Stokesshift

Stokesshift, more commonly called the Stokes shift, is the difference in wavelength or energy between light absorbed by a molecule and the light it subsequently emits as fluorescence or phosphorescence. The term is named after Sir George Gabriel Stokes. In practice, the shift is often described as the difference between the maxima of the emission spectrum and the excitation spectrum of a fluorophore.

The physical origin lies in nonradiative relaxation after absorption. After a molecule absorbs a photon and reaches an excited electronic state, it typically relaxes vibrationally to the lowest vibrational level of that state before emitting a photon. This relaxation lowers the energy of the emitted light, producing a longer-wavelength (lower-energy) emission compared with the absorbed light. Solvent reorganization and other environmental factors can also contribute to the shift, especially for polar or highly solvated systems.

Measurement is typically accomplished with a spectrofluorometer. The Stokes shift can be reported as a wavelength difference (Δλ = λmax,emission − λmax,excitation) or as an energy difference (ΔE = hc(1/λmax,excitation − 1/λmax,emission)). The magnitude of the shift varies widely with the molecule, solvent, temperature, and state (solution, solid, or aggregated material).

Applications of Stokes shift include characterizing fluorophores, designing optical probes and imaging agents, and interpreting solvent effects or environmental polarity. A larger Stokes shift helps separate excitation and emission signals, reducing background and self-absorption in fluorescence measurements.