Phosphorescence

Phosphorescence is the result of a radiative (light emitting) transition involving a change in the spin multiplicity of (in most cases) a molecule from excited state singlet to excited state triplet. This transition is quantum mechanically forbidden as is the transition that leads to light emission. These forbidden transitions are kinetically slow, which introduces a delay between photo-excitation (exposure to light of one wavelength) and emission (release of light of a different wavelength). So-called "glow in the dark" materials are phosphorescent materials with a very long (seconds, minutes, even hours) delay between excitation and emission. Most phosphorescent compounds have triplet lifetimes on the order of milliseconds.

Equation

:S_0 + hv o S_1 o T_1 o S_0 + hv prime

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Where S is a singlet and T a triplet whose subscript denotes the excited state (zero is ground state). Transitions can occur at higher energy levels, but the first excited state is denoted for simplicity.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~