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

: Shevtsov V, Malmi P, Ylinen E, Punkkinen M

Publisher: ELSEVIER SCIENCE BV

: 2000

Physica B: Condensed Matter

PHYSICA B

: PHYSICA B

: 284

: 385

: 386

: 2

: 0921-4526

DOI: https://doi.org/10.1016/S0921-4526(99)02777-5

The ortho-para conversion in solid H-2 doped with less than 0.1% of O-2 impurities is studied by pulsed NMR between 1.5 and 10.9 K. The rate of O-2-catalyzed conversion (CC) reveals remarkable temperature dependence with three distinct regions. For T > 8 K the CC rate is almost temperature independent 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 diffusion (proceeding, most probably, via tunneling to nearby vacancy positions) determines the CC rate, resulting in an activation-type variation with temperature. However, at T < 4 K the temperature dependence of the rate becomes much weaker again, and that may be ascribed to another mechanism of ortho-H-2 diffusion. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Physical sciences