Knowledge Bank

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