Main valence band transitions

The theoretical studies on imidazole are scarce. A recent MRCI study by Machado and Davidson [137] considered valence and Rydberg states together for the first time. On the other hand, the available experimental spectra of imidazole are limited to methanol, ethanol, and aqueous solutions and it is difficult to establish the effects of the solvent on the different transitions. The structure of the computed gas-phase spectrum differ from the spectra in a solvated medium. An unambiguous assignment of the main valence bands in the spectrum of imidazole thus requires consideration of such effects. The reaction field method, explained above, was employed with the CASSCF/CASPT2 method to analyze the influence of the solvent on the valence states of... 

In the following, we will discuss a number of different adsorption systems that have been studied in particular using X-ray emission spectroscopy and valence band photoelectron spectroscopy coupled with DFT calculations. The systems are presented with a goal to obtain an overview of different interactions of adsorbates on surfaces. The main focus will be on bonding to transition metal surfaces, which is of relevance in many different applications in catalysis and electrochemistry. We have classified the interactions into five different groups with decreasing adsorption bond strength (1) radical chemisorption with a broken electron pair that is directly accessible for bond formation (2) interactions with unsaturated it electrons in diatomic molecules (3) interactions with unsaturated it electrons in hydrocarbons ... 

Optical transitions between the valence and conduction bands are responsible for the main absorption band and are the primary measure of the band gap energy. The optical data are also used to extract information about the band tail density of states. However, the absorption coefficient depends on both conduction and valence band densities of states and the transition matrix elements and these cannot be separated by optical absorption measurements alone. The independent measurements of the conduction and valence state distributions described in Section 3.1.1 make it possible to extract the matrix elements and to explore the relation between N E) and the optical spectrum. 

Examples of the low temperature luminescence spectra are shown in Fig. 8.12. The luminescence intensity is highest in samples with the lowest defect density and so we concentrate on this material. The role of the defects is discussed in Section 8.4. The luminescence spectrum is featureless and broad, with a peak at 1.3-1.4 eV and a half width of 0.25-0.3 eV. It is generally accepted that the transition is between conduction and valence band tail states, with three main reasons for the assignment. First, the energy is in the correct range for the band tails, as the spectrum lies at the foot of the Urbach tail (Fig. 8.12(6)). Second, the luminescence intensity is highest when the defect density is lowest, so that the luminescence cannot be a transition to a defect. Third, the long recombination decay time indicates that the carriers are in localized rather than extended states (see Section 8.3.3). 

In the present report, three subjects are to be reviewed (i) The triplet structure of excitons and the upper valence band (ii) the full set of the fundamental optical functions in the 0-30 eV energy range for polarizations E II c, E J. c, and their theoretical analysis and (iii) the main parameters of the elementary transverse and longitudinal transition components in the 0-30 eV energy range for polarizations E c, E L c, and their theoretical analysis. 

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