Natural ortho–para conversion in solid H2 and D2

The rate of ortho–para conversion in solid H2 and D2 is determined by intersection of neighboring molecules and the nature of the crystal vibrational spectrum. Natural orth—para conversion was studied in solid H2 and D2 for o—H2 concentrations between 65 and 75% at 4.2 and 13.7°K and for 20-90% of p-D2 at 4.2 °K. The conversion constant k in the equation dc/dt = −kc2 has been found to be k = 17×10−3h−1 in H2 at 4.2 °K. Within the range 4–14°K, the constant was found to be temperature independent which indicates that thermal phonons provide negligible contribution to the o–p conversion process at these temperatures. The concentration dependence of the conversion rate in D2 is close to linear, i.e., dc/dt = k' c with k' = 5.3×10−4h−1. The discrepancy between theory and experiment observed for o-p conversion in solid D2 (the calculated k is 1.8×10−3h−1) is attributed to the Debye phonon spectrum employed in the calculation. The agreement for H2 (k = 19×10−3h−1) is considered as accidental.

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