Fluorescent yield

Zaera F, Fischer D A, Shen S and Gland J L 1998 Fluorescence yield near-edge X-ray absorption spectroscopy under atmospheric conditions CO and FI2 coadsorption on Ni(IOO) at pressures between 10 and 0.1 Torr Surf. Sc/. 194 205-16... 

The sensitivities of particular spectroscopic teclmiques to specific chemical features are described more fully in tire next section. Perhaps tire most common and versatile probes of reaction dynamics are time-resolved UV-vis absorjDtion and fluorescence measurements. Wlren molecules contain cliromophores which change tlieir stmcture directly or experience a change of environment during a reaction, changes in absorjDtion or fluorescence spectra can be expected and may be used to monitor tire reaction dynamics. Altliough absorjDtion measurements are less sensitive tlian fluorescence measurements, tliey are more versatile in tliat one need not rely on a substantial fluorescence yield for tire reactants, products or intennediates to be studied. 

Kolubayev T, Geacintov N E, Paillotin G and Breton J 1985 Domain sizes in chloroplasts and chlorophyll-protein complexes probed by fluorescence yield quenching induced by singlet-triplet exciton annihilation Biochimica Biophys. Acta 808 66-76... 

For holes in the /th shell, the fraction of the holes that result in x-rays when that hole is filled with an outer electron is called the fluorescent yield, CO, for example COj and CO. The quantity CO has been computed theoretically, but the best values come from a simultaneous evaluation of the measured and theoretical values. The value of COj varies smoothly with the atomic number Z, and the fluorescence yields for each L subsheU are smaller than the COj at the same Z. Table 14 gives values of the K and shell binding energies, COj, CO, and relative emission probabiUties of the and Kp x-rays as a function of... 

Table 14. Selected Values of the K and Electron-Binding Energies, K- and L2-Shell Fluorescent Yields, and /K x-Ray Intensity Ratio ...

Atomic number, Z Binding energy keV Fluorescent yield X-ray intensity ratio, IN/Kb... 

A particular strength of Equation (7) is that the intensity ratio is formed between mea-surements of the same X-ray energy in both the unknown and standard. This procedure has significant advant es First, there is no need to know the spectrometer s efficiency, a value that is very difficult to calibrate absolutely, since it appears as a multiplicative factor in both terms and therefore cancels. Second, an exact knowledge of the inner shell ionization cross section or fluorescence yields is not needed, since they also cancel in the ratio. 

Once an inner shell vacancy is created in an atom the atom may then remrn toward its ground state via emission of a characteristic X ray or through a radiationless Auger transition. The probability of X-ray emission is called the fluorescence yield. 

In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films. In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc. Two general methods used for making these corrections are the empirical parameters method and the fimdamen-tal parameters methods. 

The angular dependence of the fluorescence yield in the ne borhood of the critical angle should be considered in detail to establish the chemical nature of surface impurities, as well as for quantitation in terms of their concentrations (Figure 1). 

With VPD preconcentration, the angular dependence of the impurity fluorescence yield foUows the curve for residue impurities, as shown in Figure 1, in contrast to the plated-impurity case using direct TXRF. 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

A tabulation of the ECPSSR cross sections for proton and helium-ion ionization of Kand L levels in atoms can be used for calculations related to PIXE measurements. Some representative X-ray production cross sections, which are the product of the ionization cross sections and the fluorescence yields, are displayed in Figure 1. Although these A shell cross sections have been found to agree with available experimental values within 10%, which is adequate for standardless PKE, the accuracy of the i-shell cross sections is limited mainly by the uncertainties in the various Zrshell fluorescence yields. Knowledge of these yields is necessary to conven X-ray ionization cross sections to production cross sections. Of course, these same uncertainties apply to the EMPA, EDS, and XRF techniques. The Af-shell situation is even more complicated. 

Figura 1 Calculated K X-ray production cross sections for protons using the tabulated ECPSSR Ionization cross sections of Cohen end Harrigan, and the fluorescence yields calculated es In Johansson et al. (1 barn h IIT cm l.

Hydrophilic liquids can also cause stabilization and amplification of fluorescence Thus, Dunphy et al employed water or ethanol vapor to intensify the emissions of their chromatograms after treatment with 2, 7 dichlorofluorescein [260] Some groups of workers have pointed out that the layer matenal itself can affect the yield of fluorescent energy [261 —263] Thus, polyamide and cellulose layers were employed m addition to silica gel ones [245] The fluorescence yield was generally increased by a factor of 5 to 10 [264], but the increase can reach 100-fold [234, 265]... 

The Fluorescence Yield. The Auger Effect. Satellite Lines... 

To illustrate the concept of fluorescence yield, we turn again to the K spectrum. Assume that an element is irradiated with an x-ray line energetic enough to excite the K spectrum. If the irradiation is continued, a steady state will soon be reached in which the rate at which holes are produced in the K shell (i.e., the rate at which atoms in the K state are produced) is just balanced by the combined rates of the various processes causing such holes to disappear. Let n1, n2,. . . , % be the individual rates rii at which the filling of holes leads to the production of the i lines in the K spectrum. The fluorescence yield, for this simple case is... 

The fluorescence yield (Equation 1-19) is the fraction of photons absorbed according to Equation 4-12 that will actually appear as Ka lines. The fluorescence yield needed here is the product of wkcco) ( 0.4) and 0.9. The former applies to all K lines, of which only 0.9 are Ka lines, the other 0.1 being K/3. 

In his treatment, Sherman makes use of emission coefficients t that perform the function of the empirical k in Equation 6-4. Both quantities are proportional to. the product of absorption coefficient, of fluorescence yield and of (1 — 1/r), where r is the absorption-jump ratio involved (4.4). 

Table 2. Fluorescence Yields, w, and Absorption-Jump Ratios, r...

By flourescence techniques, it was observed that the fluorescence yield and lifetime of 1,8-anilinonaphthalenesulfonate decrease with an increase in the aqueous core of AOT-reversed micelles, while the position of the emission maximum shifts to longer wavelengths [64], These changes in the electronic properties were attributed to the peculiar effective polarity and viscosity of the micellar core and to their evolution with R. 

The recombination of trapped electrons and holes produces the fluorescence. Adsorbed oxygen scavenges electrons producing O2" which also is adsorbed. OJ is a much better quencher than Oj. Its accumulation under illumination therefore leads to the decrease in fluorescence intensity. During the dark period disappears. During the illumination in the presence of oxygen, the colloid undergoes photoanodic dissolution (see Sect. 3.2). The ZnS particles become smaller in this way, and this finally leads to an increase in fluorescence yield as already described for CdS. 

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