Hönl-London factors for multiplet transitions in Hund’s case a or b

Hönl-London factors are the factors that give the dependence of spectroscopic line intensities on the rotational quantum numbers. Recently it has been shown that most tabulations of Hönl-London factors for singlet-singlet transitions do not allow for the special cases of Π-Σ and Σ-Π transitions, where a consideration of the parity symmetrisation shows that an extra factor of 2 is required. The present paper extends these considerations to multiplet transitions, in either pure Hund’s case a or pure Hund’s case b.