Ortho-para conversion in solid hydrogen catalyzed by O-2 impurities

The ortho-para conversion in solid H-2 doped with less than 0.1% of O-2 imp urities is studied by pulsed NMR between 1.5 and 10.9 K. The rate of O-2-ca talyzed conversion (CC) reveals remarkable temperature dependence with thre e distinct regions. For T > 8 K the CC rate is almost temperature independe nt and is limited by the conversion time tau(cc) = 10.5 s of the ortho-H-2 neighbors nearest to an O-2 impurity. Within 4.6 < T < 7 K the ortho-Hz dif fusion (proceeding, most probably, via tunneling to nearby vacancy position s) determines the CC rate, resulting in an activation-type variation with t emperature. However, at T < 4 K the temperature dependence of the rate beco mes much weaker again, and that may be ascribed to another mechanism of ort ho-H-2 diffusion. (C) 2000 Published by Elsevier Science B.V. All rights re served.