Excitation, electronic classifications

The photovoltaic effect is initiated by light absorption in the electrode material. This is practically important only with semiconductor electrodes, where the photogenerated, excited electrons or holes may, under certain conditions, react with electrolyte redox systems. The photoredox reaction at the illuminated semiconductor thus drives the complementary (dark) reaction at the counterelectrode, which again may (but need not) regenerate the reactant consumed at the photoelectrode. The regenerative mode of operation is, according to the IUPAC recommendation, denoted as photovoltaic cell and the second one as photoelectrolytic cell . Alternative classification and terms will be discussed below. 

It will be realized that the values of n and m of A will depend on the metal site symmetry and n will only have even values for states of the same parity. In a frequently overlooked paper Eisenstein [554] tabulated the symmetry classifications of the metal ion and ligand orbitals for most of the point group site symmetries of interest. These classifications are often very useful in constructing a molecular orbital energy diagram. Predictions regarding the number and classification of the excited electronic states can then easily be made with the help of such diagrams. We will, however, resist the temptation to reproduce those tables here, in order to conserve space, as they are easily available. 

If the ir - 77 transition leads to a non-planar structure the first excited electron configuration (31), in which the excited states are labelled by the D2h species classification,... 

Properties of excited electronic states of polyatomic molecules may be conveniently classified as stationary or nonstationary. A comparison of the present knowledge about properties in these two classifications is interesting. Theoreticians and spectroscopists have been rather successful in developing good descriptions of energy levels and geometries of excited states, and hence the stationary type is in a comparatively advanced state of development. For example, the spectroscopic work of... 

Experimentally, by XPS surface analysis involves irradiation on the solid under vacuum with X-ray photons and emitted by electron classification of energy. The sample is prepared in ultrahigh vacuum environment (pressure <10 Torr) and excited by a beam of X-rays with magnesium or aluminum anode. Since the mean free path of electrons is very small, the detected electrons originate from the outermost atomic layers of the material analyzed, giving superficial sensitivity technique. 

There is a third method proposed for classifying colorants which is in terms of the mechanism of the electronic excitation process. According to this method, organic colorants may be classified as donor acceptor, polyene, cyanine or n-rc chromogens. While this method of classification is undoubtedly of importance theoretically, it is arguably of lesser practical importance, since the vast majority of commercial organic dyes and... 

CLASSIFICATION OF MOLECULAR ELECTRONIC TRANSITIONS AND EXCITED STATES... 

Figure 11.10 Symmetry classification and correlation of orbitals for the disrotatory closure of butadiene. Closure with electron pairs remaining in their original levels would lead to the excited state indicated by the orbital occupancy on the right.

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

The interpretation of d-d spectra requires the description of excited states arising from the reallocation of electrons to orbitals, relative to the ground state. CF theory makes it possible - using well-established methods that were developed c. 1930 for the classification of atomic states... 

B. Vibrational Structure of Electronic Transitions 1. Normal vibrations and their symmetry classification An electronic band system belonging to a polyatomic molecule normally contains a large number and variety of transitions in which vibrational quantum changes are superimposed on the electronic jump. The analysis, besides supplementing infrared and Raman evidence of the ground state frequencies, yields values for the fundamental frequencies of the excited state and is one of the principal sources of information as to its structure. 

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