Dissociative recombination of highly enriched para- H ₃ ⁺

The determination of the dissociative recombination rate coefficient of H ₃ ⁺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- H3+ and para- H ₃ ⁺. A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H3+ in the diffuse interstellar medium, where H ₃ ⁺ 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 ₃ ⁺ using a supersonic expansion source that produced ions with Trot ∼60-100 K. We observed an increase in the low energy recombination rate coefficient of the enriched para- H ₃ ⁺ by a factor of ∼1.25 in comparison to H ₃ ⁺ produced from normal H ₂ (ortho:para=3:1). The ratio of the rate coefficients of pure para- H ₃ ⁺+ to that of pure ortho- H ₃ ⁺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 ₃ ⁺.