Temperatures Indicated by Intensity Distributions in Band Spectra

It is generally supposed that the intensity of any particular line in a band spectrum may be calculated from the Maxwell-Boltzmann law, assuming the gas to be in temperature equilibrium. From the results of this experimental work, it appears that under some conditions of excitation other factors than the temperature of the gas govern the intensity distribution. Measurements on the negative bands of ${\mathrm{N}}_2^{+}$ excited in low voltage arcs in the pure gas show that at 1100°K and 2800°K the distribution of intensities among the rotational lines is exactly the same, whilst the admixture of helium with nitrogen alters the distribution completely and gives a much lower indicated temperature. Similar results are found with the first negative bands of C${\mathrm{O}}^{+}$. The differences are accounted for by the conversion of translational energy of the electrons into rotational energy of the molecules when the bands are excited by electron impacts in the pure gas. When the bands are excited by impacts of the second kind in gas mixtures, no change in rotational energy occurs and thus the indicated temperature is much lower. The distribution of intensity among the vibrational states of the molecules has been examined for the complete system of negative bands of ${\mathrm{N}}_2^{+}$. While almost all of the energy is concentrated in the low vibration states when the bands are excited by electron impacts, the energy is spread out towards the higher vibration states when the excitation is by collisions of the second kind with metastable helium atoms.