[en] Comparison of bands belonging to the violet and red systems of CN showed that in emission the (1, 0) band λ 7873 (red system) is stronger than the (0, 0) band λ 3883 (violet system) at temperatures of the electric furnace below 2300° C. Approximate equality of the main structure of the two bands is attained at 2300° C; λ 3883 becomes somewhat stronger than λ 7873 at 2600° C. At the high temperature, approximately 7300° C, of the carbon arc in air, the λ 3883 band is about two hundred times stronger than the λ 7873 band. Self-reversal appears easily in the violet band, but not in the red. The two systems react differently to a change in pressure, the red system increasing in intensity more rapidly than the violet when the pressure increases.
In absorption, the (0, 0) and (1, 0) bands of the red system are much weaker than the violet bands at any temperature of the furnace. A list of the stronger absorption lines of the λ 7873 band is given.
The relative intensities of the two systems in emission in the electric furnace at different temperatures and in the arc are explained by the Boltzmann populations of the upper levels in thermodynamic equilibrium. These populations differ strongly because the upper electronic level, A2Π, of the red system is much lower (e.p., 1.35 v.) than the upper level, B2∑, of the violet system (3.2 v.). From the equal intensity of the strongest lines of the two systems in emission at T = 2300° C, an approximate value of 1400 is found for the ratio of the emission transition probabilities of the strongest violet lines to those of the strongest red lines. The corresponding estimated value of the ratio of the absorption transition probabilities is 87.5, explaining the weakness of the red system in absorption in the laboratory.
These considerations show that the red bands of CN should not be expected in fluorescence in comets and that previous identifications must be revised accordingly. No line of the red system of CN will be found in interstellar absorption. The intensity of the red bands in absorption in certain carbon stars in which the violet bands are weak indicates that the atmospheres of these stars have much less continuous absorption in the red than in the violet.
