High-temperature limit for the O2-catalyzed ortho-para conversion rate in solid hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of molecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the temperature range 4.2-10.9 K. Conversion rate was mainly determined by the paramagnetic oxygen impurities. The catalyzed conversion rate constant increases with 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 conversion 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 coordination 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.