Phosphorescence
This is the same process as fluorescence but the emission of light persists after the exciting source is removed. An example of this is things that "glow in the dark". The phenomena of electron spin needs to be understood to continue with this section.
Upon excitation by a particular wavelength of UV-vis radiation, an electron will be promoted from the singlet ground state, S0, to the singlet excited state, S1. If fluorescence were to occur, then the electron would lose it's energy and return to the ground state, S0, immediately.
However, if the ground level of the S1 has the same energy as a level in the triplet state, the electron can cross over to the triplet state: intersystem crossing. Because of the rule DS=0, we expect that the electron, now in level T1, cannot return to level S0. But this transition can occur slowly, giving a steady emission of radiation.
This is illustrated by the following diagram:
