Mutual absorption

It is clear from Figure 7.18 that progressions and sequences are not mutually exclusive. Each member of a sequence is also a member of two progressions. However, the distinction is usefiil because of the nature of typical patterns of bands found in a band system. Progression members are generally widely spaced with approximate separations of cOg in absorption and co" in emission. In contrast, sequence members are more closely spaced with approximate separations of cOg — co". 

The phase-twisted peak shapes (or mixed absorption-dispersion peak shape) is shown in Fig. 3.9. Such peak shapes arise by the overlapping of the absorptive and dispersive contributions in the peak. The center of the peak contains mainly the absorptive component, while as we move away from the center there is an increasing dispersive component. Such mixed phases in peaks reduce the signal-to-noise ratio complicated interference effects can arise when such lines lie close to one another. Overlap between positive regions of two different peaks can mutually reinforce the lines (constructive interference), while overlap between positive and negative lobes can mutually cancel the signals in the region of overlap (destructive interference). 

Peaks in homonuclear 2D /-resolved spectra have a phase-twisted line shape with equal 2D absorptive and dispersive contributions. If a 45° projection is performed on them, the overlap of positive and negative contributions will mutually cancel and the peaks will disappear. The spectra are therefore presented in the absolute-value mode. 

Similarly, the first-order expansion of the p° and a of Eq. (5.1) is, respectively, responsible for IR absorption and Raman scattering. According to the parity, one can easily understand that selection mles for hyper-Raman scattering are rather similar to those for IR [17,18]. Moreover, some of the silent modes, which are IR- and Raman-inactive vibrational modes, can be allowed in hyper-Raman scattering because of the nonlinearity. Incidentally, hyper-Raman-active modes and Raman-active modes are mutually exclusive in centrosymmetric molecules. Similar to Raman spectroscopy, hyper-Raman spectroscopy is feasible by visible excitation. Therefore, hyper-Raman spectroscopy can, in principle, be used as an alternative for IR spectroscopy, especially in IR-opaque media such as an aqueous solution [103]. Moreover, its spatial resolution, caused by the diffraction limit, is expected to be much better than IR microscopy. 

In systems where several carotenoids are involved, the absorption of each carotenoid is governed by interactions among them carotenoids compete for absorption (Furr and Clark 1997). For example, (3-carotene supplementation reduced absorption of dietary lutein and lycopene in humans (Micozzi and others 1992). Tyssandier and others (2002) found that the absorption of dietary lycopene was reduced when a portion of spinach or pills of lutein were additionally administered to the volunteers. Similarly, the absorption of dietary lutein was reduced by consumption of tomato puree or lycopene pills (Tyssandier and others 2002). Furusho and others (2000) demonstrated that liver retinol accumulation in Wistar rats was significantly reduced when a fixed amount of (3-carotene was replaced by a mixture of (3- and a-carotene, suggesting that each one of these carotenoids mutually inhibits the utilization of the other. The proportion of (3-and a-carotene in the mixture used in that study (Furusho and others 2000) simulated that of carrots. 

Students will be familiar with the absorption or emission of electromagnetic radiation as the basis for spectroscopic methods. Electromagnetic radiation itself is perceived as mutually perpendicular oscillating electric and magnetic fields. The total energy of the radiation, which has a number of components, is determined by the relationship shown in equation 3.1 ... 

Absorption bands arising from adjacent protons are split into multiplet peaks by a mutual interaction of the spins. The effect is due to small variations in the effective field experienced by a proton when neighbouring nuclei can occupy two or more energy levels or spin states. It is transmitted through the intervening bonds by a tendency for electron and nuclear spins to be paired. 

Consider the case of two single (methine) protons HA and Hx attached to adjacent carbon atoms and with quite different chemical shifts (Figure 9.32(a)). The field experienced by HA is increased or decreased slightly by the two allowed spin states of Hx, designated and l, and which in the gross sample are virtually equally populated. This results in the absorption band for HA splitting into a doublet whose peak intensities are in the ratio 1 1. The effect is mutual in that the two almost equally populated spin states of HA cause the Hx absorption to split into an identical doublet. The spacing... 

However, the best approximation is found by using the intersection point of the mutually normalized absorption and emission spectra (as was the practice in Forster s laboratory). In the case of large Stokes shift, it may be difficult to determine an intersection point it is then preferable to take the average of the wave-numbers corresponding to half-heights of the absorption and emission bands, which is a better approximation than the average of the wavenumbers corresponding to the maxima. 

In multicomponent systems A"0 can be written as a sum of the individual absorption coefficients A ot = 2TA , where each AT,(A ) depends in a different way on the wavelength. If one or more of the components are fluorescent, their excitation spectra are mutually attenuated by absorption filters of the other compounds. This effect is included in Eqs. (8.27) and (8.28) so that examples like that of Figure 8.4 can be quantified. The two fluorescent components are monomeric an aggregated pyrene, Mi and Mn. The fluorescence spectra of these species are clearly different from each other but the absorption spectra overlap strongly. Thus the excitation spectrum of the minority component M is totally distorted by the Mi filter (absorption maxima of Mi appear as a minima in the excitation spectrum ofM see Figure 8.4, top). In transparent samples this effect can be reduced by dilution. However, this method is not very efficient in scattering media as can be seen by solving Eqs. (8.27 and 8.28) for bSd — 0. Only the limit d 0 will produce the desired relation where fluorescence intensity and absorption coefficient of the fluorophore are linearly proportional to each other in a multicomponent system. 

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