Many compounds absorb ultraviolet or visible light and undergo electronic transition from low electronic energy levels to high electronic, energy levels. This Absorption of light requires 10-15 sec. The instant re-emission of the absorbed energy is called Fluorescence. While delayed re-emission of the absorbed energy is called Phosphorescence.
For most molecules the electrons are paired in the ground state i.e. pair of electrons have opposite spins this is called singlet state. As the spin is paired up, they do not show any magnetic moment. But if the electrons have the same spin the magnetic moment of both the electrons gets combined and they behave like a tiny magnet. This state where the electron pair is having same spin are called as triplet state.
In molecule there are several electronic energy levels and electrons are present at the ground state in the singlet state i.e. with opposite spins. When molecule absorbs U.V. or Visible light, one electron from ground state undergoes to an excited state. Now excited state is also having several vibrational levels, so the excited electron loses its energy by intermolecular collision and it comes to the lowest vibrational level of excited state. Finally the electron from the lowest vibrational level of excited state returns to ground state by emitting the radiation or light of lower energy or higher wavelength. This overall process of absorption, intermolecular collision and emission takes 10-4 to 10-8 sec which is very small thus it looks like instant re-emission of light. But the energy of radiation emitted is always less than the energy of radiation absorbed. Or in other words wavelength of emitted radiation will be always higher than the wavelength of radiation absorbed. This is Fluorescence phenomenon.
In case of Phosphorescence the inversion of electron spin takes place. That means when the energy from the UV or visible light is absorbed the electron from the ground state jumps to excited state, at the same time the spin of the electron gets reversed and it is now in triplet state (i.e. both electrons have same spin.) In triplet state there is less chance of electron coming back to ground state because pairing of electron with same spin is not favourable. Thus it takes more time to reverse the spin and come back to ground state. The time taken by electron is from 10-4 to several seconds. Thus, it seems that the emitted radiation is delayed. In some cases even when we turn off the light sources, radiation keeps coming.This delayed re-emission of radiation is called phosphorescence. Here again the energy of radiation emitted is always less than the energy of radiation absorbed. Or in other words wavelength of emitted radiation will be always higher than the wavelength of radiation absorbed.
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