The excitation of chlorine in active nitrogen

Abstract

Excitation of the 258 nm emission of chlorine in active nitrogen has been studied. The intensity was shown to be proportional to [N] 2 over an order of magnitude variation of [N]. In N 2 carriers within the pressure range 0.1 – 1.5 kPa the intensity increased smoothly with increasing [Cl 2], finally reaching a limiting plateau; at a pressure of 0.83 kPa the limiting 258 nm photon yield was measured as (9.0 ± 3.0) × 10 2 [N] 2 einstein dm −3 s −1, with [N] in mol dm −3: this corresponded to only about 0.2% of N atom recombinations. The variation of intensity with total pressure indicated that the N 2 energy-donating species was populated mainly by a pressure independent process, while an [N 2] dependent process competed with Cl 2 in its removal. When the nitrogen carrier was progressively replaced with argon, the limiting 258 nm intensity divided by [N] 2 was invariant up to 70% argon but was substantially enhanced with further increase of the argon mole fraction, the enhancement factor being about 2 at 95% argon. On the basis of known kinetic behaviours in active nitrogen, none of the energetically feasible species N 2(A 3∑ + u) (υ′ ⩾ 6), N 2(B 3Π g) or N 2(B′ 3∑ − u) could have been acting as the donor species to Cl 2. Accordingly we advance a tentative mechanism for the excitation of the 258 nm emission with N 2(W 3Δ u), apparently the only remaining possibility, as the donor species. The kinetic behaviour of N 2(W 3Δ u) itself in active nitrogen is unknown since no emission from this state has been detected. Hence this study may provide the first insight into this.