Molecular motion in solid H2 at high pressures

Solid molecular hydrogen has been studied with proton nuclear magnetic resonance in a diamond anvil cell. Pressures from 18 to 68 kbar were used, resulting in melting temperatures from 160 to 350 K and relative densities/0 as high as 3. At temperatures above 0.7Tmelt, translational self-diffusion narrows the resonance line. The pressure variation of the activation enthalpy H yields an activation volume of 5.70.6 cm3/mol or 63% of the molar volume, a reasonable value for a vacancy diffusion mechanism. The smooth variation of H/kTmelt with density also suggests that the diffusion mechanism remains the same for /0 between 1 and 3. The spin-lattice relaxation is controlled primarily by molecular reorientation. The observed density dependence T15/3 indicates that molecular electric quadrupole-quadrupole interactions cause reorientation, even at the high temperatures and densities of this work.