A comment on Hönl-London factors

The literature on the Hönl-London factors of diatomic molecules, the factors in intensity formulas dependent on the rotational quantum numbers, is somewhat confused for the case of perpendicular transitions. We derive a formula, Eq. (23), for allowed singlet-singlet transitions between levels of definite parity, and show that it is consistent with standard sum rules. As an illustration, the formula is applied to the calculation of lifetimes in the B¹Π_u state of Na₂, showing that the corrected formula gives results in good agreement with experimental measurements.