Forbidden Band Systems in Nitrogen. I. The Vegard-Kaplan System in Absorption

Two bands belonging to the Vegard‐Kaplan system ${\mathrm{(A}}^3{\Sigma }_u^{+}{\mathrm{-X}}^1{\Sigma }_g^{+})$ of nitrogen have been found in absorption in paths of 9‐ 13 meter‐atmospheres. These bands at 1689 A (7–0) and 1726 (6–0) are incompletely resolved but show intensity alternation and distribution in agreement with theory. The positions of the band origins and each rotational line agree closely with those calculated from the known molecular constants. Two other forbidden systems have also been found in absorption. These are the $a^{\text{′1}}{\Sigma }_u^{-}{\mathrm{-X}}^1{\Sigma }_g^{+}$ (four bands at 1444 A, 1414 A, 1360 A, and 1331 A) and probably the ${}^3{\Delta }_u{\mathrm{-X}}^1{\Sigma }_g^{+}$ (one band at 1484 A); they are about three times stronger than the observed Vegard‐Kaplan bands The transition probabilities of the known forbidden systems in nitrogen are discussed briefly.