Theory of ortho-para conversion in hydrogen adsorbed on metal and paramagnetic surfaces at low temperatures.

Abstract

In order to explain the experimental results on Cu(100), Ag(111), Ag thin films, graphite, and ${\mathrm{H}}_{2}$ bubbles in Cu, the ortho-para conversion rates of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ adsorbed on metal and paramagnetic surfaces at low temperatures have been considered. The conversion rates due to magnetic dipole-dipole, Fermi contact, and spin-orbit interaction between the conduction electrons, and nuclear spins of ${\mathrm{H}}_{2}$ (${\mathrm{D}}_{2}$) are calculated to elucidate the role of the metal surface. Although the rates on clean metal surfaces are found to be too slow to account for the observed rates on Ag, they may explain the catalytic conversion on ${\mathrm{H}}_{2}$ bubble surfaces at 1.3 K. Additionally, effects of impurities and defects on the surface are investigated by calculating the conversion rate in two-dimensional solid ${\mathrm{D}}_{2}$ (${\mathrm{H}}_{2}$) by emission of one (two) phonon(s). Fast conversion rates observed on Ag and graphite surfaces as well as on the surfaces of ${\mathrm{H}}_{2}$ bubbles may be accounted for by paramagnetic impurities or defects. On Grafoil, both in (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} commensurate and incommensurate solid phase, a temperature-independent conversion rate is predicted if the mobility of the molecules is high enough to prevent concentration gradients.