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.

Spin doubling and $\Lambda$-doubling. A rough evaluation of the difference ${F_1}^{\prime \prime }\mathrm{}\left(K\right)-{F_2}^{\prime \prime }\mathrm{}\left(K\right)={\gamma }_0(K+\frac{1}{2})$ is possible, and gives ${\gamma }_0=+0.0071\mathrm{} \mathrm{and} 0.0082$ and for $v^{\prime \prime }=3\mathrm{} \mathrm{and} 2$, respectively. For the states $v^{\prime }=4, 5 \mathrm{and} 8$, the $\Lambda$-doubling is regular, and of the form recently discussed by Mulliken and Christy. We find $p_0=0.00621, 0.00853, 0.01872$, $q_0=-0.000252, -0.000365\mathrm{} \mathrm{and} -0.000435\mathrm{}$ for $v^{\prime }=4, 5 \mathrm{and} 8$.

Perturbations. Three strong perturbations are found in $v^{\prime }=6\mathrm{}$, in $T_{1c}$ at $J=13\mathrm{}\frac{1}{2}$, in $T_{1d}$ at $J=25\mathrm{}\frac{1}{2}$ and in $T_{2d}$ at $J=27\mathrm{}\frac{1}{2}$. These are exactly as expected theoretically from the three corresponding perturbations found in the violet system by Rosenthal and Jenkins, and from an extrapolation of the unperturbed levels. An anomalous $\Lambda$-doubling in ${{}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}}^{\mathrm{}\left(7\right)\mathrm{}}$ is explained as a perturbation, with the difference that here the perturbing levels of ${{}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}}^{\mathrm{}\left(7\right)\mathrm{}}$ and $a^2{\Sigma }^{\mathrm{}\left(12\right)\mathrm{}}$ do not cross, but merely approach closely.

Vibrational structure. From measurements of the $R_2$ heads of 17 other bands, and correction to origins, the following expression is obtained for band origins: $\nu =11,043.20+1788.66\mathrm{} (v^{\prime }+\frac{1}{2})-12.883\mathrm{} {(v^{\prime }+\frac{1}{2})}^2-2068.79\mathrm{} (v^{\prime \prime }+\frac{1}{2})+13.176\mathrm{} {(v^{\prime \prime }+\frac{1}{2})}^2.$ Perturbations of the band origins by 1 or 2 ${\mathrm{cm}}^{-1\mathrm{}}$ occur for $v^{\prime }=6\mathrm{} \mathrm{and} 7$, and are evidently of similar origin to the rotational perturbations in these states. The vibrational numbering used here for the ${}_{}{}^2{}_{}{}^{}\Pi$ state is that of Asundi and Ryde. This numbering, hitherto in doubt, has now been definitely established by a comparison of the observed vibrational intensity relations with those predicted by the wave mechanics with the method of Condon.