Valence changes vibrational spectrum

Since the slope, E, of the Urbach absorption reflects the shape of the valence band tails, it follows that varies with the structural disorder. For example, one measure of the disorder is the average bond angle variation, which is measured from the width of the vibrational spectrum using Raman spectroscopy (Lannin 1984). Fig. 3.22 shows an increasing E with bonding disorder, which is caused by changes in the deposition conditions and composition (Bustarret, Vaillant and Hepp 1988 also see Fig. 3.20). The defect density is another measure of the disorder and also increases with the band tail slope (Fig. 3.22). A detailed theory for the dependence of defect density on is given in Section 6.2.4. 

A natural development of this work was to study the effects of structural perturbations like conformational changes on the valence bands of stereoisomers. We have considered the four conformations relevant to the interpretation of the vibrational spectrum of polyethylene (J ). The theoretical calculations on those four conformers of polyethylene T, TG, G and TGTG revealed so important differences in the shape of densities of states that... 

In assigning the electronic spectra of transition-metal complexes, the first criteria are the number, positions and intensities of bands. These can be compared with computed predictions, and with the spectra of related species for which assignments are firmly established. In some cases, vibrational progressions can be observed, and analysis of the nature of the vibrations involved can give us an idea of both the symmetry and any changes in stmcture associated with the transition, just as it can for a band in a valence photoelectron spectrum (Section 9.4.2). 

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