Photochemistry

An electronically excited species may react with a second species to produce a new product, or set of products (path iv). For example, the products of the ultraviolet dissociation of ozone (reaction 5) are themselves electronically excited: O₃ + hν → O ^* 2 + O^*. These excited fragments may react with other atmospheric molecules such as water: O^* + H₂O → OH + OH. Or they may react with ozone: O ^* 2 + O₃ → 2O₂ + O. These reactions do not readily occur for the corresponding non-excited species, confirming the importance of electronic excitation in determining reactivity.