Intensities of absorption bands

For thin-film samples, abrupt changes in refractive indices at interfrees give rise to several complicated multiple reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quantitative work in thin films. Note, however, that these interferences are functions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useful information beyond that of chemical bonding potentially may be extracted from the FTIR speara. 

The IR spectra of thin films from an alternating copolymer of tri-n-butyl-stannyl methacrylate and maleic anhydride at various stages of UV irradiation were also studied l09). It can be expected that cross-linking results in hindered rotational and oscillatory mobility of the backbone chain and of side chains in the macromolecule. Indeed, a decrease in the peak intensity of most absorption bands is observed in the IR spectra of an irradiated copolymer 1770 and 1840 cm-1 (vc=0 anhydride), 1406 cm-1 (Sq, in CH2—Sn), 1285 and 1080 cm-1 (vc 0 c), 1115, 850, 750cm-1, etc. At the same time, there is an increase in the intensity of absorption bands near 1720 (vc=0 in — COOSnR3) and 1580 cm-1 (vc Q in -C=Q... SnRa). 

Figure 4 illustrates the positions and relative intensities of absorption bands given by functional groups when D 0 is the solvent. Although the bylk Fe-0-Fe and/or 6(Fe-OHJ vibration peaks at 895 and 800 cm arethe same, three new bands.appear (1188-1219, 2256, and 2604 cm ). The band at 3145 cm caused by bulk OH stretching is now in closer agreement with the position and... 

In order to explore this point, it is necessary to have estimates for the magnitude of V. One approach to estimating V is based on the integrated intensity of absorption bands for optically induced electron transfer as for the transition illustrated in equation (9).16 From this type of analysis V a 120 cm-1 for the particular case shown in equation (9).15 A second approach to estimating V, which is purely theoretical and based on orbital overlap calculations, typically gives values in the range 100-200 cm-1 for outer-sphere reactions.12 36 37... 

Fig. 19. Intensity of absorption bands of pyridine chemisorbed on protonic and Lewis acid sites (148).

Several factors affect intensities of crystal field spectra. In addition to enhancement by increased temperature and pressure discussed in chapter 9 ( 9.4), intensities of absorption bands depend on first, the spin-state or number of unpaired electrons possessed by a transition metal ion second, whether or not the cation is located at the centre of symmetry of a coordination site and third, interactions with next-nearest-neighbour cations. 

Intensities of absorption bands are governed by probabilities of electronic transitions between the split 3d orbital energy levels. The probabilities are expressed by selection rules, two of which are the spin-multiplicity selection rule and the Laporte selection rule. 

Table 3.6. Relative intensities of absorption bands in transition metal-bearing...

Intracrystalline Fe2+-Mg2+ distributions in natural and synthetic orthopyroxenes have been determined from intensities of absorption bands in their polarized spectra (Goldman and Rossman, 1977a Steffen et al., 1988). Molar extinction coefficients of crystal field bands centred at 10,500 to 11,000 cm-1 and 4,900 to 5,400 cm-1 originating from Fe2+ ions located in pyroxene M2 sites ( 5.5.4) enabled the iron contents to be estimated from the Beer-Lambert law equation, eq. (3.7). 

Optical absorption spectroscopy No intervalence transitions in visible region One mixed-valence transition in visible region absorption bands intensify at elevated pressures and low temperatures One or more mixed-valence transitions in visible region temperature lowers intensity of absorption bands, pressure intensifies Opaque metallic reflectivity in visible region... 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

Covalency of ligands clearly influences the positions and intensities of absorption bands in crystal field spectra of oxides and silicates, so that it is pertinent to discuss the types of covalent bonds that exist when transition elements are present in mineral structures. In this section, the more qualitative aspects of molecular orbital theory are described. 

Figure 5 shows the dose dependence of normalized optical density at the wavelength of 200, 215 and 275 nm. The intensities of absorption band around 200 and 215 nm increase with increasing absorption dose in linear form, while that around 275 nm is nearly constant. The results may indicate that these radicals and unsaturated bond contribute to the increase of electrical conductivity for the irradiated samples. 

This change in the charge density in the series mentioned above is also seen from the partial moments of the C—H bond ranging from —0.4 D (H negative) to + 0.4D (H positive), as deduced from the intensities of absorption bands in the infrared (Mecke). ... 

Empirical correlation of intensities of absorption bands with the structure of complexes in solutions have been made for lanthanide complexes. It has been recognized that forced electric-dipole transitions of low intensities, in some cases lower in intensity than those of magnetic-dipole transitions, may indicate that the ligand field has point group symmetry with a center of inversion. This criterion has been used in the determination of the ligand field by symmetry of Eu3+ aquo ion [202], The absorption band intensity ratios have been used to show the octahedral structure [49] of lanthanide hexahalide complexes, LnXg. ... 

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