Magnetic Rotation Spectra of Diatomic Molecules

The general theory of the Faraday effect associated with a single molecular absorption line is presented. The effect of the perturbation of the transition probabilities by the field in producing asymmetrical rotation is discussed, and calculated for alkali band systems and for ${\mathrm{I}}_2$. The observations on asymmetrical rotation in ${\mathrm{I}}_2$ agree with the predictions. The longitudinal Zeeman patterns and the changes in these patterns produced by the field are related to the vector model by use of the criterion that transitions which involve little change in the direction of $J$, the rotational axis, are the most probable ones. It is shown that all known facts about the ${}_{}{}^1{}_{}{}^{}\Sigma \leftarrow {}_{}{}^1{}_{}{}^{}\Sigma$ magnetic rotation systems of ${\mathrm{Na}}_2$ and ${\mathrm{K}}_2$ can be explained by assuming the upper state to be perturbed by an unknown ${}_{}{}^3{}_{}{}^{}\Pi _0^{}{}_{}{}^{}$ state. The constants have been calculated for this predicted perturbing state in ${\mathrm{Na}}_2$.