Fluorescence yield mode

Because of its very nature (photons in and photons out using transmission or fluorescence yield mode), XAS can be applied in situ under general conditions, as opposed to XPS, which requires ultra-high vacuum (UHV) conditions. 

Are the XAFS data going to be collected in transmission or fluorescence yield mode By the nature of the experiment transmission cells are simpler in design than those for fluorescence. The complexity increases further if the cell has to be capable of allowing both transmission and fluorescence XAFS spectra to be collected. 

Micro-XANES were obtained for Cu and Mn on the same beamline with the same experimental set-up as for the XRF The Fe spectra were collected as a single scan in fluorescence yield mode at selected points within the cross-section... 

The a-diketones show both phosphorescence and fluorescence emission, not only in a glass at 77°K but also in fluid solutions at room temperature, a property which has made these compounds useful in energy transfer studies.25-28 Fluorescence, however, is quite weak with intersystem crossing and decay through the triplet state being the principle mode of decay. The absolute fluorescence yields of biacetyl and benzil in solution are reported to be 0.22% and 0.27%, respectively, while the measured phosphorescence yield... 

XANES spectroscopy has been used to study the composition and mechanism of antiwear tribofilm formation. The absorption XANES spectra were recorded in total electron yield (TEY) versus fluorescence yield (FY) detection to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. The application of XANES surface TEY mode which analyzes the top 5 nm layer, and the FY technique which analyzes the 50 nm layer of the bulk, taken together, give a marvelous opportunity to study nondestructively the antiwear tribofilms. Both techniques can be used under a wide variety of conditions e.g., the formation of tribofilms at different rubbing times, load, concentrations, temperatures and surface roughness (Kasrai et al., 1993 and 1996 Koningsberger and Prins, 1988 Martin et al., 2001 Yin et al., 1997a). 

As can be seen from Fig. 10, the Auger M line intensity becomes, through the mode 32, large for large ROR probability or for small AE defined in Fig. 2. This leads, in the first approximation, to reduction in the fluorescence yield Yxi and then also reduction in the intensity ratio of Xl/XO as shown in Fig. 11, though the fluorescence yield is, strictly speaking, a function both of Auger and X-ray yields. Thus chemical bond effects in the satellite intensities of the XI lines in the F Ka fluorescent X-ray spectra emitted from the alkali-metal fluorides have now successfully been explained. 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

Fluorescent photon counting is an alternative to electron yield (EY) [41]. The strongest advantage of fluorescence yield (EY) is that the mean free path of photons through solids is much longer than that of electrons. Therefore, EY mode reveals x-ray absorption from deeper into the film bulk ( 100 mn) than EY modes can measure (=10 nm). A comparison of EY and FY NEXAFS spectra can reveal surface... 

Figure 19. Is NEXAFS spectra of metal free (left), and Ni containing porphyrines (top). Spectra shown on the top are collected in electron yield mode, and the spectra shown on the bottom are collected using fluorescence. Spectrum of 2-methylbenzimidazole is shown for a comparison (lower spectrum in bottom figure). One of the distinct features between the metalated and metal-free porphyrine is the absence of peak at 401.1 eV in the former. [Used with permission from Vairavamurthy and Wang 2002. and Mitra-Kirtley et al. 1993. American Chemical Society.]...

Other nuclear parameters, such as decay mode probability (e.g. X % p, y % EC), internal conversion coefficients, fluorescence yields and transition mnltipolarities. 

It has been empirically established [2] that the presence of torsional modes (intramolecular rotations about bonds) accelerates Sq IC in dyes. For example, the dye phenolphthalein has essentially zero fluorescence yield, while fluorescein has 90% yield (Fig. 9.6) The torsional modes in phenolphthalein are frozen out by the presence of the oxygen bridge in fluorescein. In the absence of such torsional modes, most of the <- Sq electronic energy difference in IC is... 

A core-ionized atom has two possibilities to lower its energy, namely Auger decay and X-ray fluorescence (described in more detail in Chapter 7). The Auger yields for processes following core hole creation in the K and L shell are sketched in Fig. 3.25 (right). Obviously, Auger processes are the dominant decay mode in light elements. 

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