What is photochemistry?

Photochemistry deals with the study of chemical reaction which are caused by the absorption of light radiations (photons).  Only the absorbed radiations can cause a chemical change.  The light radiations of the visible and ultraviolet regions lying between 800 nm and 200 nm wave lengths bring about such reactions.  The related photons should be of sufficient energy (given by hv)  to raise the atom or the molecule from the ground electron from a bonding molecular orbital (BMO) to an antibonding molecular orbital (ABMO).  In the excited state, the atom or the molecule is more likely to undergo a chemical reaction than in the ground state.

     In ordinary thermal reactions, the activation energy results from the random intermolecular collisions.  In photochemical reactions, on the other hand, activation energy is acquired by the absorption of photons of light associated with a particular amount of energy.  By using monochromatic light of a particular wave length in the visible or ultraviolet region, it is possible to excite particular atom or a molecule in a reaction mixture instantaneously to an excited state.  Thus in a photochemical reaction only a single selected species (atom or molecule) can be promoted to an excited state independent of the other species present in the reaction system.  In contrast, thermal translational, rotational and vibrational energies of all the molecules resulting in the excitation of all the molecules to almost the same extent.  For example exposure of a mixture of hydrogen and bromine to radiations of wave lengths between 450 nm and 550 mn, results in excitation of only the bromine molecules whereas exposure to thermal radiations (heat) would excitation of both the bromine and the hydrogen molecules.