rotational sum rule

rotational sum rule

[rō′tā·shən·əl ′səm ‚rül]
(spectroscopy)
The rule that, for a molecule which behaves as a symmetric top, the sum of the line strengths corresponding to transitions to or from a given rotational level is proportional to the statistical weight of that level, that is, to 2 J +1, where J is the total angular momentum quantum number of the level.