What is Fluorescence? Is it the same as Phosphorescence?
When an atom or molecule absorbs a photon of light or is otherwise supplied with a sufficient quantum of energy (e.g. by interaction with energetic electrons or as a result of chemical reaction) it reaches an electronically-excited state. For simplicity we will confine the discussion to molecules excited by absorption of light. The state first formed on excitation with a photon of light is almost always a so-called ‘excited singlet’ The term ‘singlet’ means that the net spin of the electrons involved in the excited state transition is zero. This excited singlet state can subsequently give rise to other excited states, particularly the so-called ‘triplet’ state. Excitation of molecules by means other than photon absorption can lead to direct formation of such ‘triplet’ states. An electron can have a spin of +1/2 or -1/2. In the ground state of most molecules electrons are paired and must have opposite spins if they occupy the same orbital. The net spin of the ground state is therefore z
