Forbidden Band Systems in Nitrogen. III. The Y 3Σu—←X 1Σg+ System in Absorption

Four bands of the Y 3Σu—←X 1Σg+ system of nitrogen have been observed in absorption in a path of 3.4 meter-atmospheres at 1518 A (0–0), 1484 A (1–0), 1453 A (2–0) and 1423 A (3–0). These bands consist of an unresolved Q branch (QQ, QP, QR), and SR and 0P branches. From a comparison of the experimental and calculated intensity distribution, it is shown that the upper state is 3Σu— rather than the alternative assignment of 3Δu. It is further shown that the Y 3Σu— state is identical with the upper state of certain infrared bands recently found in the active nitrogen afterglow. Those bands are thereby identified as Y 3Σu—→B 3Πg. The upper state was designated ``Y'' by Kistiakowsky and Warneck. The constants of Y 3Σu— as obtained here are: ωe=1517.69 cm—1; ωexe=12.22 cm—1; Be=1.472 cm—1; αe=0.0161 cm—1; T00=65850.6 cm—1; re=1.2788 A. The transition Y 3Σu—←X 1Σg+ receives intensity from perturbations of Y 3Σu— by 1Σu+ and 1Πu states lying at higher energies.