Infrared (2 to 8 μm) fluorescence of the W 3Δu→B 3Πg and w 1Δu→a 1Πg systems of nitrogen

Eleven transitions in the W 3Δu→B 3Πg (W–B) and w 1Δu→a1Πg (w–a) systems of nitrogen have been observed in the infrared including the previously unobserved (1,0) and (2,1) W–B features at 6.5 and 7.65 μm, respectively. The fluorescence spectra were observed in a cryogenic reaction chamber at pressures of ∼3 mTorr (0.4 Pa), following expansion of flowing N2/Ar mixtures excited by microwave discharges at ∼1 Torr. Einstein coefficients for the w–a system, calculated using a published transition moment function, predict the radiative lifetimes of the lower vibrational levels of the w 1Δu state to be a factor of 3 longer than earlier estimates. Using a spectral simulation and linear least-squares fitting technique, the published W–B and calculated w–a branching ratios are verified for the transitions observed across the 2 to 4 μm region. The observed vibrational/electronic state distributions are not characteristic of those expected for direct excitation, but appear to result from extensive collisional coupling among excited states of nitrogen which occurs in the high pressure region prior to expansion.