Ortho- to Para-Hydrogen Conversion and Molecular Diffusion in the High Pressure Compound Ar(H2)2

Ortho-para hydrogen conversion kinetics has been measured in the stoichiometric Ar(H2)2 compound, at pressures from 3 to 35 GPa, from the intensity of the rotational Raman transitions. The conversion rate increases with pressure by almost a factor 50 in the investigated pressure range. A comparison is made with pure hydrogen, where a similar increase is also observed. For long times and low ortho concentration, the conversion kinetic deviates from the law derived for uniform distribution of the ortho-hydrogen molecules in the crystal. The magnitude of this effect, due to the limited diffusion rate of the hydrogen molecules in the crystal, allows the measurement of the diffusion coefficient.