Basics of fluorescence

The phenomenon of fluorescence is based on the absorption of light by a molecule. Whatever the wavelength of the incident radiation, an excited molecule returns spontaneously to its fundamental state through different pathways of de-excitation. One of these pathways is the fluorescence shown in the Jablonski diagram.

Jablonski diagram: absorption stage leading to the excited singlet state, leading to the fluorescence stage. The next state is the singlet ground state, passing to the triplet state leading to the phosphorescence stage