The term photochemistry is interpreted broadly to include a range of both chemical and physical changes that result from interaction of matter with electromagnetic radiation from the infrared range far into the ultraviolet. A photochemical process is characterized by the participation of an excited electronic state in the chemical or physical transformation. An excited state is a configuration of a molecule or ion with an energy higher than the most stable configuration (the ground state) of the molecule or ion. An electronic excited state involves the pattern of electron distribution in the atom or molecules that is different from the ground state electron distribution. In order for a chemical transformation to occur, the excited state must have an energy greater that is than the ground state by an amount similar to that of the energy of a chemical bond (on the order of one hundred to several hundred...