Photochemistry first received some systematic attention wen over one hundred years ago but it did not receive any great attention until
after World War II. Free atoms and free radicals produced by photo-
chemical means have been used for many years to study single steps
which may form parts of complex mechanisms, but, in a way, the more
fascinating problems of complex molecules which undergo reaction
after absorption of radiation, without at any time passing through the
stage of atoms and radicals, have only occupied the attention of
chemists during recent years.