Strongly allowed transitions Franck-Condon

The main spectroscopic consequence of the combined action of electron transfer and vibronic interaction is the occurrence of the so-called electron transfer optical absorption (intervalence band), which is shown by the arrows in Fig. 10. The shape and intensity of the intervalence band in the PKS model is defined by the ratio t /(v /cu). In the case of weak transfer the Franck-Condon transitions are almost forbidden, and at the same time, the Stokes shift can be significant. Therefore the MV dimers of Class I are expected to exhibit weak and wide intervalence bands. On the contrary, in the Class III compounds the Franck-Condon transition is allowed, and the Stokes shift is zero. For this reason, intervalence optical bands in delocalized MV dimers are strong and narrow. When the extra electron jumps over the spin cores in a multielecton MV dimer d" — > 1) [85-87] we are dealing with... 

There are two physical reasons for the enhancement in RRS, the Franck—Condon enhancement and the vibronic enhancement Both mechanisms are complicated, a detailed description is given in [30], An appHcation of RRS is the investigation of biological molecules like metalloporphyrins and carotenoids. These molecules have very strong electronic transitions in the VIS. The vibrations of the chromophoric part become resonance enhanced but the vibrations of the surrounding protein matrix do not This allows observation of the chromophoric site without spectral interference from the surrounding protein. RRS is suitable... 

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