A photobiological response is predicated upon some chemical modification of the biological system through the action of absorbed light energy. Chemical reactions that result from irradiation of chemical systems with light have traditionally been called photochemical reactions, and their study comprises that science called photochemistry. With the advent, during the last two decades, of many techniques that allow direct measurements on reactive intermediates, and on molecules in electronically excited states, the term photochemical reaction has taken on a more specific definition: a photochemical reaction starts in one of the electronically excited states of a reactant and ends with the appearance of the first ground-state product(s). That is, photochemical reactions comprise that class of electronic relaxation processes that do not lead back to the starting molecule.(1-3) With this definition comes the insight provided by the notion that photochemical reactions differ from conventional thermal reactions in that photochemical reactions always involve a molecule possessing an excited electron.
KeywordsQuantum Yield Triplet State Photochemical Reaction Action Spectrum Excited Molecule
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