The Red CN Band System

Rotational Structure of red CN bands. The rotational structure of the (4,1), (5,2), (6,1), (6,2), (7,2), (7,3) and (8,3) bands of this system is measured on plates from the first order of a 21-foot grating. To excite the spectrum, carbon tetrachloride vapor was mixed with active nitrogen. The lines of each band are arranged in eight branches, giving a structure characteristic of a ${}_{}{}^2{}_{}{}^{}\Pi \to {}_{}{}^2{}_{}{}^{}\Sigma$ transition in which the spin doubling in ${}_{}{}^2{}_{}{}^{}\Sigma$ is small. Missing lines show the ${}_{}{}^2{}_{}{}^{}\Pi$ state to be inverted. Good agreement is obtained for the constants of the lower state with those of the violet CN system. The rotational constants of the ${}_{}{}^2{}_{}{}^{}\Pi$ state are: $A=-52.2\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B_v=1.6990-0.01746\mathrm{} (v+\frac{1}{2})$ ${\mathrm{cm}}^{-1\mathrm{}}$, $D_v=-[6.133+0.0127\mathrm{} (v+\frac{1}{2}\left)\right]\times {10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $I_e=16.28\mathrm{}\times {10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, $r_e=1.236\mathrm{}\times {10}^{-8\mathrm{}}$ cm. The ${}_{}{}^2{}_{}{}^{}\Pi$ terms are represented accurately by the Hill and Van Vleck formula. Microphotometer measurements of photographic densities in the 8,3 band are given, and agree qualitatively with the theory. They show the ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{3}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}\Sigma$ band to be stronger than the ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}\Sigma$ band.