Past, Present, and Future of Fluorescence

Abstract

It is important to define fluorescence. There are several ways to describe this term: (1) the emission of electromagnetic radiation, especially of visible light, stimulated in a substance by the absorption of incident radiation and persisting only as long as the stimulating radiation is continued. (2) Emitting light during exposure to radiation from an external source. (3) Luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate reradiation usually at a different wavelength and that ceases almost at once when the incident radiation stops. The common denominators for this term are a source of radiation, the absorption of this radiation, and the emission of a modified radiation that ends immediately as the source stops emitting radiation. Chemically speaking, fluorescence is brought about by absorption of photons in the singlet ground state promoted to a singlet excited state. The spin of the electron is still paired with the ground state electron. As the excited molecule returns to ground state, it involves the emission of a photon of lower energy, which corresponds to a longer wavelength, than the absorbed photon and therefore lower energy, than the absorbed radiation.