Extension of the Violet CN Band System to Include the CN Tail Bands

Measurements of fine-structure lines of tail bands.---Wave-numbers of band lines accurate to about 0.02 ${\mathrm{cm}}^{\ensuremath{-}1}$ are given for 13 of the CN tail bands as excited by the reaction of acetylene with active nitrogen. The table also includes estimated intensities of the lines. Each band has a doublet $P$ and a doublet $R$ branch like the stronger violet CN bands, but the doublet separations vary markedly, being undetectable in the majority of the bands. Large perturbations, which affect several lines of each branch, are also found.Assignment of tail bands to the violet CN system.---The combination principle is applied to determine the relative vibrational quantum numbers for the tail bands. The constants ${B}^{\ensuremath{'}}$ and ${B}^{\ensuremath{'}\ensuremath{'}}$ are then evaluated for each band, and by plotting these against vibration quantum number, $n$, it is found that they represent a continuation of the constants of the ordinary violet CN bands to the higher values of $n$. The tail bands are thus merely the high-$n$ members of the ordinary CN sequences, and those measured are shown to be the (11,9), (12,10), (13,11), (9,8), (10,9), (11,10), (12,11), (10,10), (11,11), (12,12), (13,13), (14,14) and (15,15) bands of the violet system. With this assignment, the tail bands give ${B}^{\ensuremath{'}\ensuremath{'}}=1.894\ensuremath{-}0.0181{n}^{\ensuremath{'}\ensuremath{'}}$ as compared to $1.891\ensuremath{-}0.0173{n}^{\ensuremath{'}\ensuremath{'}}$ obtained by Kratzer from the low-$n$ bands of the system. The band-origins are calculated, and the vibrational term differences obtained therefrom satisfy the combination principle exactly. The linear extrapolation of ${\ensuremath{\omega}}^{\ensuremath{'}\ensuremath{'}}$ checks the above assignment of vibrational quantum numbers. Both ${B}^{\ensuremath{'}}$ and ${\ensuremath{\omega}}^{\ensuremath{'}}$ show a non-linear variation with $n$. There is evidence for perturbations in the vibrational terms.New features of band-spectrum structure.---According to the above results, the violet CN system includes some bands shaded to the violet and others shaded to the red (the tail bands), with an intermediate type in which the lines have practically equal spacing on a frequency scale. Also the band-origins of a given sequence, with increasing $n$, pass through a maximum of frequency, the result being that a sequence may form a "head" composed of bands. One of the rotational perturbations is investigated, and the deviations of the terms from their expected values in this case give a typical "resonance" curve, resembling that found by Schr\"odinger in the Al II line spectrum. A diagram of the vibrational intensity distribution in the violet CN system is discussed. Certain anomalous bands of longer wave-length are found which do not fit into the system.Relation to theory of $^{2}\mathrm{S}$\ensuremath{\rightarrow}$^{2}\mathrm{S}$ bands.---In agreement with Mulliken's predictions, the first line of the $R$ branch is found to be double, that of the $P$ branch not being resolved. The intensity difference for the doublet components for low $m$ is observed, and the estimated relative intensities are in satisfactory quantitative agreement with the theory.