Ultrafast relaxation ground electronic states

Figure VE-1 Ultrafast relaxation of photoexcited carriers into polaronic states with their associated midgap electronic states. We show schematically the band diagrams and corresponding absorption spectra for a non-degenerate ground state polymer.

Following the time-resolved results of FuP et al in the previous section, we shall presently describe computational efforts to describe and explain the ultrafast relaxation phenomena and dynamics inferred by experimental study. We shall focus on two paradigm systems of chemical importance, namely Cr(CO)5 and Fe(CO)5 (both 18 electron complexes). The photodissociation, and subsequent ultrafast relaxation to the singlet electronic ground state surface are fundamental to the photochemistry of these species. 

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

Electron phonon interactions induce two distinct processes in QDs dephasing and relaxation. Superposition of electronic states, created by the coulomb interaction during photoexdtation and subsequent time evolution, dephases into incoherent mixtures of states. Dephasing is ultrafast if it involves electronic states with substantially different energies and spatial densities. Examples of this include superposition of SEs and MEs and ground and excited states. ME fission into independent SEs occurs by dephasing that is much slower. This is because MEs are typically formed by SEs that are close in energy. 

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... 

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