Forbidden transitions

From Glossary of Meteorology

forbidden transitions

Spectroscopic transitions that have very low probability due to the violation of certain selection rules.

Such transitions are of low intensity and are not important in the atmosphere unless the molecule involved has a high abundance, for example, O2. If the transition to the ground state is forbidden, such as O(1D) to O(3P), then the excited atom can have a lifetime long enough to engage in important reactions.