Franck-Condon principle window

Spectroscopy provides a window to explain solvent effects. The solvent effects on spectroscopic properties, that is, electronic excitation, leading to absorption spectra in the nltraviolet and/or visible range, of solutes in solution are due to differences in the solvation of the gronnd and excited states of the solute. Such differences take place when there is an appreciable difference in the charge distribution in the two states, often accompanied by a profonnd change in the dipole moments. The excited state, in contrast with the transition state discussed above, is not in equilibrium with the surrounding solvent, since the time-scale for electronic excitation is too short for the readjustment of the positions of the atoms of the solute (the Franck-Condon principle) or of the orientation and position of the solvent shell around it. 

Because the probabilities of absorption and emission depend on dipole moments in the same states, there exists a straightforward (linear) relationship between the molar absorption coefficient and the rate constant of the spontaneous emission (the higher the probability of absorption, the higher the emission) [7]. However, the observed fluorescence intensity is often much weaker than that expected, because the competitive nonradiative processes can deplete the excited state much faster than fluorescence. Hence, according to the Franck-Condon principle, the molecule finishes in a higher vibrational level of the ground state So- Then, a fast vibrational relaxation takes place that causes the intrinsic Stokes shift (the red shift of fluorescence with respect to absorption) [8]. One more fact is important and should be kept in mind for further discussion the absorption and emission of a photon by a particular molecule are two almost infinitely fast events, but they are separated by a time window of nanoseconds. 

Fig. 1. The principle of pumjvprobe spectroscopy by means of transient two-photon ionization A first fs-laser pulse electronically excites the particle into an ensemble of vibrational states creating a wave packet. Its temporal evolution is probed by a second probe pulse, which ionizes the excited particle as a function of the time-dependent Franck Condon-window (a) shows the principle for a bound-bound transition, where the oscillative behaviour of the wave packet will appear (b) shows it for a bound-free transition exhibiting the exponential decay of the fragmentizing particle, and (c) shows the process across a predissociated state, where the oscillating particle progressively leads into a fragmentation channel.

When a molecule is excited to a continuum state, at least one of the bonds in the molecule will start to stretch. If the molecule is then further excited, before dissociation can occur, the effect is to widen the Franck-Condon window, compared with the ground state, in at least one coordinate. A good example of how this principle can be used to explore the higher excited states of molecules is provided by work on CH3I. 

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