It would certainly help spruce up the articles on phosphorescence, fluorescence, intersystem crossing, etc, which look like they could use some help .-- Bennybp 18 : 16, 21 July 2007 ( UTC)
22.
In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher " spin multiplicity " ( " see term symbol " ), usually a triplet state.
23.
As the spin / orbital interactions in such molecules are substantial and a change in spin is thus more favourable, intersystem crossing is most common in heavy-atom molecules ( e . g . those containing iodine or bromine ).
24.
However the El-Sayed s rule states that the rate of intersystem crossing, e . g . from the lowest singlet state to the triplet manifold, is relatively large if the radiationless transition involves a change of molecular orbital type.
25.
However, when the molecule is distorted away from its equilibrium geometry, the potential surfaces of the triplet and singlet states intersect, allowing for intersystem crossing to the singlet state, which is unbound and dissociates to two ground-state CO molecules.
26.
Alternatively, an excited singlet state electron ( S1 ) can undergo spin inversion and populate the lower-energy first excited triplet state ( T1 ) via intersystem crossing ( ISC ); a spin-forbidden process, since the spin of the electron is no longer conserved.
27.
The optical excitations conserve the spin state, but there is a high probability of the states \ left | ^ 3 \ text { E }, \ pm1 \ right \ rangle decaying non-radiatively to the singlet state \ left | ^ 1 \ text { A } _ 1 \ right \ rangle, a phenomenon called intersystem crossing ( ISC ).
28.
While absorption of a photon can occur for any energy corresponding to the difference between two singlet states, the excited electronic state relaxes to the lowest energy singlet excited state through fluoresce, radiating a photon and returning to the singlet ground state, or it can move to the lowest energy triplet excited state ( a state where two unpaired electrons have the same spin ) by a second non-radiative process termed intersystem crossing.
