Kasha's rule is a principle in the photochemistry of electronically excited molecules. The rule states that photon emission (fluorescence or phosphorescence) occurs in appreciable yield only from the lowest excited state of a given multiplicity. It is named for American spectroscopist Michael Kasha, who proposed it in 1950.
Read more about Kasha's Rule: Description and Explanation, Kasha–Vavilov Rule
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