Electronic excitation Franck-Condon principle

The excited state has the same geometry of the nuclei as the ground state a short time after excitation (Franck-Condon principle). The molecule immediately starts to relax toward the new equilibrium geometry by descending the vibrational levels without radiation and, at the same time, heating the probe. This process is referred to as internal conversion. Internal conversion also includes decay from higher excited electronic states down to the first excited singlet state (Sj). 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Even where the promotion is to a lower vibrational level, one that lies wholly within the 2 curve (such as Vi or V2), the molecule may still cleave. As Figure 7.2 shows, equilibrium distances are greater in excited states than in the ground state. The Franck-Condon principle states that promotion of an electron takes place much faster than a single vibration (the promotion takes... 

Electronic transitions in a solute take place very fast, i.e., almost immediately in comparison with the movement of the molecules as a whole and vibrations of atoms in organic molecules. Hence, absorption and fluorescence can be denoted in Fig. 5 by vertical arrows, in accordance with Franck-Condon principle. Both these processes are separated by relaxations, which are intermolecular rearrangements of the solute-solvent system after the excitation. 

Fig. 21. Top The general Jablonski diagram for the flavin chromophore. The given wavelengths for absorption and luminescence represent crude average values derived from the actual spectra shown below. Due to the Franck-Condon principle the maxima of the peak positions generally do not represent so-called 0 — 0 transitions, but transitions between vibrational sublevels of the different electronically excited states (drawn schematically). Bottom Synopsis of spectra representing the different electronic transitions of the flavin nucleus. Differently substituted flavins show slightly modified spectra. Absorption (So- - S2, 345 nm S0 -> Si,450nm 1561) fluorescence (Sj — S0) 530 nm 156)) phosphorescence (Ty Sq, 605 nm 1051) triplet absorption (Tj ->Tn,...

FIGURE 17.13 An illustration of the Franck-Condon principle. In this case, the transition is from v = 0 in the electronic ground state to the state with id = 3 in the excited electronic state. 

As shown in Fig. 6, there is a correlation between absorption spectrum and emission spectrum. Taking into consideration the Franck-Condon principle, which states that there is no motion of the atoms during an electronic transition, one has to differentiate between the two following possibilities in the one the geometry of the excited state is similar to the one of the ground state (same interatomic distances),... 

A molecule exhibits a great difference in the speeds of electronic transitions and vibrational atomic motions. The absorbtion of photon and a change in the electronic state of a molecule occurs in 10 15—10—18 s. The vibrational motion of atoms in a molecule takes place in 10 1 s. Therefore, an electronically excited molecule has the interatomic configuration of the nonexited state during some period of time. Different situations for the exited molecule can exist. Each situation is governed by the Franck-Condon principle [203,204],... 

In spectroscopy we may distinguish two types of process, adiabatic and vertical. Adiabatic excitation energies are by definition thermodynamic ones, and they are usually further defined to refer to at 0° K. In practice, at least for electronic spectroscopy, one is more likely to observe vertical processes, because of the Franck-Condon principle. The simplest principle for understandings solvation effects on vertical electronic transitions is the two-response-time model in which the solvent is assumed to have a fast response time associated with electronic polarization and a slow response time associated with translational, librational, and vibrational motions of the nuclei.92 One assumes that electronic excitation is slow compared with electronic response but fast compared with nuclear response. The latter assumption is quite reasonable, but the former is questionable since the time scale of electronic excitation is quite comparable to solvent electronic polarization (consider, e.g., the excitation of a 4.5 eV n — n carbonyl transition in a solvent whose frequency response is centered at 10 eV the corresponding time scales are 10 15 s and 2 x 10 15 s respectively). A theory that takes account of the similarity of these time scales would be very difficult, involving explicit electron correlation between the solute and the macroscopic solvent. One can, however, treat the limit where the solvent electronic response is fast compared to solute electronic transitions this is called the direct reaction field (DRF). 49,93 The accurate answer must lie somewhere between the SCRF and DRF limits 94 nevertheless one can obtain very useful results with a two-time-scale version of the more manageable SCRF limit, as illustrated by a very successful recent treatment... 

This idea may be summarized from within the Franck-Condon principle. Because the atomic nuclei are relatively massive and effectively immobile, the transition is from the ground state to the excited state lying vertically above it. We say that the electronic excitation is vertical, which explains why the arrow drawn on Figure 9.13 is vertical. 

The multiplicity of excitations possible are shown more clearly in Figure 9.16, in which the Morse curves have been omitted for clarity. Initially, the electron resides in a (quantized) vibrational energy level on the ground-state Morse curve. This is the case for electrons on the far left of Figure 9.16, where the initial vibrational level is v" = 0. When the electron is photo-excited, it is excited vertically (because of the Franck-Condon principle) and enters one of the vibrational levels in the first excited state. The only vibrational level it cannot enter is the one with the same vibrational quantum number, so the electron cannot photo-excite from v" = 0 to v = 0, but must go to v = 1 or, if the energy of the photon is sufficient, to v = 1, v = 2, or an even higher vibrational state. 

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... 

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. 

We have seen how the position and intensity of is affected by the energy difference between the electronic energy levels. The Franck-Condon principle states that electronic transitions involve the movement of electrons, including those of the solvent, but not the movement of atoms. When the solvent electrons can rearrange to stabilize the excited state of a molecule, the energy difference between the electronic levels of the molecule is lowered and the absorption moves to higher wavelength. 

The chemistry of the excited states of molecules induced by light absorption in the visible and ultraviolet range is the normal realm of photochemistry. Because of the great rapidity of internal conversion processes in which highly excited electronic states are converted to lower electronic states with the energy difference distributed among the various vibrational modes as dictated by the Franck-Condon principle, the photo-... 

By absorbing excited radiation the electrons are raised from the ground state to the excited state. These transitions take place so rapidly that no displacement of the atomic nuclei occurs (Franck-Condon principle). The space coordinate thus remains unchanged and the transitions can be represented by vertical lines. Because the excited system is not immediately in a state of equilibrium after absorption of energy, it first moves towards the lowest vibrational level with loss of energy to the lattice... 

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