Excitation Studies on the N2(1+) and N2+(1—) Systems in Shock-Heated N–N2 Mixtures

Abstract

The rate of excitation of the N₂(1+) and N_2⁺(1—) band systems was observed in shock‐heated N–N₂ mixtures in which the initial mixture ratio N/N₂ ranged from 0 to 0.27. A pulsed discharge prior to diaphragm rupture produced the N atoms; their concentration was determined by monitoring the decay of the Lewis—Rayleigh afterglow for about 50 msec subsequent to the discharge and up to the arrival of the shock. The shock waves produced temperatures in the range 7000°—18 600°K close to the shock front, after rotational relaxation. The radiation intensities from the N₂(1+) (6600 to 8000 Å) and N_2⁺(1—) (0, 1) (4272 Å) bands were monitored as functions of time behind the shock front. The initial slopes of the radiation‐time histories were compared with theoretical slopes. The rate‐constant expression for exciting ground‐state nitrogen X ¹Σ_g⁺ to the A ³Σ_u⁺ state by collisions with N atoms was found to be k_N = 1.9×10⁻⁶T${}^{-\frac{3}{2}}$ exp(—E_XA/kT) cm³/particle·sec, where E_XA = 6.168 eV. This yields an effective cross section of 2.7×10⁻¹⁸ cm² at 12 000°K. The analogous rate constant for collisions with N₂ was determined to be kN₂≤0.01 k_N. These rate constants were found consistent with unsuccessful attempts to obtain Vegard—Kaplan spectra in the discharged but unshocked gas. Attempts to fit the N_2⁺(1—) data were not completely successful. It is most probable that the N_2⁺ is formed by the associative ionization of a ground‐state atom and an excited atom.