A substrate A can be promoted from the electronic ground state to an electronically excited state (*A) by the absorption of a quantum of light (process 6-I).

Subsequently, the excites deactivates along routes that typically, according

to the Kasha’s rule, lead to the formation of the lowest excited singlet state (S1) or the lowest excited triplet state (T1) of the molecule. In the most general sense, *A, when unperturbed, can decay via radiative (6-II), nonradiative (6-III), and reactive intramolecular (6-IV) processes:

' +A A * r νhk→ (6-II) heat +A A * nr→k (6-III) productsA * rx→k (6-IV)

The probabilities of the various unimolecular processes (6-II)-(6-IV) have

the dimensions of a first-order rate constant. The lifetime of unperturbed *A, τ0, is defined as τ0 = 1/k0, where k0 = kr + knr + krx.