Coherency dyad

The definition of the coherency and Stokes vectors explicitly exploits the transverse character of an electromagnetic wave and requires the use of a local spherical coordinate system. However, in some cases it is convenient to introduce an alternative quantity, which also provides a complete optical specification of a transverse electromagnetic wave, but is defined without explicit use of a coordinate system. This quantity is called the coherency dyad... 

It is important to remember, however, that when the eohereney dyad is applied to an arbitrary electromagnetie field, it may not always have as definite a physical meaning as, for example, the Poynting vector. The relationship between the coherency dyad and the actual physical observables may ehange depending on the problem at hand and must be established carefully whenever this quantity is used in a theoretical analysis of a specific measurement proeedure. For example, the right-hand sides of Eqs. (13.2) and (13.3) may become rather meaningless if the products p n and n p do not vanish. 

Figure 15. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the bridge states lie above the D level consequently the electron is transferred in one coherent jump and is never localized within the bridge. The distance dependence behavior is exponential decay.

Figure 40. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the virtual bridge states lie well above the donor level (A is large) and, consequently, the electron is never localized within the bridge instead, the electron is transferred from donor to acceptor in one coherent jump. The distance dependence behavior is exponential decay. Molecular wire behavior The virtual bridge states are energetically comparable to the donor level (A is very small). In this case, the electron may be thermally injected into the bridge and becomes localized within the bridge, whereupon it moves from the donor to the acceptor incoherently as a defect, such as a polaron. The distance dependence behavior is Ohmic (varies inversely with distance).

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