In all cases, the emitted light has a lower energy (lower frequency, longer wavelength) than the absorbed radiation; the difference in these energies is known as the Stokes shift. In some cases under intense illumination it is possible for one electron to absorb two photons allowing for the emission of radiation of a "higher" photon energy (shorter wavelength) than the absorbed radiation; such two-photon absorption is not referred to as fluorescence. A molecule that is excited through light absorption or other process (e.g. from a chemical reaction) can transfer its energy to a second 'sensitized' molecule, raising it to an excited state from which it will then fluoresce.