What causes the Stokes shift in fluorescence?

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energy levels of a luminescent centre — luminescence Figure 1: Energy levels of a luminescent centre (see text).

The Stokes shift in fluorescence is caused by the relaxation of a molecule's nuclei toward their new positions following photoexcitation.

When a molecule absorbs a photon, the change in electron configuration occurs almost instantaneously, while the nuclei remain in their ground-state positions. Because the excited state has a different molecular geometry than the ground state, the nuclei subsequently relax toward their new equilibrium positions. This structural relaxation lowers the total energy of the excited state. Consequently, when the molecule eventually emits a photon to return to the ground state, the emitted light has lower energy—and thus a longer wavelength—than the light originally absorbed. This difference between the excitation and emission wavelengths is defined as the Stokes shift.1