Photochemistry is the branch of science which deals with chemical reactions, reactions which occurred by the absorption of light waves which are called photons of energy. Only the radiations which fall under the region 800 nm to 200 nm can cause such reactions. Also the photons should contain sufficient amount of energy to raise the atoms from ground to excited state. In excited state atoms or groups of atoms undergoes chemical reaction more rapidly as compared to ground state.
When an atom goes from ground state to excited state, atoms essentially get promotes from bonding molecular orbital called BM0 to anti bonding orbital called ABMO.