Energy Exchange with Electronically Excited Molecules

16 Energy Exchange with Electronically Excited Molecules 16.1 VET Energy Transfer 

It seems to follow from the experimental data on vibrational relaxation of electronically excited molecules that, as a rule, the vibrational (and rotational) relaxation of these, molecules is faster than that of the same molecules in the ground electronic state. There are several reasons for it. 

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Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

Summarizing the results obtained up to now with semiconductor electrodes and excited dye molecules one can say that it is possible by using different semiconductors to get an approximate information on the energy position of the electron exchange orbitals of excited molecules in solution. In future it should become possible to study photochemical reactions with other reactants by competition with the electrode reaction (quenching for instance) and to analyse in this way... 

In recent years, the first applications of DFT to excited electronic states of molecules have been reported. In the so-called time-dependent DFT (TDDFT) method, the excitation energies are obtained as the poles of the frequency-dependent polarizability tensor [29], Several applications of TDDFT with standard exchange correlation functionals have shown that this method can provide a qualitatively correct description of the electronic excitation spectrum, although errors of the order of 0.5 eV have to be expected for the vertical excitation energies. TDDFT generally fails for electronic states with pronounced charge transfer character. 

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