Fluorescence

Fluorescence occurs 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, unlike phosphorescence. 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. The energy loss is due to vibrational relaxation while in the excited state.

A Stokes shift is the difference between the absorption and emission peaks of a molecule.