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 ${}_{}{}^1{}_{}{}^{}\Sigma \leftarrow {}_{}{}^1{}_{}{}^{}\Sigma$ transition as hitherto supposed. It is concluded that they belong to a ${}_{}{}^3{}_{}{}^{}\Pi _{0u}^{}{}_{}{}^{}\leftarrow {}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ transition, with a case $c {}_{}{}^3{}_{}{}^{}\Pi _0^{}{}_{}{}^{}$ state whose $\Lambda$-type doubling is so wide that the state is split into two separate states which simulate a ${}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ and a ${}_{}{}^1{}_{}{}^{}\Sigma _u^{-}{}_{}{}^{}$ state. The observed bands involve however only the state which behaves like ${}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, the ${}_{}{}^1{}_{}{}^{}\Sigma _u^{-}{}_{}{}^{}$-like state and the ${}_{}{}^3{}_{}{}^{}\Pi _1^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\Pi _2^{}{}_{}{}^{}$ states being unknown. The classification ${}_{}{}^3{}_{}{}^{}\Pi _0^{}{}_{}{}^{}$ really has little meaning, one needs for case $c$ here a classification in which only $\Omega$ 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.