Excited states energy surfaces

After UV photoexcitation the DNA and RNA bases return to the electronic ground state at an ultrafast time scale of about one picosecond [1], Their short excited-state lifetimes imply an intrinsic stability against structural photoinduced changes. The characterization of the excited-state energy surfaces by means of stationary points, conical intersections and relaxation paths has been of fundamental importance for the understanding of the mechanisms taking place in the ultrafast deactivation of these bases [2-10], In particular, theoretical investigations have shown the existence... 

At the end of the 1980s improved quantum chemical methods and faster computers became available which were suitable for computing excited state energy surfaces. In 1990 these methodologies were used to investigate the reaction path of the photochemical cycloaddition of ethylene [19,20]. It was shown that... 

The calculation of excitation energies and transition moments for unsaturated organic molecules has been one of the more successful applications of multiconhgurational quantum chemistry since the hrst application to the benzene molecule in 1992 [34]. Many hundred molecules have been studied. The CASSCF method allows optimization of excited state energy surfaces and this has been used to compute vibrationally resolved electronic spectra [35,36]. The method is used by several research groups for studies of photochemical reactions, including the localization of conical intersections, etc. 

Chapter 12 ventures into the realm of photochemistry, where structural concepts are applied to following the path from initial excitation to the final reaction product. Although this discussion involves comparison with some familiar intermediates, especially radicals, and offers mechanisms to account for the reactions, photochemistry introduces some new concepts of reaction dynamics. The excited states in photochemical reactions traverse energy surfaces that have small barriers relative to most thermal reactions. Because several excited states can be involved, the mechanism of conversion between excited states is an important topic. The nature of conical intersections, the transition points between excited state energy surfaces is examined. 

Ground-state and lowest excited-state energy surfaces for stilbene have been calculated by a semiempirical procedure.12 Calculations show that in the first... 

Barbatti M, Paier J, Lischka H (2004) Photochemistry of ethylene a multireference configuration interaction investigation of the excited-state energy surfaces. J Chem Phys 121 11614... 

In an emission spectrum, the light intensity emitted from a probe after absorption is recorded (Figure 12.8). The emission is measured perpendicular to the incoming ray. Due to processes on the excited state energy surfaces that are faster than the emission, the intensity pattern is very different from the absorption spectrum. The fluorescence spectrum is recorded in the energy region where fluorescence can be expected due to the Stokes shift. Other emissions may be due to phosphorescence or charge transfer. 

The quantum chemical calculations are the only source of direct information on the structure and the energetics of transition states. Although these cannot in principle be observed experimentally since the Schrodinger equations has no stationary solutions in the points not corresponding to minima on the PES, one may, according to the Bersuker theorem [28], still envisage an indirect experimental approach to a transition state structure. The fact is that over the saddle point of a PES, which corresponds to a transition state, there always exists such an excited state energy surface which at this point exhibits a minimum. Hence one may attempt to study this transition state by appropriate spectral methods. 

Fig. 8.7 Dependence of the excited-state energy surfaces on a generalized solvent coordinate in cases of strong (A) and weak (B) exciton interactions. Hu and 7/22 (dashed lines) are the excited-state energies of individual molecules, which are assumed to be identical. Eb+ and (solid lines) are the energies of the two exciton states as given by Eqs. (8.9b-8.9d). (Exciton state Fb+ is assumed arbitrarily to have a higher energy than I b- ) Th ground-state energy is off scale at the bottom. In the units of energy used for the ordinate scale, H12 is 0.4 in (A) and 0.04 in (B)...

Fig. 11.23B) is determined by the fastest component in the time domain, which is the initial drop in (X(O)IX(O) near t = 0. In the semiclassical wavepacket model, the speed of this drop depends on the steepness of the excited-state energy surface at the point where the wavepacket is created [132],... 

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