Dissociative recombination of highly enriched para-H

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The determination of the dissociative recombination rate coefficient of H has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho-H and para-H. A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H in the diffuse interstellar medium, where H has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched (~83.6%) para-H using a supersonic expansion source that produced ions with T_rot~60–100  K. We observed an increase in the low energy recombination rate coefficient of the enriched para-H by a factor of ~1.25 in comparison to H produced from normal H₂ (ortho:para=3:1). The ratio of the rate coefficients of pure para-H to that of pure ortho-H is inferred to be ~2 at low collision energies; the corresponding ratio of the thermal rate coefficients is ~1.5 at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of H. ©2009 American Institute of Physics