Low Temperature Physics: 45, 676 (2019); https://doi.org/10.1063/1.5103261
Fizika Nizkikh Temperatur: Volume 45, Number 6 (June 2019), p. 789-803    ( to contents , go back )

Hydrogen atom catalyzed ortho-to-para conversion in solid molecular hydrogen

A.I. Strom, K.L. Fillmore, and D. T. Anderson

Department of Chemistry, University of Wyoming, Laramie, WY 82071–3838, USA
E-mail: danderso@uwyo.edu
pos Анотація:

Received April 9, 2019, published online April 26, 2019

Abstract

Infrared spectroscopy is used to investigate the process of molecular hydrogen ortho-to-para (o/p) conversion in solid hydrogen samples doped with small concentrations (10–50 ppm) of hydrogen atoms (H-atoms) as an impurity. The H-atoms are generated using the in situ 193 nm photolysis of N2O dopant molecules. For hydrogen crystals with relatively low initial ortho-H2 fractions (Xo ≤ 0.03), the o/p conversion kinetics at temperatures of 1.8 and 4.0 K follow kinetic equations developed previously for H-atom catalyzed o/p conversion. The measured H-atom catalyzed o/p conversion kinetics indicates the H-atoms are mobile under these conditions in agreement with previous ESR measurements. It has been proposed that the H-atoms diffuse by a quantum tunneling mechanism that is described as chemical diffusion. Detailed fits of the measured o/p conversion kinetic data allow the initial H-atom concentration after photolysis to be extracted assuming literature values for the H-atom recombination rate constant (H + H → H2). The measured o/p conversion kinetics show the observed o/p conversion is much less than expected based on the previously measured H-atom recombination rate constant and thus suggest that the H-atoms do not diffuse randomly through the crystal but rather diffuse preferentially in regions of high para-hydrogen content. The estimated H-atom concentrations from this study are consistent with previous ESR measurements but in conflict with kinetic studies of H-atom reactions with various dopants such as N2O.

Key words: solid hydrogen, quantum solid, ortho-para conversion, quantum diffusion, nuclear spin conversion, quantum mechanical tunneling.

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