Forbidden Band Systems in Nitrogen. II. The a′1Σu−←X1Σg+ System in Absorption

Four bands of the ${a^{\prime }}^1{\Sigma }_u^{-}{\mathrm{\leftarrow X}}^1{\Sigma }_g^{+}$ system in nitrogen have been observed in absorption at high resolution in a path of 3.4 meteratmospheres, at 1444 A (1–0), 1414 A (2–0), 1358 A (4–0), and 1331 A (5–0). They consist of Q branches only (ΔJ=0). The observation of these bands, highly forbidden by the usual selection rules, is explainable by perturbations with the nearest ¹II_u states lying between about 98 000 cm^—1 and 109 000 cm^—1 above the ground state. The rotational analysis of each of the bands resulted in rotational constants and vibrational intervals for a′¹Σ_u^— which agree very well with those obtained from the emission system $x^1{\Sigma }_g^{-}{\mathrm{\to a}}^{\text{′1}}{\Sigma }_u^{-}$ . The electronic energy (T₀₀) of state a′¹Σ_u^— is now precisely located at 67738.18 cm^—1 (8.398 ev); this value combined with other data gives also the following electronic energies (T₀₀): w¹Δ_u, 71697.68 cm^—1 (8.889 ev); x¹Σ_g^—, 113210.98 cm^—1 (14.036 ev); and y¹II_g, 114165.18 cm^—1 (14.154 ev). T₀₀ of a′¹Σ_u^— is close to that of a ¹II_g: 68951.21 cm^—1 (8.548 ev). The new energy for y¹II_g is 0.339 ev lower than the minimum value deemed possible by Lofthus and Mulliken on the basis of an observed predissociation at v=0, J=10 of state y. That predissociation is now reinterpreted as most probably an accidental one resulting in the formation of ²P+⁴S atoms instead of ²D+²D atoms as formerly assumed. The intensity distribution in the ${a^{\prime }}^1{\Sigma }_u^{-}{\mathrm{\leftarrow X}}^1{\Sigma }_g^{+}$ system is found to be approximately proportional to J(J+1)(2J+1) exp—[B₀″J(J+1)hc/kT], where B₀″=1.9898 cm^—1 and T=300°K, in agreement with theory. From appearance pressures of the five forbidden systems of nitrogen now known in absorption, it is possible to estimate the transition probabilities. From these, together with a recent molecular beam determination of the mean lifetime of the state a¹II_g, it is possible to estimate the lifetimes of the metastable states: A³Σ_u⁺, 2.6×10^—2 sec, a′¹Σ_u^—, 4.0×10^—2 sec. By comparison of the transition probability of the atmospheric oxygen bands ${\mathrm{(b}}^1{\Sigma }_g^{+}{\mathrm{\leftarrow X}}^3{\Sigma }_u^{-})$ with that of the Vegard‐Kaplan bands of nitrogen ${\mathrm{(A}}^3{\Sigma }_u^{+}{\mathrm{\leftarrow X}}^1{\Sigma }_g^{+})$ , together with the known lifetime of b¹Σ_g⁺, the value 6.5×10^—3 sec is found for the lifetime of A³Σ_u⁺, in fair agreement with the value given above.