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 +.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry