Franck-Condon principle structure

Radiative transitions may be considered as vertical transitions and may therefore be explained in terms of the Franck-Condon principle. The intensity of any vibrational fine structure associated with such transitions will, therefore, be related to the overlap between the square of the wavefunctions of the vibronic levels of the excited state and ground state. This overlap is maximised for the most probable electronic transition (the most intense band in the fluorescence spectrum). Figure... 

On the other hand, it is pleasing to see that the organic chemist s standard evidence for structural identification such as NMR and IR spectral data can be computed quite accurately UV/vis spectra can also be computed with time-dependent methods, which, however, cannot yet be used to optimise the excited states as the Franck-Condon principle is a common assumption in electron spectroscopy, this is no serious drawbackfor computing... 

CPL and CD are based upon similar aspects of molecular structure. It is important to realize, however, that, even if the same states are involved, these measurements do not usually supply redundant information. From the Franck-Condon principle, CPL is a probe of excited state geometry, and CD is a probe of ground state geometry. CPL measurements have some advantages over the measurement of CD, as well as some inherent limitations. The most serious limitation is, quite obviously, that the optically active molecule under study must contain a luminescent chromophore with a reasonable quantum yield. Although this severely limits the range of possible applications of CPL, it does result in a specificity and selectivity that is not present in CD or absorption experiments. 

In the case of non-equilibrium conditions, the crossing point is located for a frozen solvent structure. During an electron transition the Franck-Condon principle is applicable and the solvent nuclei remain fixed during the transition. Consequently, the solvent structure is in equilibrium with the charge distribution of the solute in its ground state. The crossing point search procedure is performed in presence of this solvent structure. 

Many ionization potentials have now been calculated for simple and complex molecules using more sophisticated self-consistent field treatments and, when the effect of electron correlation is considered, extremely good results may be obtained (e.g. Hush and Pople, 1954). Because ionization is rapid, the Franck-Condon principle applies in the calculation of ionization potentials, and the structure of the ion immediately after formation is essentially that of the molecule. On vibration, the geometry of the ion may change. 

Let us consider the last point. The reader is already familiar with two important implications of the timescale separation between electronic and nuclear motions in molecular systems One is the Bom-Oppenheimer principle which provides the foundation for the concept of potential energy surfaces for the nuclear motion. The other is the prominent role played by the Franck-Condon principle and Franck-Condon factors (overlap of nuclear wavefunctions) in the vibrational structure of molecular electronic spectra. Indeed this principle, stating that electronic transitions occur at fixed nuclear positions, is a direct consequence of the observation that electronic motion takes place on a timescale short relative to that of the nuclei. 

Figure 2.10 Illustration of the Franck Condon principle. The bottom diagrams illustrate the vibrational structure of the absorption bands...

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