Davydov split
The Davydov splitting describes a splitting of the energy levels of the electronic system in molecular crystals . It is a consequence of the exchange interaction within a unit cell and leads to a Z-fold splitting of the exciton level if Z is the number of molecules per unit cell. It is named after Alexander Sergeyevich Davydov .
Occurrence
The Davydov splitting occurs in molecular crystals that consist of more than one molecule per unit cell. The prerequisite is that the bonds between the molecules are significantly weaker than the bonds within the molecules, which is particularly the case in organic solids, where the molecules are only bound to one another by the Van der Waals interaction .
Derivation
One looks at an exciton level in an initially isolated molecule. If you put Z molecules of this kind together, the level under consideration splits up Z-fold due to the exchange interaction, whereby the energetic degeneration is canceled. The resulting levels are sometimes referred to as Davydov levels . The energy difference between two Davydov levels is called . The quantum mechanical dipole moment of the associated states is i. A. different.
If N such identically constructed unit cells are now combined to form a crystal, each Davydov level fans out N-fold. N is typically in the order of 10 23 . If this fanning out is smaller than the splitting of the Davydov levels, then Z separate Davydov bands arise , one band per Davydov level. In this case it is called the Davydov split .
proof
The Davydov splitting can be detected with all common spectroscopic methods from solid state physics . Optical detection using photons from the visible spectral range ( absorption spectroscopy , fluorescence spectroscopy ...) is particularly suitable, since Davydov levels with different dipole moments can be detected separately with the help of polarized light .
literature
- Marcus Schwoerer, Hans Christoph Wolf: Organic Molecular Solids. Wiley-VCH, Weinheim 2005, ISBN 3-527-40539-9 .