Inter-valence transitions

There are many compounds containing two or more transition-metal atoms in different oxidation states, which have a range of interesting properties. Inter-valence compounds have been classified according to the ease of electron transfer between the distinct sites. In Class I compounds, electron transfer between the centers is slow, and the sites behave as if they were effectively independent. In Class II species, the centers perturb each other but the sites remain distinct, while in Class III species the sites are indistinguishable and electron delocalization is effectively complete. In Class II compounds inter-valence change-transfer (IVCT) transitions are possible, giving bands in their electronic spectra that can be very intense. For example, the 

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Because of the different nature of those transitions, different photochemical products are formed depending on the excitation wavelength. Excitation into the inter-valence transition at 700 nm does not initiate any photoreactivity. However, irradiation with light <450 nm populates an MLCT state, resulting in transient oxidation of the metal centre, reduction of the N2 molecule, and dinitrogen splitting, with quantum yields 0.002 and 0.003 under 254 nm or 365 nm irradiation respectively. 

In terms of this sort of classification, electronic transitions can be of five types (a) ligand-based transitions, (b) metal-based transitions, (c) metal-to-ligand transitions, (d) ligand-to-metal transitions and (e) inter-valence transitions. These will be considered in turn in the following sections. 

Unlike metals, semiconductors and insulators have bound valence electrons. This aspect gives rise to inter band transitions. The objective of this and the next section is... 

Illumination of a semiconductor electrode generates excited electrons in the conduction band and holes in the valence band. Some of them recombine with each other radiatively, resulting in emission of luminescence called photoluminescence (PL). The radiative recombination may occur directly between the conduction and valence bands (inter-band transition) or via certain impurity or defect levels within the band gap at which either electrons or holes, or both are trapped, as schematically illustrated in Fig. 5(a). Also,... 

The luminescence mainly originates from the inter-band transitions, which are divided into direct and indirect transitions according to the transition modes. If the electrons jump at the same point between the VBM (valence band maximum) and the CBM (conduction band minimum), this transition is direct. In contrast, there is indirect transition. The semiconductors silicon (Si) and gallium arsenide (GaAs) are typical examples as shown in Fig. 6.6. They have an indirect and direct band gap with the values 1.95 and 0.17 eV, respectively. When the crystal size becomes smaller, e.g., forming quantum dots, the Si becomes a better self-activated luminescence material. 

The electromagnetic theory is based on the assumption that electrons are allowed to move freely being excited by an external field. Contrary to that, the positively charged ion cores are immobile. Thus, it is clear from this precondition that metals with free electrons exhibit the strongest optical effects and are best suited to study the electronic properties of resonant particle plasmons. These metals have partially filled conduction bands but completely filled valence bands. Ideally free electron metals response is based on the conduction electrons whereas in reality most metals exhibit various inter-band transitions. 

In this chapter, we have studied a list of inter- and intra-row main group (five, six, seven and eight valence electrons species) as well as few transition metal compounds. We attempted to explain their unusual shortness on the basis of their detailed electronic structures. 

Intense efforts in the last decade have exhaustively mapped the electronic and superconducting properties of intercalated alkali fullerides and the occurrence of the metal-antiferromagnetic insulator transition as a function of inter -fullerene separation, orientational order/disorder, valence state, orbital degeneracy, low-symmetry distortions and metal-C60 interactions [6-12]. 

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