High-temperature limit for the O-2-catalyzed ortho-para conversion rate insolid hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of m olecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the tempera ture range 4.2-10.9 K. Conversion rate was mainly determined by the paramag netic oxygen impurities. The catalyzed conversion rate constant increases w ith temperature in the range 4.2-8 K and becomes independent of temperature above 8 K. The high-temperature limit is 1.15(8) s(-1). The catalyzed conv ersion rate was shown to become temperature independent when the ortho-H-2 diffusion time tau d falls below the conversion time tau(cc) in the first c oordination sphere of O-2 impurity. The obtained value of tau(cc) = 10.5(8) s was used to identify the position of O-2 in H-2 lattice as substitutional . The temperature dependence of the ortho-H-2 diffusion rate near the high temperature limit was determined.