Photoexcitation and Relaxation Processes in Solution

Photoexcitation as the induction mechanism for CT reactions has been studied extensively and the undergoing processes are nowadays well understood [75]. It is also a cheap mechanism for generating free charge-carriers, which in turn can be used elsewhere. 

To fully understand the photoexcitation processes we should ask ourselves how light is absorbed by a molecular species. Understanding this phenomenon gives insight into the parameters that trigger CT processes in molecules. As we have elucidated in the previous sections, the transfer of electrons from one end of a large molecule to another is based on electronic interactions between the molecular subunits. In fact, this interaction may be induced by the absorption of light. 

The nature of the molecular system implements a change in the physical mechanism of the photoabsorption process. Once again we may, however, employ the golden rule expression. We use it in a general sense to express the absorption rate in the form of  

For a direct absorption, x and g label the excited and ground states, respectively. These states are, in turn, coupled by a dipole moment operator p, and are assumed to be of Born-Oppenheimer type, i.e. the electronic contributions are separated from those of the nucleus. With these assumptions, the interactions between these states result from vibrational overlaps between the ground and excited state with the transition probability Wg x and the density of the final states pf. In general terms, we now can evaluate the transition probability, which mainly depends upon two parameters  

the strength of the coupling between the initial and the final states, 

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