Negative absorption

The MCD spectra of furan and pyrrole are similar and quite different to those of the foregoing heterocycles. Both are soft chromophores and give rise to negative absorptions. 

The n t effect of the presence of other elements is conveniently assessed by comparing the intensity of the analytical line in their presence with the intensity calculated from Equation 7-1. The net effect may be to increase the intensity over that calculated (positive), or to decrease it (negative). Individual effects may result from the following causes (1) Presence of an element with absorption coefficient smaller than that of E positive absorption effect). (2) The reverse of this situation negative absorption effect). (3) Presence of an element a characteristic line... 

Fig. 7—2. Spectral data to illustrate absorption and enhancement effects for three transition elements. (To avoid crowding, only part of the cobalt absorption curve is shown.) See Table 7-1. Case B. Substitution of A1 for Fe decreases absorption of incident beam and has little effect on analytical line. Net positive absorption effect. Case C. Substitution of Pb for Fe decreases absorption of primary beam but greatly increases absorption of analytical line. Net negative absorption effect. Case D. Note wavelength relationship indicated in figure. Enhancement impossible. Case E. Note wavelength relationship in figure. Enhancement occurs.

The earlier discussion (7.3) leads one to expect absorption and enhancement effects in these high-temperature alloys. For example, the results for nickel will be very sensitive to the iron content. Nickel Ka has a wavelength about 0.1 A shorter than that of the iron K edge (Figure 7-2), and a slight increase in the iron content consequently means an increased absorption of nickel Ka, which tends to reduce the apparent nickel content (negative absorption effect). Furthermore, this increased absorption of nickel Ka will enhance iron Ka, whence an... 

Figure 8-3 is explained as follows. With an increasing content of alumina, the intensity of silicon Ka decreases progressively below the value to be expected from Equation 7-1. The deviation is a negative absorption effect that can be calculated according to Equation 7-6. The intensity of aluminum Ka, on the other hand, behaves quite differently as the silica content increases. This intensity then increases... 

It will be clear that EMIRS and SNIFTIRS spectra are difference spectra and can be somewhat complex ( ). Typically they will contain positive absorption bands from species present in excess at potential El compared to potential E2 and negative absorption bands from species whose polulation changes oppositely with potential. In addition, bands which shift with potential will appear as a single bipolar band either with one lobe of each sign, figure 2, (or even more complex structures with three lobes). 

Four different laboratories have built IR kinetic spectrometers for use with organometallic compounds. A fundamental feature of all these spectrometers is that the detector is AC coupled. This means that the spectrometers only measure changes in IR absorption. Thus, in the time-resolved IR spectrum, bands due to parent compounds destroyed by the flash appear as negative absorptions, bands due to photoproducts appear as positive absorptions, and static IR absorptions, due to solvents, for example, do not register at all. The important features of these spectrometers are listed in Fig. 2. Since three spectrometers have a line-tunable CO laser as the monochromatic light source, we begin with the CO laser. Then we look in more detail at spectrometers designed for gas phase and solution experiments. 

In a celebrated paper, Einstein (1917) analyzed the nature of atomic transitions in a radiation field and pointed out that, in order to satisfy the conditions of thermal equilibrium, one has to have not only a spontaneous transition probability per unit time A2i from an excited state 2 to a lower state 1 and an absorption probability BUJV from 1 to 2 , but also a stimulated emission probability B2iJv from state 2 to 1 . The latter can be more usefully thought of as negative absorption, which becomes dominant in masers and lasers.1 Relations between the coefficients are found by considering detailed balancing in thermal equilibrium... 

A problem encountered with atomic absorption is that emission from the flame may fall on the detector and be registered as negative absorption. This can be eliminated by modulating the light source, either mechanically or electronically, and using an a.c. detector tuned to the frequency of modulation of the source. D. C. radiation, such as emission from the flame, will then not be detected. A high intensity of emission, however, may overload the detector, causing noise fluctuations. 

As yet, the only work that has been reported on a single crystalline surface has been done by Hoffmann and Robbins . They used the same approach as Burrows eta/. to study the methanation reaction (CO-I-3H2- CH4 + HjO) on a Ru(001) surface. Figure 21 shows the spectra of the C—O stretch mode taken at 50 torr of a mixture of CO and Hj at temperatures up to 600 K. The decrease in peak height at SOO K indicates the buildup of a passivating carbon layer. The negative absorption around 2140 cm is due to imperfect canceling of the gas phase CO band. 

Fig. 19, an unapodized spectrum [response function (sin nx)/nx = sinc(x)] is shown in trace (b). For such a spectrum there will be sidelobes and negative absorption if the natural linewidths are narrower than the full width of the sine-shaped response function. These are seen in Fig. 19, where the linewidth is three points and the response function width eight points. Here the phrase instrument response function may have a slightly different definition, but the meaning is clear. For such a response function, the direct deconvolution methods fall short. 

Figures 26 and 27 show the results of using deconvolution to remove the sidelobes present in a Fourier transform spectrum. These two spectral sections are taken from the v2 absorption band of ammonia at about 848 and 828 cm-1, respectively. The unapodized spectrum exhibits a resolution of 0.125 cm-1, and is shown in trace (a). The spacing of the lines in Fig. 26 is such that the sidelobes when added together partially cancel, minimizing their effect. Sidelobes and the apparent negative absorption between the lines are both still present. The spacing of the lines in Fig. 27 is such that the sidelobes add constructively, accentuating their effect and producing...

Fig. 26 Fourier transform spectrum of v2 of ammonia. Trace (a) is a section of the infrared absorption spectrum of ammonia recorded on a Digilab Fourier transform spectrometer at a nominal resolution of 0.125 cm-1. In this section of the spectrum near 848 cm-1 the sidelobes of the sine response function partially cancel, but the spectrum exhibits negative absorption and some sidelobes. Trace (b) is the same section of the ammonia spectrum using triangular apodiza-tion to produce a sine-squared transfer function. Trace (c) is the deconvolution of the sine-squared data using a Jansson-type weight constraint.

H/ H-ROESY (-NOESY) Cross peak.s and diagonal peaks appear in absorption. The diagonal peaks are the most intense and are best suited for phase adjustments. They should be phased for negative absorption in ROESY spectra and in NOESY spectra measured for small molecules, giving cross peaks in positive absorption in both cases. For NOESY spectra of large molecules (e.g. biomolecules), both diagonal and cross peaks should be phased to positive absorption. 

After a short period of use in the average engine, changes start to occur. Initially, a loss of the zinc based antiwear/antioxidant additive ZDDP is observed by negative absorptions at 1000 cm 1 and 715 cm 1. Oxidative degradation of oil follows soon after and this is observed by positive absorptions, represented by carbonyl, hydroxy, nitro and C-O- species. The ER spectroscopy of lubricants can reflect additive depletion and the formation of oxidation products (Coates and Setti, 1984 Coates etal., 1984). 

Figure 6.18 Transient absorption spectra, uncorrected for luminescence, observed after 532 nm excitation of a [Ru(deebpy)(bpy)2]2+-modified Ti02 film in neat CH3CN under argon. The apparent negative absorption change observed beyond 570 nm is due to emission. This part of the spectrum is included to illustrate the correspondence between absorption and luminescence kinetics. Reprinted with permission from C. A. Kelly, F. Farzad, D. W. Thompson and G. J. Meyer, Langmuir, 15, 731 (1999). Copyright (1999) American Chemical Society...

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