| People involved: | Tõnu Pullerits |
| Former members: | Ben Brüggemann, Pär Kjellberg, Sergey Polyutov |
| Involved facilities: |
This technique has the following projects (and possibly other techniques) related to it:
In case of two well separated molecules one usually describes the excited states as molecular excitations and the transfer takes place via Förster-type incoherent hopping. If the molecules are densely packed together as in many photosynthetic antenna systems, excitonic coupling between the neighboring molecules becomes strong and the elementary excitations are more properly thought of as coherent superpositions of the monomeric excited states. We say that the excited states are delocalized over many molecules. In this case ordinary Förster theory breaks down and one should use some flavor of exciton relaxation theory. In contemporary scientific literature the prevailing such description is Redfield relaxation theory and its modifications. In some sense the two ways of looking at the excitation transfer - the Förster and the Redfield theory - are complementary. In Förster theory the electron-phonon coupling is treated exactly (in fact one uses experimental spectra which include all orders of electron-phonon coupling) whereas the exciton coupling between the molecules acts as perturbation. The whole theory is then based on the Fermi Golden Rule (first order perturbation theory). In Redfield theory, however, the intermolecular electronic coupling is diagonalized out resulting in delocalized excitonic excited states. The price one pays is that now the coupling to the phonons and vibrations is treated as a perturbation and usually included up to the first order. Therefore, in the Redfield theory only single-phonon and single vibration processes are included. Various Modifications to the Redfield theory exist which provide additional refinements to the standard formalism.
We have used Refield theory to simulate excitonic processes model systems and in light harvesting complexes. Combination with nonlinear spectroscopy methods provides a direct link with the experiments carried out in our department.