Pre-edge region

The optical absorption of some semiconductors or insulator materials shows a series of peaks or features at photon energies close to but lower than the energy gap (the pre-edge region). These features correspond to a particular type of excitation called... 

FIGURE 1.20 XAS spectra of D4h CuCl42 4 Panel (a) shows the orientation of the X-ray beam with the complex and the angle (cf>) by which the complex was rotated. Insets show a blowup of the pre-edge region and a plot of the pre-edge intensity as a function of cf> ... 

Fig. 19. Resonant scattering, absorption and fluorescence of iodine in a 100 pm foil of polyacetylene, CHJ j, near the Lj-absorption edge (X = 2.72 A) at h = 0.1 The very high dispersion of fluorescence and absorption at the short wavelength side of the absorption edge limits the measurement of resonance scattering to the pre-edge region of the spectrum. ( + ) f (E) S,...

Figure 31. XANES and EXAFS analysis of the surface structure in 1.9-nm nanoparticles of T102. Top XANES analysis focussing on the pre-edge region. Intensification of the features in this region are consistent with distorted or reduced Ti coordination. Bottom EXAFS analysis shows reduced peak area and coordination in the same samples with XANES intensification. Both analyses point to 5-coordinated Ti on particle surfaces. Capping the particles with ascorbic acid removes the distorted sites. After Chen et al. (1999).

Fig. 2 The basic features of a metal K-edge X-ray absorption spectrum highlighted on the spectra of two classical coordination compounds D4h CuCl42 (pale line) and CuCl42- (dark line). The inset shows the pre-edge region is expanded at the bottom [22]...

Figure 9.9 The contributions to a Bragg reflection at three wavelengths through an absorption edge / and f vary rapidly with wavelength but FP is constant. The Friedel related reflection, mirrored through the real axis is also shown. The wavelengths shown follow the sequence Ai>A2>A3 where li corresponds to the minimum of A3 to the white line position (/" maximum) and to a position in the pre-edge region. From Helliwell (1984) with the permission of the Institute of Physics.

Determination of local coordination geometry. The position and intensity of the peaks in the pre-edge region do not solely depend upon the oxidation state of the absorber transition metal, but also upon the shape of the site (coordination polyhedron) where the absorber is located in the structure (Galas and Petiau 1983). An increase in coordination number provokes a positive energy shift, while the intensity of the peak is proportionally reduced (Waychunas et al. 1983). 

Figure 25. Experimental Fe A -edge spectra for the powders of two natural tri-octahedral micas and one natural brittle mica. The pre-edge regions are shown as inset.

Fig. 2 shows the XANES spectra of TS(x,y) and Ti02/Si02(z) samples. For comparison, the spectrum of Ti02 (anatase) is also shown. The Ti02 exhibited multiple peaks in the pre-edge region (4960-4980 eV), indicating the presence of titanium in octahedral coordination (Fig. 2a). On the other hand, all of TS(x,y) and Ti02/Si02(z) samples showed an intense pre-edge peak at 4967.1 eV (Fig.2b-g) as the characteristic of tetrahedrally coordinated titanium [19].

As described above, the pre-edge region and the edge position Eq contain interesting information on the local electronic structure at the absorbing atom. As the host lattice of most microporous compounds is electronically quite inert , this type of information is most interesting for non-frame-work species. 

In addition. Fig. 9 shows CuiC edge spectra of differently treated Y zeolites loaded with copper species [67]. The spectrum of Fig. 9e was obtained on a sample of zeoHte Y that had been ion-exchanged with Cu(N03)2 solution. The positions of the three features clearly show that copper exists as Cu + ions in this sample. The spectrum in Fig. 9f was measured on a sample of zeolite Y in its protonic form treated with gaseous CuCl at elevated temperatures. Obviously, the oxidation state of the copper species in this sample is -i-l, possibly with minor traces of Cu +. Reduction of this sample with CO led to a substance that gave the spectrum depicted in Fig. 9g. The additional absorption in the pre-edge region is due to the formation of Cu(0). 

Fig. 11. CoK edge XANES spectra of a-d Co in different oxidic coordination environments and e of a sample of CoAPO-20. Insets represent enlarged pre-edge regions. Co in a symmetrical octahedral environment in CoO b Co in a strongly distorted octahedral environment in C02B2O5, c Co in the spinel CoGa204 with 60% of the Co in tetrahedral and 40% of the Co in o ctahedral coordination d Co in the spinel C0AI2O4 with 80% of the Co in tetrahedral and 20% of the Co in octahedral coordination and e Co in CoAPO-20 it can be estimated that ca. 60% of the Co in this sample is in tetrahedral coordination (after [42])...

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