Thermal and Recombination Emission of NO2

Abstract

The visible emission from electronically excited NO₂ has been studied both with thermal excitation in an electric oven and with recombination excitation from the nitric oxide–atomic oxygen reaction. In both cases the emission is continuous with weaker, diffuse bands superimposed. The correlation of these bands with NO₂ absorption bands has been extended. The thermal emission continuum begins near 3300 Å and increases in intensity to the end of our range at 8600 Å. The recombination emission continuum begins near 3800 Å and has a broad maximum at 6200 Å, in general agreement with previous workers. The absolute intensity of the thermal emission has been obtained at 100‐Å intervals between 4600 and 8600 Å for temperatures between 972 and 1335°K. Excitation energies, which are obtained from the variation of intensity with temperature, range from 66 kcal at 4600 Å to a minimum of 44 ± 1 kcal near 8300 Å. This latter value is $E_0$ , the minimum energy of the emitting state of NO₂. Radiative lifetimes are obtained for thermal emission which range from 100 to 1 μsec depending on emission wavelength and which approximately agree with fluorescent lifetimes for comparable excitation energies. The results are compared with shock tube measurements and are discussed in relation to air emissivity and opacity problems.