Time-resolved kinetic studies of the N-2(+)+N-2 association reaction

In this work, we apply a technique in which N-2(+) ions are formed by reson ance enhanced multiphoton ionisation (REMPI) and monitored by laser-induced fluorescence (LIF) to measure reaction kinetics. Using this approach, we a re able to measure pressure-dependent rate constants for the N-2(+) + N-2 M --> N-4(+) +M association reaction over the pressure ranges 1-13 Torr N- 2 and 10-950 Torr He. In these ranges we find evidence for fall-off behavio ur in the kinetics. Constraining the high pressure limiting rate constant ( k(infinity)) to the ion-induced dipole capture rate constant for N-2(+) + N -2 collisions (8.3 x 10(-10) cm(3) 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) x 10(-29) cm(6) s(-1) and F-cent = 0.465( +/-0.024) and for M = N-2 we find k(o) = 5.83(+/-0.40) x 10(-29) cm(6) s(-1 ) and F-cent = 0.517(+/-0.117). Both the low pressure limiting rate constan ts and the observed fall-off behaviour are analysed using statistical treat ments for the reaction based upon the formation of a linear N-4(+) energise d adduct. These treatments are found to interpret our data very satisfactor ily.