Stokes shift

The Stokes shift (english Stokes shift) - also known as Stokes' rule or Stoke 's law - is the shift in the wavelength or frequency of light ( electromagnetic radiation ) between absorption and emission. It occurs, for example in the fluorescence and Raman effect.

Discovery

The Stokes shift is due to the mathematician and physicist Sir George Gabriel Stokes. He recognized the 1852 law that the light emitted by fluorescent back light materials greater than the electromagnetic wavelength absorbed by the material, causing the fluorescence exciting light. For substances that fluoresce by incident light, the back light emitted is therefore shifted to a longer wavelength region, which is known as redshift.

Description

In the experiment carried out by Stokes shift can be the result of two effects describe. Electrons are located after the absorption or emission rarely in the vibronic ground state of the electronically excited state and the ground electronic state, resulting in a non-radiative relaxation in the vibrational ground state. However, in most cases, the stronger is the effect Lösungsmittelrelaxation.

Fluorescent dyes are generally considered to be dipoles. The non-excited system is usually present in equilibrium. Characterized the electronic ground state is energetically preferred. After absorption, the system is no longer in equilibrium, since the dipole of the dye has changed. Once the system is back in balance, the electronically excited state of the ground electronic state, however, is preferable to its energy level is therefore deeper than before, higher. Wherein the energy difference is small between the states in the system than in the non-excited excited, so that when the emission of less energy is released when it was added in the absorption.

In some cases it may occur notwithstanding of this fundamental rule also that the emitted light is not again changed in its wavelength. In these cases, one speaks of a resonance fluorescence occurs.

Mathematical Description

The Stokes shift is the difference in energy between the incoming and outgoing photons:

Per molecule when only one photon is absorbed, the wavelength of the emitted photons is larger than the depth:

The energy loss of the photon is transformed into heat or vibration energy of the absorbing particle:

In the optical wavelength region, the Stokes shift is often expressed as the difference of the wavelength in units of nanometers (nm ) or a wave number in the units of cm -1:

Anti- Stokes shift

If, during the emission of the photon, however, a pre-existing in the material excitation destroyed ( in solids, for example, a phonon ), it is called anti - Stokes shift. Here, the wavelength of the emitted photons is shorter than that of the incident. This is for example in nonlinear Raman spectroscopy in the form of coherent anti-Stokes Raman scattering ( engl. coherent anti -Stokes Raman scattering CARS) exploited for material investigation.