Optipedia • SPIE Press books opened for your reference.
Excerpt from Field Guide to Spectroscopy
Phosphorescence is similar to fluorescence except that after excitation, there is a violation of the ΔS = 0 selection rule as the system undergoes a radiationless intersystem crossing into the triplet manifold. (Alternatively, there could be excitation directly into the triplet manifold by an intense light source, like a laser.) Once in the triplet manifold, relaxation to the singlet ground state is also a spin-forbidden process; while it does happen, it is relatively slow. Most phosphorescence occurs in the 10−4–102 s timeframe. In order for phosphorescence to be favored, the system must be protected from other potential mechanisms of de-excitation. Intermolecular collisions or the presence of dissolved oxygen causes quenching, which lowers the efficiency of the phosphorescence process. Phosphorescence and fluorescence can occur in the same system.
Phosphorescence is typically lower in energy than fluorescence because the triplet state involved is usually lower in energy. Because heavy-atom molecules are less ideal, systems having heavy atoms are more likely to violate the spin selection rule and show phosphorescence.