Franck-Condon excitation

Absorption of a photon by an alkene produces a (tt,Jt ) vertical (Franck-Condon) excited state in which the geometry of the ground state from which it was formed is retained. Since the (it,it ) state has no net n bonding, there is little barrier to free rotation about the former double bond. Thus, relaxation takes place rapidly, giving a nonvertical (it,it ) state with a lower energy and different geometry to the vertical excited state. 

Dissociation at a surface appears to be analogous to dissociation in the gas phase. The impinging electron causes a Franck-Condon transition to an electronic state which subsequently dissociates. This one-dimensional Franck-Condon excitation model is illustrated schematically in Fig. 31. The cross section for the electronic transition is probably comparable to gas phase excitation processes. After excitation the particle, which is now in a repulsive state, begins to move away from the surface. If it has sufficient energy it may escape from the surface. If not the fragments remain adsorbed. Moreover, radiationless de-excitation may occur... 

S0 is the unperturbed vapour phase energy level of the molecule in the ground state, in solution it is depressed by an amount proportional to R0. On excitation, the molecule is promoted to the Franck-Condon excited te (FC). In the excited state, the dipole moment may not only have a... 

At perpendicular geometries, this excited state of charge-separated nature usually exhibits an energy minimum.2,7,14,35 Thus, a molecule, which starts off in the planar conformation by Franck-Condon excitation, can spontaneously relax in the excited state by a twisting motion toward a... 

Franck-Condon excitation of ethylene and subsequent relaxation... 

The effects of increased solvent polarity on the light absorption energy from the ground to the (Franck-Condon) excited state can be hypsochromic, if the ground state is more stabilized than the excited state or bathochromic if the opposite relative stabilization takes place. 

Having, thus demonstrated the delocalized nature of the Franck-Condon excited states, it was a challenge to probe experimentally the dynamics of energy transfer in the DNA helices. 

The ensemble of the experimental results briefly reviewed here, e.g. steady-state absorption and fluorescence spectra, fluorescence decays, fluorescence anisotropy decays and time-resolved fluorescence spectra, allow us to draw a qualitative picture regarding the excited state relaxation in the examined polymeric duplexes. Our interpretation is guided by the theoretical calculation of the Franck-Condon excited states of shorter oligomers with the same base sequence. 

Fig. 11 Illustration of the excited state relaxation derived from experimental results obtained for poly(dA).poly(dT) by steady-state absorption and fluorescence spectroscopy, fluorescence upconversion and based on the modeling of the Franck-Condon excited states of (dA)io(dT)io. In red (full line) experimental absorption spectrum yellow circles arranged at thirty steps represent the eigenstates, each circle being associated with a different helix conformation and chromophore vibrations.

FIGURE 3. (a) Potential energy diagram for a diatomic molecule, illustrating the Franck-Condon excitation, (b) Intensity distribution among vibronic bands as determined by the Franck-Condon principle. (Modified from Ref. 2.)... 

Although the transient spectrum of a radical-ion pair was recorded in a picosecond-nanosecond time domain in the flash photolysis of Co(III) alkylcobalamins,123 there has been no direct spectroscopic observation of the LMCT excited states. The observed photochemical behavior may be simply described by the following sequence of events (1) absorption of radiation produces a Franck-Condon excited state, which (2) rapidly loses its excess vibrational energy (k > 10ns ) to form the thermalized excited state, followed by (3) product formation and internal conversion to the ground state. The existence of LMCT excited states with a finite lifetime in the... 

Figure 13-8. Relevant schematic potential energy curves for the near UV photophysics of the most stable tautomers of guanine. The region of the first singlet tht excited state surface accessible by Franck-Condon excitation is indicated in bold. Excited state internal conversion through a conical intersection (Cl) with S0 is illustrated by curved arrows. Vertical arrows indicate fluorescence emission. The eventual role of excited nir singlet states cannot be ruled out, especially at high energy excess in the excited state (see text)...

Dipolar solute in a polar solvent. Since the ground-state solvation results largely from dipole-dipole forces in this case, there is an oriented solvent cage around the dipolar solute molecules, resulting in a net stabilization of their ground state. If the solute dipole moment increases during the electronic transition the Franck-Condon excited... 

Again, the excited state referred to here is the Franck-Condon excited state, which has a solvent shell identical to that of the ground state. 

When considering the solvent dependence of the position of emission bands, the finite relaxation time tr for the rearrangement of the solvent molecules surrounding the solute molecule in the Franck-Condon excited state and the finite lifetime Te of the molecule in the excited state have to be taken into account. 

In the case of tr > Ts the emission will occur before any rearrangement of solvent molecules in the solvation shell takes place. The initial state of the emission process is the Franck-Condon excited state and the final state is the equilibrium ground state. Hence, the wavenumber of emission will be equal to the wavenumber of the corresponding absorption. In the case of tr Te (c/ Fig. 6-7) , reorientation of the solvent molecules can take place after electronic excitation and a relaxed excited state is obtained in which another solvation equilibrium has been established. It is from this equilibrium state that fluorescence occurs at room temperature. By analogy, there is a... 

Fig. 6-7. Effect of solvent reorientation in the excited state on the fluorescence band of a dipolar molecule with dipole flip on excitation. S[ and Sq are the Franck-Condon excited and ground states, respectively Si and Sq are the corresponding equilibrium states tr < Te.

The comparison of the Mfiu values with those of Mabs allows one to obtain information about the changes in the electronic structure and molecular conformation between the Franck-Condon excited state initially reached upon excitation and the solvent-equilibrated fluorescent state [14]. Electronic transition dipole moments are mainly determined by the direct interactions between the lowest CT state and the ground state (So), and by the contributions from the locally excited configurations [14, 54, 56, 57]. For example, for the fluorescent CT state one can obtain... 

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