09-26-2021-2045 - Stokes shift

Stokes shift is the difference (in energy, wavenumber or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition.^[1] It is named after Irish physicist George Gabriel Stokes.^[2][3][4] Sometimes Stokes shifts are given in wavelength units, but this is less meaningful than energy, wavenumber or frequency units because it depends on the absorption wavelength. For instance, a 50 nm Stokes shift from absorption at 300 nm is larger in terms of energy than a 50 nm Stokes shift from absorption at 600 nm.

When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited state. One way for the system to relax is to emit a photon, thus losing its energy (another method would be the loss of energy as translational mode energy (via vibrational-translational or electronic-translational collisional processes with other atoms or molecules)). When the emitted photon has less energy than the absorbed photon, this energy difference is the Stokes shift.

The Stokes shift is primarily the result of two phenomena: vibrational relaxation or dissipation and solvent reorganization. A fluorophore is a dipole, surrounded by solvent molecules. When a fluorophore enters an excited state, its dipole moment changes, but surrounding solvent molecules cannot adjust so quickly. Only after vibrational relaxation do their dipole moments realign.^{[citation needed]}

Absorption and emission spectra of rhodamine 6G with ~25 nm Stokes shift

^{https://en.wikipedia.org/wiki/Stokes_shift}