Bonding NEXAFS studies

Cuf1101-HC00 The decomposition of formic acid on metal and oxide surfaces is a model heterogeneous reaction. Many studies have since shown that it proceeds via a surface formate species. Thus on Cu 110) adsorbed formic acid is found at low temperature. On heating to 270 K deprotonation occurs, giving rise to the surface formate, which in turn decomposes at 450 K with evolution of H2 and C02- In previous studies, particularly with vibrational spectroscopy, it had been demonstrated that the two C-0 bonds are equivalent and that the symmetry is probably C2v [19]. A NEXAFS study by Puschmann et al. [20] has subsequently shown that the molecular plane is oriented perpendicular to the surface and aligned in the <110> azimuth. 

With various alkyl-substituted cyclopentanes, ring-opening selectivity with Ir/A Os depended on the number of CH2—CH2 bonds, which argues for the dicarbene mechanism, in which the intermediate is a l,l,2,2-a4 species. The chemisorbed state of cyclopentene was reported to be more or less perpendicular to the Ir(l 11) surface by a NEXAFS study, in support of this mechanism. 

Previous NEXAFS studies on n-octadecyltrichlorosilane (OTS-C18) on silicon [29] and mica [30] indicate that monolayer films of OTS are highly oriented. The angular dependence of the resonances is indicative of a preferential molecular orientation of the polymeric C-C chains perpendicular to the surface (standing up), and the C-H bond plane is parallel to the surface. It should be noted that the orientation in the form of tilt angle or order calculated from NEXAFS are average values. 

A recent combined LEED, UPES, NEXAFS, and theoretical study of the ethyne/Ni(110) system (64) suggested a structure with the C-C bond parallel to the direction of the surface-atom rows and bonded to two metal atoms on adjacent ridges. This structure implies an HCCH plane perpendicular to the surface and is therefore unfortunately at variance with the spectroscopic conclusions, at least at near-saturation coverage. Experimental spectroscopic work at lower coverage, analogous to that carried out on Pd(110) (30) would be advantageous. 

The EXAFS technique has been especially useful for metalloproteins. It has often provided the first clues as to the identity of atoms (O, N, S) surrounding a metal atom and either covalently bonded to it or coordinated with it (Chapter 16). Interpretations are often difficult, and a common approach is to try to simulate the observed spectrum by calculation from a proposed structure.118 Tautomerism in crystalline Schiff bases (see Eq. 23-24) has been studied by nearedge X-ray absorption fine structure (NEXAFS) employing soft X-rays.119... 

The purpose of this paper is to introduce the technique of NEXAFS spectroscopy to scientists and engineers interested in the analysis of polymers and metal-polymer interfaces. NEXAFS is just coming into its own as a powerful tool for studying bonding interactions and molecular orientation of fairly complicated systems. By presenting background material and examples of applications to metal-polymer systems, it is hoped that the reader will be left with a basic understanding and an impression of the potential of this technique. 

A variation of XANES or NEXAFS has been used to determine the structure of molecules chemisorbed on surfaces. In this approach photoemitted electrons excite molecular orbitals in the chemisorbed molecules. By varying the polarization of the incident photons, molecular orientation can be determined from selection rules for excitation. The bond lengths can be determined from a quasi-empirical correlation between bond-length and the shift in the molecular orbital excitation energy. This technique has been used to study the chemisorption of several hydrocarbon molecules on different metal surfaces./17/... 

The theoretical results obtained in the present work, strongly support the conclusions of Sheppard [46] and of Lehwald et al [67]. The results in Table 10 show that the molecule binds strongly on a Ni(lOO) surface, the geometry and the electronic structure of the molecule are highly perturbed by the adsorption process, and the di-CT adsorption mode is more stable than the 7t mode, by approximately 91 kJmoT. The calculated CC bond length for the di-a mode, 1.46 A, is identical to the experimental value determined by NEXAFS [68]. An analysis of the electronic structure shows that on both adsorption complexes the degree of the [Pg.236]

Fan, L.J., Yang, Y.W., and Tao, Y.T., Molecular orientation and bonding of ter-thiophene-thiol self-assembled on An(lll) A combined NEXAFS and XPS study, J. Electron Spectrosc. Relat. Phenom. 144, 433-436, 2005. 

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