Absorption Spectra in the Visible Range

Among the types of structural information that can be obtained are the determination of the local site symmetry and ionization state for transition metal or rare earth ions present as dilute impurities in optically transparent host crystals. This type of study is important not only as a measure of the strength and precise symmetry of the internal crystalline field operated typically by a polyhedron of anions or a contained cation, but it also provides data on the influence on multiplet splittings, and a measure of the broadening of certain transitions as a result of excitations to orbitals of different mean radial extension. Such information has an immediate practical application in the evaluation of host crystals for optically pumped lasers. It has also 

Pure alkali halide crystals are uncolored but it is possible to color them by heating in alkali vapor or by radiation damage. F centers (electrons trapped at anion vacancies) and other light-absorbing defects are introduced in this manner. Orientations of the aspherical electron distribution in F centers can be detected by observing the change in resonant absorption with the plane of polarization of light. 

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Examination of the absorption spectra, in the visible range, of alcoholic extracts of the reaction-products revealed that that from tryptophan was the simplest. Hence it appears that tryptophan is the most satisfactory readily-available -indole derivative for use in the estimation of 2-desoxy-ribose and probably also of other 2-desoxypentoses. 

The spectral shifts of the iron complexes of transferrins occurring upon modification of amino groups were particularly prominent with succinylated derivatives. Absorption spectra in the visible range for succinylated chicken ovotransferrin are seen in Fig. 17. The changes were a shift in the maximum from 465 mp to shorter wavelengths, and an increase in absorption at approximately 400 mp. Similar but slightly different spectral shifts have been obtained by treatment of metal free transferrins with hydrogen peroxide (82) (Komatsu and Feeney, 1966 un-... 

A possible interpretation of these observations can be done taking into account features of optical absorption for this system described earlier [4]. A key point of the absorption spectra in the visible range was shown to be the excitonic maxima, which are characteristic for tellurides and their solid solutions after the heat treatment. Occurrence of excitons is more probable in tellurides and Te-rich solid solutions possessing a smaller Bohr exciton radius than in selenides. The heating promotes phase transitions in nanoparticles (chalcopyrite-sphalerite). In the selenides the transition can appear at higher temperatures. Thus, the appearance of PL maxima we can associate with excitons in the nanoparticles. 

Despite the fact that optical applications require thin films of poly(3-alkylthiophene)s, the photochemistry of these materials has been characterized in solution but only scarcely in the solid state. The UV/Vis spectra of these films of poly(3-butylthiophene) show an absorption band in the visible range corresponding to a n—n transition whose energy depends on 7r-electron delocalization. 

For Cu+ (d °) and open-shell metal ions, the absorption spectra in the visible spectral range are totally different, and weak bands (e of the order of 10 M cm ) are found up to 600-700 nm. Cu.5+ exhibits the expected wide MLCT absorption (see above), whereas the bands observed for Ni-+ (d ) and Co + (d ) are assigned to metal-centered (MC) or ligand-to-metal-charge-transfer (LMCT) transitions. Finally, the Pd + (d ) catenate has to be considered a special case since it is actually a... 

When the substrate or the reaction product or both has a characteristic absorption spectra in the visible or ultraviolet ranges, it is possible to calculate the correction factor by measurement of the optical density variations in identical experimental conditions at two selected wavelengths. For example at 334 nm, NAD+ does not show any absorption, whereas... 

In order to follow progress of elimination, reactions were also performed on thin films in a special sealed glass cell which permitted in situ monitoring of the electronic or infrared spectra at room temperature (23°C). Typically, the infrared or electronic spectrum of the pristine precursor polymer film was obtained and then bromide vapor was introduced into the reaction vessel. In situ FTIR spectra in the 250-4000 cm-- - region were recorded every 90 sec with a Digilab Model FTS-14 spectrometer and optical absorption spectra in the 185-3200 nm (0.39-6.70 eV) range were recorded every 15 min with a Perkin-Elmer Model Lambda 9 UV-vis-NIR spectrophotometer. The reactions were continued until no visible changes were detected in the spectra. 

Fluoride is also used as an anionic dopant. An early study demonstrated that F substitutes surface OH species leading to an increase in the degradation of phenol at least three times faster than an undoped sample [56]. Other anions are also effective as dopants in reducing the Ti02 bandgap. Chloride, for example, was shown to give active photocatalysts in the visible range thanks to the red shift in the absorption spectra (Eg = 3 eV) but also increased surface acidity [57]. 

The ESA spectra of this series of A-n-A dyes are shown in Fig. 20. They exhibit broad and intense bands in the visible range (400-600 nm for G37,400-630 nm for G38,450-630 nm for G74, and 450-700 nm for G152) and weak bands in the NIR as revealed in Fig. 20 for G38. We observe that lengthening of the conjugation chain leads to a 30-40 nm red shift of the ESA peaks, which is similar to the behavior of D-rc-D polymethine dyes. This red shift ( 30-40 nm) is much smaller than for the linear absorption bands ( 100 nm). Another experimental feature is connected with the redistribution of the ESA magnitude from the shorter to the... 

The ESA spectra of asymmetrical dyes in toluene are shown in Fig. 25. They show broad structureless bands in the NIR region (750-1,100 nm for G19, 850-1,100 nm for G40, and 950-1,100 nm for G188) and more intense transitions in the visible range (400-550 nm for G19, 400-600 nm for G40, and 450-650 nm for G188). Similarly to symmetrical anionic polymethine dyes (Fig. 20), the increase of conjugation length leads to a small red shift of ESA spectra, and to an enhancement of ESA cross sections and the ratio between the ESA and linear absorption oscillator strengths by approximately a factor of two. More detailed experimental description and quantum-chemical analysis can be found in [86]. 

Figure E4.3 shows the room temperature absorption spectra of an insulator (LiNbOs), a semiconductor (Si), and a metal (Cu). (a) Determine the spectrum associated with each one of these materials, (b) From these spectra, estimate the energy-gap values of Si and LiNbOj and the plasma frequency of Cu. (c) What can be said about the transparency in the visible range for each of these materials ... 

Spectroscopy provides a window to explain solvent effects. The solvent effects on spectroscopic properties, that is, electronic excitation, leading to absorption spectra in the nltraviolet and/or visible range, of solutes in solution are due to differences in the solvation of the gronnd and excited states of the solute. Such differences take place when there is an appreciable difference in the charge distribution in the two states, often accompanied by a profonnd change in the dipole moments. The excited state, in contrast with the transition state discussed above, is not in equilibrium with the surrounding solvent, since the time-scale for electronic excitation is too short for the readjustment of the positions of the atoms of the solute (the Franck-Condon principle) or of the orientation and position of the solvent shell around it. 

Comparison of the absorption spectra of stilbene (Fig. 15.2c) and azobenzene (Fig. 15.2 d) shows another interesting feature. The replacement of the two double-bonded carbon atoms by two nitrogen atoms leads to additional, intensive n—>n transitions. In this case, the additional absorptions lay in the visible wavelength range (i.e., above 400 nm, see Table 15.2). Substituted azobenzenes are widely used dyes, and it has been recognized for quite some time that large amounts of such compounds enter the environment (Weber and Wolfe, 1987). 

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