Phosphorescence
From The Science of Spectroscopy
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Phosphorescence is a luminescent process similar to fluorescence in that it an emission of light by molecules which cannot relax by vibrational relaxation. The essential differences between the two techniques are that phosphorescence is a slower process, lasting for milliseconds up to several seconds, rather than for only a few microseconds with fluorescence. The second essential difference is that phosphorescence requires a change of spin of the electron, from a triplet state to the singlet state before it can relax. Both processes can be quenched by collision of the excited molecules with other molecules, which absorb the excess energy. This radiationless process is known as external conversion. Often, phosphorecence can only be seen if the chromophore is cooled or immobilized.
The animation below shows this process in two steps:
The diagram shows the arrangement of the energy levels within a molecule. The exciting source radiation could be any UV or visible source with sufficient energy to cause absorption to occur.
The electrons which absorb the radiation are promoted to an excited state, which then relaxes by vibrational relaxation to a lower energy excited state.
Once the molecule is in the excited state it will try to lose the excess energy. Most molecules lose this energy by vibration, and return to the ground state. However, some molecules, particular those molecules that have rigid chemical structures, like X shown below, it cannot relax vibrationally to any great extent, and instead relaxes by emission of its excess energy in the form of light. This is known as fluorescence provided that there is no net change in the spin of the electron, ie from a singlet state.
The animation below shows the emission process:
