Initial excited state geometry

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

A possible isomerization pathway for the = 6 cluster has been proposed on the basis of the quantum chemical calculations of Conbariza et al. [59] and Kim et al. [33]. In the predicted most stable structure for the ground state [59], the 1 ion lies on the surface of a V-shaped solvent network. It has been assumed that the initially excited state has a similar geometry, in which the excess electron is weakly bound by the net dipole moment of the solvent network. The supposed form after isomerization resembles the stable half-cage structure for the water hexamer anion [33], in which the excess electron is confined by dangling hydrogens of waters. 

In this appendix69 some details and results of calculations of the Forster transfer in the planar geometry is presented, the donor being a semiconductor quantum well (QW), while the surrounding organic material plays the role of acceptor (7), (8). Three different possibilities are considered, corresponding to the initial excited state in the semiconductor being a free Wannier-Mott exciton, a localized Wannier-Mott exciton, or a dissociated electron-hole pair. 

Reoptimization of the geometry of each excited state from the initial ground-state geometry leads to the results summsirized in columns 2 to 3 of Table 7.8. The A state optimizes smoothly... 

Direct irradiation leads to isomerization via singlet-state intermediates. The isomerization presumably involves a twisted singlet state that can be achieved from either the cis- or the trans-isomer. The temperature dependence of the isomerization further reveals that the process of formation of the twisted state involves a small activation energy. This energy is required for conversion of the initial excited state to the perpendicular geometry associated with the state. Among the pieces of... 

The calculation of UV/vis spectra, or any other form of electronic spectra, requires the robust calculation of electronic excited states. The absorption process is a vertical transition, i.e. the electronic transition happens on a much faster timescale than that of nuclear motion (i.e. Bom-Oppenheimer dynamics, more correctly referred to as the Franck-Condon principle in the context of electronic spectroscopy). The excited state, therefore, maintains the initial ground-state geometry, with a modified electron density corresponding to the excited state. To model the corresponding emission processes, i.e. fluorescence or phosphorescence, it is necessary to re-optimize the excited-state nuclear geometry, as emission in condensed phases generally happens from the lowest vibrational level of the emitting excited state. This is Kasha s Rule. 

At the instant of excitation, only electrons are reorganized the heavier nuclei retain their ground-state geometry. The statement of this condition is referred to as the Fmnck-Condon principle. A consequence is that the initially generated excited state will have a non-minimal-energy geometry. 

This time period is too short for a change in geometry to occur (molecular vibrations are much slower). Hence the initially formed excited state must have the same geometry as the ground state. This is illustrated in Figure 1.2 for a simple diatomic molecule. The curves shown in this figure are called Morse curves and represent the relative energy of the diatomic system as a... 

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