Electronic States in the Visible Halogen Bands

It is shown on the basis of theoretical considerations that the well-known visible iodine absorption bands and the analogous bands of the other halogens almost certainly cannot correspond to a ¹Σ←¹Σ transition as hitherto supposed. It is concluded that they belong to a ³Π₀ᵤ←¹Σg⁺ transition, with a case c ³Π₀ state whose Λ-type doubling is so wide that the state is split into two separate states which simulate a ¹Σᵤ⁺ and a ¹Σᵤ^- state. The observed bands involve however only the state which behaves like ¹Σᵤ⁺, the ¹Σᵤ^--like state and the ³Π₁ and ³Π₂ states being unknown. The classification ³Π₀ really has little meaning, one needs for case c here a classification in which only Ω values and symmetry properties (+ or -, $g or u$ are significant. According to the above interpretation, the upper level of the halogen bands is paramagnetic, this is in harmony with certain magnetic phenomena observed with these bands. Electron configurations for the normal and excited states are suggested in the case of fluorine.