Time-resolved kinetic studies of the N₂⁺ + N₂ association reaction

In this work, we apply a technique in which N₂⁺ ions are formed by resonance enhanced multiphoton ionisation (REMPI) and monitored by laser-induced fluorescence (LIF) to measure reaction kinetics. Using this approach, we are able to measure pressure-dependent rate constants for the N₂⁺ + N₂ + M ? N₄⁺ + M association reaction over the pressure ranges 1-13 Torr N₂ and 10-950 Torr He. In these ranges we find evidence for fall-off behaviour in the kinetics. Constraining the high pressure limiting rate constant (k8) to the ion-induced dipole capture rate constant for N₂⁺ + N₂ collisions (8.3 × 10^-10 cm³ s^-1), we extract low pressure limiting rate constants (k°) and fall-off broadening parameters(F_cent). For M = He, we find k° = 2.02(±0.20) × 10^-29 cm⁶ s^-1 and F_cent = 0.465(±0.024) and for M = N₂ we find k° = 5.83(±0.40) × 10^-29 cm⁶ s^-1 and F_cent = 0.517(±0.117). Both the low pressure limiting rate constants and the observed fall-off behaviour are analysed using statistical treatments for the reaction based upon the formation of a linear N₄⁴ energised adduct. These treatments are found to interpret our data very satisfactorily.