Polyatomic molecules Franck-Condon principle

Duschinsky F 1937 On the interpretation of electronic spectra of polyatomic molecules. I. Concerning the Franck-Condon Principle Acta Physicochimica URSS 7 551... 

Fortunately the Franck-Condon principle, which states that an electronic transition occurs so rapidly that there is a high probability of forming the upper state with the same bond distances and angles as the lower state, means that many bands will be of very low intensity or absent. Nevertheless the resolution of polyatomic spectra for molecules of low symmetry is usually impossible. It is useless to try to determine the onset of predissociation, or even of direct dissociation, from the appearance of such absorption spectra. Unfortunately the photochemical literature is replete with mis-statements based on the appearance of absorption spectra. 

In those favorable cases of predominantly direct ionization in which the photoionization cross-section curve consists of a series of steplike thresholds, the situation is particularly simple. Each step corresponds to the production of ions in a vibrationally excited state as shown in Fig. 3 for NO. For polyatomic molecules with many normal vibrational modes, the vibrational structure can get hopelessly complicated yet often only a few normal modes are predominantly excited and these can often be identified from the observed spacing of steps. A particularly simple case is shown in Fig. 10. Photoionization of the pyramidal NH3 molecule produces an NH3 ion which has a planar equilibrium configuration. By the Franck-Condon principle, one expects predominant excitation of the symmetric out-of-plane bending mode of the NH3 ion. This expectation is confirmed by the energy spacing of the observed steps, which corresponds to production of the ion with successively higher vibrational excitation predominantly in this particular normal mode. 

Symmetry of Vibronic Transitions in Polyatomic Molecules and the Franck-Condon Principle. 

The Franck-Condon principle is applicable to polyatomic molecules also. However, as might be expected, the potential energy diagrams get more complicated, in part because there are now 3N — 6 vibrational degrees of freedom and therefore... 

As with diatomic molecules, vibrational and rotational transitions in polyatomic molecules take place along with electronic transitions. The Franck-Condon principle applies, so that the final state will usually be an excited vibrational state as well as an excited electronic state. Since there are several normal modes in any polyatomic molecule the simultaneous electronic, vibrational, and rotational transitions can give very complicated spectra. The most important selection rule is the same for all molecules and atoms The total spin quantum number is the same for the final as for the initial state ... 

In the case of diatomic parent molecules, the fragmentation can be treated quantitatively in terms of the Franck-Condon principle. For the dissociation of small polyatomic molecules, spectroscopic and quantum-theoretical ideas (correlation rules) can be used to determine the dissociation path and predict the resulting electronic states. With the advent of ab initio Cl (configuration interaction) calculations, detailed information on the potential energy surfaces of the different states and on their interactions is becoming available. However, to deal with large... 

Doktorov, E.V, Malkin, I.A., and Manlco, VI. (1975) Dynamical symmetry of vibronic transitions in polyatomic molecules and the Eranck-Condon principle. /. Mol. Spectrosc., 56, 1 Doktorov, E.V, Malkin, I.A., and Manlco, VI. (1977) Dynamical symmetry of vibronic transitions in polyatomic molecules and the Franck-Condon principle. /. Mol. Spectrosc., 64, 302. 

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