Origin of Fluorescence and Phosphorescence Spectra:

When a radiation in the UV-visible region interacts along with a molecule, it causes transition between the quantized electronic energy stages. Have you ever thought what happens to the excited molecules so acquired? These relax back to the ground state through using different processes. In a few of the processes the absorbed energy is provide off as heat to the surroundings and in a few it is emitted as radiation. While such a relaxation is accompanied through the emission of a radiation it is known as luminescence. There are two kinds of luminescence phenomena namely, phosphorescence and fluorescence that arise from the excited state generated through absorption of electromagnetic radiation. Fluorescence is the emission of visible light through a substance which has absorbed light of a different wavelength. An emitted photon has a longer wavelength. Phosphorescence is related to fluorescence in emitting a photon; therefore, a phosphorescent material does not immediately re-emit the radiation it absorbs. As the excitation of the molecule is because of the absorption of a photon (light), these kinds of luminescence are called photoluminescence.