Intersystem Crossing ( ISC ) is a photophysical process involving an isoenergetic radiationless transition between two electronic states having different multiplicities.
2.
The observation of electron tracks that were independent of the frequency of the incident photon suggested a mechanism for electron ionization that was caused from an internal conversion of energy from a radiationless transition.
3.
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.
4.
Radiationless transition between levels, such as between the excited " S " = 0 and " S " = 1 states, may proceed quickly enough to siphon off a portion of the " S " = 0 population before it spontaneously returns to the ground state.
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