Franck-Condon factor, direct molecular

Example The electronic and vibrational states of the oxygen molecular ion could be perfectly resolved by PES (Fig. 2.20), thus allowing to directly read out the Franck-Condon factors and to identify the (0 <— 0) transitions corresponding to... 

The A 3 + state has a radiative lifetime of 2 s, and is therefore long-lived on the time scale of the molecular beam experiments, where it was produced by electron impact on a beam of N2 diluted with Ar, cooled to liquid nitrogen temperature. Calculations of the Franck-Condon factors show that direct excitation of the A state is expected to populate many vibrational levels, and a major feature of the magnetic resonance studies was that spectra involving the first thirteen vibrational levels (v = 0 to 12) were obtained. The accurate and extensive determination of the vibrational constants, given above, was therefore of considerable importance. 

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. 

The fluorescence induced by the second laser allows the accurate determination of the molecular parameters for those excited states on which the fluorescence transitions from are terminating. The LIF method can therefore be extended by stepwise excitation to the investigation of many molecular states which may not even have been found before. Of particular interest are dissociating excited states with repulsive potential curves below bound states E. These continuous states often cannot be studied by direct absorption from the ground state because the Franck-Condon factors for the transitions may be quite small. As an example of such investigations we mention the two-step excitation of the iodine molecule I2 (Fig.8.29). Selected (V, J ) levels in the B rig state are populated by optical pumping with a cw dye laser. Starting from these levels a krypton laser excites further levels in a... 

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