Kasha's rule

Scheme of Kasha's rule. A photon with energy excites an electron of fundamental level, of energy , up to an excited energy level (e.g. or ) or on one of the vibrational sub-levels. Vibrational relaxation then takes place between excited levels, which leads to dissipation of part of the energy (), taking the form of a transition (internal conversion) towards the lowest excited level. Energy is then dissipated by emission of a photon of energy , which allows the system to go back to its fundamental state.

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 after American spectroscopist Michael Kasha, who proposed it in 1950.[1][2]

  1. ^ Characterization of Electronic Transitions in Complex Molecules. Kasha, M. Discussions of the Faraday Society, 1950, 9: p.14-19.
  2. ^ IUPAC. Kasha rule – Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by McNaught, A.D. and Wilkinson, A. Blackwell Scientific Publications, Oxford, 1997.