence phonon-assisted energy dissipation, the most important being the compression-related decrease of the conversion energy (gap closing). Contributions from the standard one-phonon channels with single and double conversion yield fairly good agreement with low-pressure data. The proposed new channel identified as responsible for the observed conversion acceleration is the one in which the conversion Hamiltonian H-ss only initiates conversion driving the system to a temporarily nonequilibrium state from which the conversion energy is dissipated via EQQ coupling into excitations within the J=1 manifold. Our mechanism predicts a strong and abrupt conversion slowdown at still higher compressions. The abrupt decrease in rate observed at a given pressure at longer times (decreasing ortho fractions) can be explained as due to the inability of slow diffusion to restore the random distribution of ortho species and due to the intrinsic inefficiency of the new channel at low c.