Three-Photon Excitation of Fluorescence

Most experiments in optical spectroscopy rely on the interaction of a single photon with the absorbing molecule. However, since 1931 it has been known that molecules can simultaneously interact with two photons. Two-photon absorption was predicted theoretically by Goppert-Mayer in 1931 [1]. This phenomenon only occurs at high light intensities, and was not studied experimentally until after the appearance of lasers in the 1960s and 1970s. Since then, two-photon spectroscopy has been used to study the excited state symmetry of organic chromophores [2–4], Two-photon excitation has been applied to study such systems as hydrocarbons [5, 6], porphyrins [7], polyenes [8, 9], protein bound chromophores [10] and indole derivatives [11,12]. Two-photon excitation has also become a tool in analytical chemistry [13,14]. Until recently, the main emphasis of two-photon spectroscopy has been to study the symmetry properties of electronic states. This emphasis was due to selection rules for electronic transitions, which are different for one-photon and two-photon processes.