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

Abstract

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 (k^∞) 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^o) and fall-off broadening parameters (F_cent). For M = He, we find k^o = 2.02(±0.20) × 10⁻²⁹ cm⁶ s⁻¹ and F_cent = 0.465(±0.024) and for M = N₂ we find k^o = 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 N₄⁺ energised adduct. These treatments are found to interpret our data very satisfactorily.