Ortho-para conversion in solid hydrogen, catalyzed by molecular oxygen impurities

Ortho-para conversion in solid hydrogen with small amounts of molecular oxygen as impurity (0.01-0.5%) was studied by NMR in the temperature range 4.2-7 K. The observed conversion rate was substantially greater than that of the natural conversion process in pure solid H-2 and revealed a strong dependence on temperature and the oxygen content in the sample. Obviously, the conversion is catalyzed by the large paramagnetic moments of O-2 impurities. Since the influence of O-2 is of very short range, the overall rate of the catalyzed conversion in the sample is determined by the relative diffusion of ortho-H-2 and the O-2 impurities. The diffusion coefficient was found to obey and Arrhenius behaviour D(T) = 3(2). 10(-11). exp[96(8)T] cm(2)s(-1). This temperature dependence is interpreted in terms of vacancy-particle exchange by quantum tunneling through the intervening barrier.