X-ray absorption measurements

X-ray absorption measurements were made on several of the more... 

Comparative x-ray absorption measurements were used in the identification of various new compounds that could contain at most the following elements carbon, hydrogen, fluorine, and chlorine. The presumed composition of each compound, known in advance, was duplicated by properly blending carbon tetrachloride, benzotrifluoride, heptane, and benzene the latter also was used as solvent for the unknown. Under conditions intended to be identical, the amount of unknown... 

When the x-ray absorption measurements were made, nothing was known about the samples except that AOT-1 and B62M-1 were the only two base stocks involved. The sixteen values of equivalent thickness obtained were interpreted substantially as in Table 3-4 by those who supplied the samples. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

X-ray Absorption Spectra (XAS). X-ray absorption measurements were performed at station 9.2 of the SRS at Daresbury (UK) with an electron beam energy of 2 GeV and a stored current varying between 290 and 160 mA. The wiggler was operational at 5.0 Tesla. Data were collected in the transmission mode from 11.37 keV to 13.43 keV (Pt Li -edge 11.564 keV, Pt Vedge 13.273 keV) with a Si (220) monochromator detuned to 50 % of the maximum intensity... 

Early stages of copper electrodeposition and coadsorption of chloride on the Au(lll) electrode surface have been studied by Wu et al. [390] applying electrochemical methods and in situ X-ray absorption measurements. The results indicate a large degree of static disorder and exclude the presence of high-symmetry structures. Krznaric and Goricnik [391]... 

G.B. Kistiakowsky P.H. Kydd, JChemPhys 23, 271-74 (1955) (The density profile of the rarefaction wave which follows a gaseous detonation was detd by using the technique of x-ray absorption measurements to follow rapid gas density changes)... 

There is no valid interpretation for the activation by OJ and by hexacyano-ferrate(III), although they fitted nicely in a reaction scheme with Cu(III) as the active species In the oxidation of an alcohol to an aldehyde Cu(III) would be reduced to Cu(I). In the subsequent reaction of Cu(I) with Oj, Cu(II)Oj was considered an intermediate yielding Cu(III) and H O. This intermediate would be in a reversible equilibrium with OJ and with the resting Cu(II)-enzyme. This resting enzyme would be oxidized by hexacyanoferrate(III) to the active Cu(III) species. There was unfortunately no indication in X-ray absorption measurements for the formation of Cu(III) with hexacyanoferrate(III) and the resting enzyme . EPR measurements indicated that Cu(II) was present in the active enzyme It was not possible, moreover, to detect Oj by the reduction of Fe(III)-cytochrome c in a galactose oxidase — galactose system... 

The situation is the same in /-Ge. Although X-ray absorption measurements [292] and TB MD calculations [293] have been carried out for supercooled /-Ge, L-L transitions have not yet been firmly confirmed. 

X-ray diffraction data were obtained using Rigaku RAD-C with a copper X-ray tube in air atmosphere. X-ray absorption measurements of Cu K-edge were performed with laboratory EXAFS equipment (Technos EXAC-820). The X-ray source with a rotating Mo target and a LaBe filament was operated at 17 kV, 100 mA (EXAFS) and 20 kV 150 mA (XANES). The samples were pressed into wafers with methyl cellulose as a binder. The measurements were carried out in air atmosphere at room temperature. EXAFS Fourier transformations were carried out over the ranges of photoelectron wave vector, k, of 2.5 -10.0 A 1. 

Fig. 2. Schematic diagram of X-ray absorption measurements by (A) transmission and (B) secondary particle (photons, electrons, ions) detectors. From Brown and Doniach (48a).

The X-Ray absorption measurements were carried out on freshly in situ sulfided catalysts at liquid nitrogen temperature at the SRS in Daresbury (EXAFS station 9.2). The sulfidation was carried out in a... 

X-ray absorption measurements of fine stmcture in solution (EXAFS)... 

The interaction between the molecule and the d-electrons of the substrate system is more obvious in the case of NO interaction with NiO. The molecule is tilted with respect to the surface normal as is experimentally evident from X-ray absorption measurements [59] as well as photoelectron diffraction investigations... 

Our own X-ray absorption measurements of Rh-Ti compounds exhibit suggestive similarities with the reduced Rh/Ti02 catalysts. First of all, as described above, the EXAFS analysis of high-temperature-reduced Rh/Ti02 (100% dispersed) catalyst provides evidence for the formation of... 

Aluminum is to be used as windows for a cell for X-ray absorption measurements with the Ag Ka line. The mass absorption coefficient for aluminum at this wavelength is 2.74 its density is 2.70 e/em. What maximum thickness of aluminum foil could be used to fabricate the windows if no more than 3.5% of the radiation is to be absorbed by them ... 

The determination of precise oxidation states has been troublesome because of the extensive spin coupling within the FeMoco. The molybdenum atom in M is thought to be low-spin Mo(IV), based on ENDOR and X-ray absorption measurements. Assigning the iron oxidation states as Mo N6Fe +/lFe + (or Mo +/5Fe +/2Fe +) is more consistent with Fe ENDOR measurements. However, a Mo +/4Fe +/3Fe + formulation better rationalizes the Mossbauer isomer shifts. All of the oxidation state conclusions, as well as ab initio evaluations of the oxidation state problem, need to be re-evaluated using a model of FeMoco with the central X. 

The X-ray absorption measurements are analogous to optical absorption or fluorescence excitation measurements. The spectrometer consists of a source of X-rays, a monochromator, an X-ray detector which monitors the incident flux, Iq, a sample compartment, and a second detector which monitors the flux of X-rays transmitted through the sample, /,. The sensitivity of the technique is extended several orders of magnitude by fluorescence detection of the absorption spectrum (FIIq). This enhanced sensitivity is essential to the measurement of X-ray absorption spectra of biological systems. The fluorescence detector is placed at right angles to the incident beam. In addition, a third X-ray detector is used downstream from /, to measure simultaneously the spectrum of a reference compound, which provides an energy reference. The essential components described... 

The quantity of both, fluorescent photons and Auger electrons, is directly related to the number of core holes created, i.e., to the absorption, and is thus a direct measure of the absorption. In cases where the usual X-ray absorption measurement in transmission is not feasible, determination of the intensity of X-ray fluorescence or of Auger electrons as a function of the energy of the primary X-ray beam will give absorption spectra. 

The solution level is kept at the upper limit during electrochemical experiments, providing complete participation of the working electrode surface in any electrochemical process. In order to minimize solution absorption of the X-rays, the level is lowered before X-ray absorption measurements. As the lower edge of the working electrode is still immersed in the electrolyte solution, electrode potential control is maintained. 

Figure 3.28 Mn K-edge X-ray absorption measurements as MnAPO-18 is calcined in oxygen. The change in edge position edge shift at 450 °C corresponds to the oxidation of framework manganese cations from 2+ to 3 +. [Figure courtesy of G. Sankar.]...

According to an X-ray powder analysis by Rieger and Parthe (1969b) TbOujSij is isotypic with the ThOrjSij-type of structure [a = 3.980(6), c = 9.966(10)]. For sample preparation, see YOu2Si2. For X-ray absorption measurements, see Padalia et al. (1983). 

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