Binary and ternary recombination of para-H₃⁺H3+ and ortho-H₃⁺H3+ with electrons: State selective study at 77–200 K

Measurements in [12pt]{minimal}{document}H₃⁺{document}H3+ afterglow plasmas with spectroscopically determined relative abundances of [12pt]{minimal}{document}H₃⁺{document}H3+ ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77–200 K) para-[12pt]{minimal}{document}H₃⁺{document}H3+ ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that [12pt]{minimal}{document}H₃⁺{document}H3+ recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution.