This lecture is a very concise review of the phenomenon of fluorescence and the associated processes. Let us move a step forward from the absorption of the UV/visible radiation. What happens to the electrons that absorb UV/visible light and occupy the high energy molecular orbitals? In a UV/visible absorption experiment, the samples continue absorbing light. This means that the higher energy molecular orbitals never get saturated. This further implies that after excitation, the molecules somehow get rid of the excess energy and return back to the ground state. The electrons can return back to the ground state in different ways such as releasing the excess energy through collisions or through emitting a photon. In fluorescence, the molecules return back to the ground state by emitting a photon. The molecules that show fluorescence are usually referred to as the fluorophores. Various electronic and molecular processes that occur following excitation are usually represented on a Jablonski diagram as shown in Figure 6.1.

 

Figure 6.1 Jablonski diagram showing various processes following absorption of light by the fluorophore