The recombination of nitrogen atoms and the nitrogen afterglow

Abstract

The recombination of nitrogen atoms has been studied photometrically in a fast flow system. The concentration of nitrogen atoms was determined by nitric oxide titration. The measured bimolecular rate constant has the form k B = A + B[M] for M = N 2 , Ar and He with a constant value of A . The surface process was proved to be second order by inducing a small first order catalytic recombination involving CN and showing by computer analysis that a first order surface process would have been measured readily in our system. Third order rate constants (expressed as d[N 2 ]/d t ) had values: k N 2 = k Ar = (1.38 ± 0.11) × 10 15 , k He = (1.92 ± 0.18) × 10 15 in cm 6 mole –2 s –1 units at 298 °K. The surface process in the 26 mm i. d. flow tube had a value of (4.4 ± 0.1) × 10 8 cm 3 mole –1 s –1 at 298 °K. In the range 196 to 327 °K, activation energies were –(975 ± 140) cal/mole for the homogeneous process and +(620 ± 50) cal/mole for the surface reaction. The intensity of the nitrogen afterglow was shown to be proportional to [N] 2 and inde­pendent of total pressure for nitrogen carrier in the range 2 to 10 mmHg. Partial replacement of the nitrogen carrier by helium or argon enhanced the nitrogen afterglow on a mole fraction basis. This effect was shown to be associated with an efficient quenching of the emitting B 3 П g state by molecular nitrogen. This view is supported by Jeunehomme & Duncan’s work on the pressure dependence of the lifetime of this state. Measurement of the absolute intensity of the afterglow when combined with their data show that about 50% of the recombination passes through the B 3 П g state. On this basis it is concluded that the A 3 Ʃ + u state and not the shallow 5 Ʃ + g state is the precursor of the afterglow. Levels of the B 3 П g state around v' = 12, 6 and 2 are populated by collision induced transition from the A state.