Excitation energy quantum chemical calculation

Bases stacked rather than hydrogen bonded have also been studied with quantum chemical methods [182, 244-247]. The nature of excited states in these systems has been debated and theoretical calculations are called to decide on the degree of excited state localization or delocalization, as well as the presence and energy of charge transfer states. The experimentally observed hypochromism of DNA compared to its individual bases has been known for decades [248], Accurate quantum chemical calculations are limited in these systems because of their increased size. Many of the reported studies have used TDDFT to calculate excited states of bases stacked with other bases [182, 244, 246, 247], However, one has to be cautious when us-... 

The timescales of population dynamics associated with the energy funnel were determined with pump-probe measurements with laser spectra centered at different wavelengths, so that different sets of pigments could be initially excited. From the pump-probe results analysis combined with quantum chemical calculations, the model of energy transfer depicted in panel (c) of Fig. 5 can be proposed. 

The measurements were carried out using polarized-light from synchrotron radiation. The angle-resolved UPS spectra were recorded for specific directions of photon incidence, photon polarization, and electron exit, chosen in order to resolve the momentum dependence of the 7t-electron energy bands which could be observed in this experiment. Details are available elsewhere63. The UPS results are analysed not only with the help of the valence effective Hamiltonian (VEH) method, but also with the help of new quantum-chemical calculations based upon the excitation model method64. The full VEH band structure is shown in Fig. 7.32. 

The quantum chemical modeling is a very useful supplement to spectroscopic experimental methods for investigation of properties of point defects, however, until recently it was used mainly for calculations of vertical excitation energies. The modeling of structural transformation in excited electronic states is still a rather complicated task, which requires state-of-the-art quantum chemical calculations. In this chapter, we first describe theoretical methods applied in ab initio and vibronic theory calculations and then demonstrate their applications in theoretical studies of various point defects in silica and germania. 

Table 7.9. Quantum-chemical calculations of the energies (eV) of the lowest electron-excited states for silicon-centered radicals3...

The vibrationally excited precursor AB/s/(fs) can decay not only via energy transfer to the bulk but also via a chemical transformation (desorption of B and reaction with the formation of D and C/s/). These chemical processes can be characterized by the chemical lifetime Tch, which can be estimated in the framework of the statistical RRKM theory (see, e.g., Refs. [50, 51]) using the reaction parameters of reagents B and A/s/, precursor AB/s/, and transition complexes determined based on the results of quantum-chemical calculations. Such estimates were performed for many reactions of interest for the growth of metal oxide films [20]. It appeared that in the wide temperature range... 

The quantum chemical calculations on Fe(CO)5 indicate that the lowest-energy accessible excited state is MC in character. The oscillator strength for the ground state to MC transition is small and most ultrafast experiments use either single or multiphoton excitation to MLCT states. The difference in product distribution depending on the excitation pulse duration points to an enhanced absorption cross section for the Fe(CO)5 excited state over the ground-state species to both pump and probe pulses. This tends to complicate the apparent photochemistry. Consequently the use of short-pulse single-photon excitation provides a better picture of the excited-state dynamics. 

Fig. 25. Quantum-chemical calculated potential energy curves for the ground singlet (So), electronically excited singlet (Sp, and excited triplet (rp states of the tetrahedral and octahedral complexes of vanadium oxide [reproduced with permission from Kazanskii (/.J2)].

More recently, Kohler et al. [218] presented an experimental study coordinated with quantum chemical calculations of the wavefunctions of higher lying excited states i.e. higher lying states at energies above the lowest (1B ) excited state. They noted two important points ... 

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