Phosphorescence
Phosphorescence
Many substances continue to emit light for quite some time even after the incident light stimulus is cut off. This is known as phosphorescence. ZnS and sulphides of IIA metals are excellents examples of such substances. The addition of a trace of a heavy metal enhances the intensities of light emission by a phosphorescencent substance. Thus presence of impurities in the form of mixed crystals is necessary for phosphorescence.In order to understand the mechanism of phosphorescence, we have to understand the excitation process first. In such excitation process, there is no tie lag between the absorption of radiation and the process of excitation.
Excitation occurs too rapidly to allow enough time for the orientation of the spin of the electrons to change. This excited electronic state having two electrons in different orbitals with opposite spin is called singlet state. If the radiations are lost without any loss of time, fluorescence is said to occur.
But if there be any time lag between the absorption and radiation and its emission, there occurs a change in the orientation of spin. It gives rise to a lower energy excited triplet states in which the two electrons have the same direction of spin. Any radiation emitted from the triplet state produces what is known as phosphorescence. It is quite probable that the formation of triplet state produces what is known as phosphorscence. It is quite probable that the formation of triplet state may be caused by loss of energy on collision with impurities necessary for phosphorescence.
Fluorescence
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