as the excited state, the process is called fluorescence. When the initial and final states have different multiplicity (spin), the phenomenon is termed phosphorescence.

When a molecule in its ground state (called S₀) is photoexcited it may end up in any one of a number of excited electronic states (S₁, S₂, S₃,...). It can occupy different vibrational states within the electronic state. These vibrational states are populated in proportion to their overlap