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

Abstract

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 H2 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 O2 impurities. Since the influence of O2 is of very short range, the overall rate of the catalyzed conversion in the sample is determined by the relative diffusion of ortho-H2 and the O2 impurities. The diffusion coefficient was found to obey an Arrhenius behavior D(T)=3(2). 10−11.exp[96(8)/T] cm2s−1. This temperature dependence is interpreted in terms of vacancy-particle exchange by quantum tunneling through the intervening barrier.