Laporte rule

Spectroscopic selection rule

The Laporte rule is a rule that explains the intensities of absorption spectra for chemical species. It is a selection rule that rigorously applies to atoms, and to molecules that are centrosymmetric, i.e. with an inversion centre. It states that electronic transitions that conserve parity are forbidden. Thus transitions between two states that are each symmetric with respect to an inversion centre will not be observed. Transitions between states that are antisymmetric with respect to inversion are forbidden as well. In the language of symmetry, g → g and u → u transitions are forbidden. Allowed transitions must involve a change in parity, either g → u or u → g.

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