NH (X3Σ-, v=1-3) formation and vibrational relaxation in electron-irradiated Ar/N2/H2 mixtures

Measurements of the dynamics of NH(X3∑−, v =1–3), created in electron-irradiated N2/H2 and Ar/N2/H2 mixtures, have been performed. Time-resolved Fourier spectroscopy was used to observe NH(v→v–1) vibrational fundamental band emission. Time-dependent populations were then determined by spectral fitting. Subsequent kinetic fitting of these populations using a single-quantum relaxation model and a power-law dependence of kv on v yielded the following NH(v =1–3) relaxation rate constants (units of 10−14 cm3 s−1): kv=1(N2)=1.2±0.5, kv=2(N2)=3.8±1.5, kv=3(N2)=7.5±2.5; kv=1(Ar)=0.2±0.1, kv=2(Ar)=0.5±0.2, kv=3(Ar)=0.8±0.3; kv=1(H2)≤50, kv=2(H2)≤100, kv=3(H2)≤150. In addition, the N2/H2 data provided a measurement of the nascent excited vibrational state distribution resulting from the reaction N(2D)+H2→NH(X,v)+H. The ratio NH(1):NH(2):NH(3) was found to be 1.0:0.97:0.81 (±0.28 in each value). Comparison of the observed nascent distribution with that of a statistical model suggests that the ratio NH(0):NH(1)=0.47. Using this derived distribution, we find the average product level 〈v〉 =1.6, and the fraction of the available product energy in vibration 〈fv〉 =0.44. The present evidence confirms that a single reaction mechanism dominates NH formation, and suggests that the reaction proceeds by direct H atom abstraction rather than the formation of a long-lived H–N–H intermediate.