Absorption and excitation spectra

FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much more sensitive than the ultraviolet method but tends to be prone to error and is less convenient to use. Small quantities of impurities may lead to serious distortions of both emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and fluorescence excitation spectra can yield useful information on the purity of an FBA. Different samples of an analytically pure FBA will show identical absorption and excitation spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been developed for the quantitative estimation of FBAs and other fluorescent additives present on a textile substrate. The procedure was demonstrated by measuring the fluorescence intensity at various excitation wavelengths of moving nylon woven fabrics treated with various concentrations of an FBA and an anionic sizing agent. It is possible to detect remarkably small differences in concentrations of the absorbed materials present [67]. 

In the case of total transfer (T = 1), these two spectra are identical after normalization at the same height. But for any value of x less than 1, the excitation band corresponding to the donor is relatively lower than the absorption band. The comparison of the absorption and excitation spectra can be done at two wavelengths 7p and 7a corresponding to the absorption maxima of the donor and the acceptor, respectively. If there is no absorption of the donor at 7A, we get... 

Figure 7.23 Absorption and excitation spectra of Rhodamine G in ethanol, (a) Corrected excitation (open circles) compared with absorption (full line), (b) Uncorrected excitation spectrum. Vertical axes, fluorescence intensity in arbitrary units...

Table 3. Energies [cm 1 ] of the lowest excited states as observed in absorption and excitation spectra of crystalline samples...

Figure 7 Electronic absorption (---) and excitation (-) spectra offac-ClRe(CO)3...

Note that, there are two kinds of spectra—namely, excitation and absorption. The absorption and excitation spectra are distinct but usually overlap,... 

The absorption and excitation spectra of 4f" 4f"-15d transitions in lanthanides have been... 

The Terbium Chloride-Aluminum Chloride Vapor System. I. Absorption and Excitation Spectra, J.A. Caird, W.T. Camall, J.P. Hessler, and C.W. Williams, J. Chem. Phys. 74, 798-804 (1981). 

For the solutions containing Trp, riboflavin, ethidium bromide, and ANS alone, normalize the absorption and excitation spectra and superimpose them. We suggest that the normalization be carried out at 280, 450, and 280 nm, for L-Trp, riboflavin, and ANS, respectively. 

From comparisons of the absorption and excitation spectra for the oxides, as shown in Table I (66) it appears that the energy decreases with an increase in the cation size from Mg to Ba in the alkaline earth metal cation series. This pattern has been satisfactorily explained by using the approach of Levine and Mark (84), whereby ions located on an ideal surface are considered to be equivalent to the bulk ions, except for their reduced Madelung constants. A more detailed analysis has been carried out by Garrone et al. (60, 79), who reinterpreted earlier reflectance spectra and suggested that there is evidence of three absorption bands corresponding to ions in live, four, and three coordination—aU three for MgO, CaO, and SrO. 

The third example is the light-harvesting complex B800-850 obtained from the wild-type Rb. sphaeroides 2.4.1. This complex contains three BChls and one carotenoid (spheroidene) per protein subunit. The fluorescence excitation was obtained by monitoring the emission at 850 nm. Fig. 3 (C) shows the absorption and fluorescence excitation spectra in the 400-620 nm region, with the two spectra normalized at 590 nm (marked with ). The excellent match ofthe absorption and excitation spectra indicates that photoexcited spheroidene transfers energy to bacteriochlorophyll with a high efficiency. 

An additional piece of evidence regarding the mechanism is the red-shift in the absorption and excitation spectra upon complexation. Complexatlon facilitates the Intermolecular exclmer formation because PEG chains are attracted to the vicinity of PAA and the local pyrene concentration increases. Thus the observed red-shift in the absorption and excitation spectra upon complexation seems to result from the pyrene-pyrene interaction which is preformed intermolecularly as a result of the specific structure of complex. Here also the hydrophobic attraction between chromophores... 

The quantum yield for energy transfer ent for compact and extended nanostars was measured by comparing absorption and excitation spectra normahzed in the perylene absorption region (430-500 nm) (Fig. 5) [45,47]. 

Fig. 6 Absorption and excitation spectra of an a extended nanostar and b compact nanostar [48]...

If tautomeric forms of a substance are present in a solution, discrepancies between the absorption and excitation spectra will be found, provided the tautomers differ in both absorption spectra and quantum yield. The dissociation of the carboxyl group in fluorescein produces changes in absorption and fluorescence spectra and quantum yield as indicated37 on Table 10J. At pH 5-7, where both tautomers are present in appreciable amounts, the absorption and excitation spectra differ appreciably. The change in quantum yield has been shown to be due to a long-lived tautomer and not to quenching... 

Figure 13. The absorption (a), excitation (b), and luminescence spectra (c) of 40-A ammonia-passivated CdS clusters in Nation film. The absorption and excitation spectra were taken at room temperature. Temperatures corresponding to the luminescence spectra are 7.6, 12, 18, 25, 35, 50, 70, 100, 140, 190, 238, and 290 K. (Taken from reference 50 with permission.)...

A fluorometric analysis results in the collection of two spectra, the excitation spectrum and the emission spectrum. The excitation spectrum should be the same as the absorption spectrum obtained spectropho tome trie ally. Differences may be seen due to instrumental factors, but these are normally small, as seen in Fig. 5.45, which shows the absorption and excitation spectra for Alizarin garnet R, a fluorometric reagent for aluminum ion and fluoride ion. The longest wavelength absorption maximum in the excitation spectrum is chosen as the excitation wavelength this is where the first monochromator is set to excite the sample. It would seem reasonable to choose the wavelength that... 

It is important to note that, to increase signal intensity, multiple lanthanide labeling of even small molecules is possible because lanthanide luminescence, in contrast to organic dye fluorescence, [15] does not undergo cmicentratimi quenching. This can be attributed to the lack of overlap between the absorption and excitation spectra due to the large Stokes shift. 

Figure 5.30 presents the excitation spectra of the Xi and X4 lines in the 410-715 nm spectral range at 300 and 100 K. At 300 K, the spectra for both lines are very similar and contain a broad band peaking at 600 nm (Fig. 5.30a) and two narrow lines at 687 and 704 nm (Fig. 5.30b). Another broad excitation band is evidently peaking at approximately 400 nm, which is out of spectral range presently studied. It is certain that there are differences in position of both broad bands between the absorption and excitation spectra (http //minerals.gps.caltech.edu). The differences were previously detected and explained by the contribution of the lower field sites to the absorption spectrum, while the excitation spectmm is due to the highest field site (Wojtowicz 1991 Platonov et al. 1998). The fuU-widths at half-magnitude (FWHM) of the broad excitation bands are approximately...

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