Ternary Association Reaction of Vibrationally Cold N₂⁺ Ions in Ambient Helium

The rate coefficients for the ternary association reaction of vibrationally cold N₂⁺ ions with N₂ and He were measured in an afterglow plasma in the temperature range of 140-250 K: k_N = (1.02 ± 0.39)(300/T)^(1.28±0.08) × 10^-29 cm⁶ s^-1. The rotational and vibrational state populations of N₂⁺ ions were probed in situ by time-resolved cavity ring-down spectroscopy. The rate coefficient for Penning ionization of N₂ by helium metastable atoms was also determined: k_m = [(2.70 ± 0.17) + (84.3 ± 5.9)e^-(877±36)/] × 10^-11 cm³ s^-1.