Fine structure determination

Figure 4.8. Least-squares BC for molecular spectra with rotational fine structure determination of Pb in the BCR 186 Pig Kidney CRM at 217.001 nm using HR-CS ET AAS and direct solid sample analysis (a) absorbance over time and wavelength after correction for continuous absorption (b) reference spectrum absorbance over wavelength integrated over time for NH4H2P04 (the dotted line represents the center pixel) (c) absorbance over time and wavelength after subtraction of the reference spectrum using least-squares BC.

Kelly SD, Kemner KM, Fein JB, Fowle DA, Boyanov MI, Bunker BA, Yee N (2002) X-ray-absorption fine-structure determination of pH-dependent U-bacterial cell wall interactions. Geochim Cosmochim Acta 66 (in press)... 

Ebert M, Mair V, Tessadri R, Hoffmann P, Ortner HM (2000) Total-reflection X-ray fluorescence analysis of geological microsamples. Spectrochim Acta 55 205-212 Eisenberger P, Lengeler (1980) Extended X-ray absorption fine-structure determination of coordination numbers limitations. Phys Rev 22 3551-3562... 

EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

Zaera F, Fischer D A, Carr R G and Gland J L 1988 Determination of chemisorption geometries for complex molecules by using near-edge X-ray absorption fine structure ethylene on Ni(IOO) J. Chem. Rhys. 89 5335-41... 

The discovery of the phenomenon that is now known as extended X-ray absorption fine structure (EXAFS) was made in the 1920s, however, it wasn t until the 1970s that two developments set the foundation for the theory and practice of EXAFS measurements. The first was the demonstration of mathematical algorithms for the analysis of EXAFS data. The second was the advent of intense synchrotron radiation of X-ray wavelengths that immensely facilitated the acquisition of these data. During the past two decades, the use of EXAFS has become firmly established as a practical and powerfiil analytical capability for structure determination. ... 

Fine structure extending several hundred eV in kinetic energy below a CEELS peak, analogous to EXAFS, have been observed in REELS. Bond lengths of adsorbed species can be determined from Surface Electron Energy-Loss Fine Structure (SEELFS) using a modified EXAFS formalism. 

M. De Crescenzi. Phys. Rev. Letts. 30,1949,1987. Use of surface electron energy-loss fine structure (SEELFS) to determine oxygen-nickel bond length changes for oxygen absorbed on Ni (100) on a function of coverage from 0 to 1.0 monolayer. 

Sealed conventional fine structure tubes with Mo, W, Cu, or Cr anodes are used as primary X-ray sources, as well as rotating anode tubes, or synchrotron radiation. The maximum energy of the X-ray quanta determines the range of elements acces-... 

Br20 a dark-brown solid moderately stable at —60° (mp —17.5° with decomposition), prepared by reaction of Bt2 vapour on HgO (cf. CI2O p. 846) or better, by low-temperature vacuum decomposition of BrOa. The molecule has C2v symmetry in both the solid and vapour phase with Br-O 185 1pm and angle BrOBr 112 2° as determined by EXAFS (extended X-ray absorption fine structure). It oxidizes I2 to I2O5, benzene to 1,4-quinone, and yields OBr in alkaline solution. 

Another use for this solvent is exemplified by 1,4,5,8-tetraazanaph-thalene, the anhydrous species of which has a predicted i Ka value of — 2.7 (the observed pA in water is + 2.51). The spectrum obtained in anhydrous dichloroacetic acid is almost identical with that of the predominantly anhydrous neutral species determined in water, but quite different from the spectrum measured in dilute aqueous acid. Moreover, addition of water to the anhydrous dichloroacetic acid solution of this base caused the fine structure present in the spectrum of the neutral species to disappear and the band due to the hydrated cation (i.e. the spectrum obtained in water at pH 0.5) to appear. Addition of water to dichloroacetic acid solutions has been used to show that the cations of 3- and 8-nitro-l,6-naphthyridine20 are hydrated in aqueous acid at pH 0.5. 

Contrary to widespread opinion, the value of Ea is not a constant quantity. As was proved previously [52], the value of E is variable, since it depends on the ordering of macromolecules in the amorphous material of the fiber. At the same time, one can suppose that this ordering will be affected by the specificity of the fine structure of the fiber, and particularly by the type of substructure of the fiber. The relationship determining the modulus Ea appropriate for a definite type of fiber substructure can be derived from Eq. (11) when appropriate values of A are assumed. In the case of the microfibrillar substructure, i.e., for A < I, typical of PET fibers stretched, but not subjected to annealing, this equation has the form [52] ... 

Fibers of a diversified draw ratio in the range 2.0-5.2 X were considered, determining the following parameters of their fine structure the crystalline and amorphous orientation functions,and/a, degree of crystallinity, X, and critical dissolution time (CDS) in seconds. The results obtained are listed in Table 11. 

PCu(ci,q) is clearly not a 5-function as has been suggested. Many more LSMS calculations would have to be done in order to determine the structure of Pcn(ci,q) for fee alloys in detail, but it is easier to see the structure in the conditional probability for bcc alloys. The probability Pcu(q) for finding a charge between q and q-t-dq on a Cu site in a bcc Cu-Zn alloy and three conditional probabilities Pcu(ci,q) are shown in Fig. 6. These functions were obtained, as for the fee case, by averaging the LSMS data for the bcc alloys with five concentrations. The probability function is not a uniform function of q, but the structure is not as clear-cut as for the fee case. The conditional probabilities Pcu(ci,q) are non-zero over a wider range than they are for the fee alloys, and it can be seen clearly that they have fine structure as well. Presumably, each Pcu(ci,q) can be expressed as a sum of probabilities with two conditions Pcu(ci,C2,q), but there is no reason to expect even those probabilities to be 5-functions. 

Another difficulty with the infrared method is that of determining the band center with sufficient accuracy in the presence of the fine structure or band envelopes due to the overall rotation. Even when high resolution equipment is used so that the separate rotation lines are resolved, it is by no means always a simple problem to identify these lines with certainty so that the band center can be unambiguously determined. The final difficulty is one common to almost all methods and that is the effect of the shape of the potential barrier. The infrared method has the advantage that it is applicable to many molecules for which some of the other methods are not suitable. However, in some of these cases especially, barrier shapes are likely to be more complicated than the simple cosine form usually assumed, and, when this complication occurs, there is a corresponding uncertainty in the height of the potential barrier as determined from the infrared torsional frequencies. In especially favorable cases, it may be possible to observe so-called hot bands i.e., v = 1 to v = 2, 2 to 3, etc. This would add information about the shape of the barrier. 

The X-ray absorption fine structure (XAFS) methods (EXAFS and X-ray absorption near-edge structure (XANES)) are suitable techniques for determination of the local structure of metal complexes. Of these methods, the former provides structural information relating to the radial distribution of atom pairs in systems studied the number of neighboring atoms (coordination number) around a central atom in the first, second, and sometimes third coordination spheres the... 

The ionization being accompanied by a vibrational excitation, the fine structure of bands can be exploited for determination of vibrational levels of an ionized system in the ground and excited states. Of course, the first (0-0) and the strongest vibrational bands are the most important because they determine adiabatic and vertical ionization potentials of radicals. 

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