Auger Effect in the High-Resolution Ce 3d-edge Resonant Photoemission

The bulk-sensitive Ce 4$f$ spectral weights of various Ce compounds including CeFe$_2$, CeNi$_2$, and CeSi$_2$ were obtained with the resonant photoemission technique at the Ce 3d-edge. We found the lineshapes change significantly with the small change of the incident photon energy. Detailed analysis showed that this phenomenon results primarily from the Auger transition between different multiplet states of the Ce $\underline{3d_{5/2}}4f^2$ (bar denotes a hole) electronic configuration in the intermediate state of the resonant process. This tells us that extra care should be taken for the choice of the resonant photon energy when extracting Ce 4$f$ spectral weights from the Ce 3$d$-edge resonant photoemission spectra. The absorption energy corresponding to the lowest multiplet structure of the Ce $\underline{3d_{5/2}}4f^2$ configuration seems to be the logical choice.Comment: 13 pages, 5 figures, submitted to Phys. Rev.

Carbonate-Induced structural perturbation of Al hydroxides

The chemistry of Al transformation has been well documented, though little is known about the mechanisms of structural perturbation of Al precipitates by carbonates at a molecular level. The purpose of the present study was to investigate the structural perturbation of Al precipitates formed under the influence of carbonates. Initial carbonate/Al molar ratios (MRs) used were 0, 0.1, and 0.5 after aging for 32 days, then the samples were analyzed by X-ray absorption near edge structure spectroscopy (XANES), X-ray diffraction (XRD), Fourier-transform infrared absorption spectroscopy (FTIR), and chemical analysis. The XRD data were in accord with the FTIR results, which revealed that as the carbonate/Al MR was increased from 0 to 0.1, carbonate preferentially retarded the formation of gibbsite and had relatively little effect on the formation of bayerite. As the carbonate/Al MR was increased to 0.5, however, the crystallization of both gibbsite and bayerite was completely inhibited. The impact of carbonate on the nature of Al precipitates was also evident in the increase of adsorbed water and inorganic C contents with increasing carbonate/Al MR. The Al K- and L- edge XANES data provide the first evidence illustrating the change in the coordination number of Al from 6-fold to mixed 6- and 4-fold coordination in the structural network of short-range ordered (SRO) Al precipitates formed under the increasing perturbation of carbonate. The fluorescence yield spectra of the O K-edge show that the intensity of the peak at 534.5 eV assigned to σ* transitions of Al-O and O-H bonding decreased with increasing carbonate/Al MR. The XANES data, along with the evidence from XRD, FTIR, and chemical analysis showed clearly that carbonate caused the alteration of the coordination nature of the Al-O bonding through perturbation of the atomic bonding and structural configuration of Al hydroxides by complexation with Al in the SRO network of Al precipitates. The surface reactivity of an Al-O bond is related to its covalency and coordination geometry. The present findings were, therefore, of fundamental significance in understanding the low-temperature geochemistry of Al and its impacts on the transformation, transport, and fate of nutrients and pollutants in the ecosystem

Vanadium L-2,3 XANES experiments and first-principles multielectron calculations: Impact of second-nearest neighboring cations on vanadium-bearing fresnoites

Transition-metal L-2,L-3 XANES spectra are widely used to determine coordination and valence of the target ion. For decades, experimental fingerprinting, i.e., the comparison with spectra obtained from known reference compounds was the way to interpret spectral features. This approach was based on the common understanding that only anions in the first coordination sphere would determine the near-edge structure, and crystalline references were selected accordingly. Using ab initio charge-transfer multiplet calculations, we demonstrate that there is also a significant impact on spectral features from the second-nearest neighbor cations. This finding is exemplified for three fresnoite-type vanadates, namely Ba2VSi2O8 (BVS), K2VV2O8 (KVV), and Rb2VV2O8 (RVV). The theoretical treatment provides evidence that for the three compounds studied it is not variable bond lengths or bond angles between vanadium and oxygen that make the V-L-2,L-3 XANES spectra different, but the interaction of the target vanadium ions with its neighboring cations (Si for BVS, V for KVV and RVV), which dominates. Therefore, we conclude that simple fingerprinting can result in misleading interpretations when interactions with second-nearest neighboring cations are not taken into account. Ab initio charge-transfer multiplet calculations of spectral shapes (theoretical fingerprinting) should be employed instead to get a deeper understanding of structure-spectra relationships, or the choice of reference spectra should take into account second-nearest neighbors. Our findings are similarly important for the interpretation of electron energy-loss near-edge (ELNES) spectra

Long-range order induced by cobalt porphyrin adsorption on aminothiophenol-functionalized Au(111): the influence of the induced dipole

Porphyrins are useful in materials science for optical, photoelectrochemical and chemical sensor applications. Solid films of oriented porphyrins on gold can be realized through a simple procedure and without synthesizing thiol-derivatized porphyrins. In order to immobilize the porphyrin rings on the surface, we prepared a 4-aminothiophenol (4-ATP) self-assembled monolayer (SAM) on gold(111) followed by axial legation, in situ, of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (CoTBPP). Ultrahigh vacuum (UHV) Scanning Tunneling Microscopy (STM) studies performed on the SAMs revealed the Au(111) herringbone structure reconstruction, probably due to adsorption/desorption processes of molecules. STM images and Scanning Tunneling Spectroscopy (STS) measurements clearly showed that the immobilization of molecules also induced an electronic perturbation on the gold surface. This effect is ascribed to the presence of oriented molecular dipole layers between the metal and the organic material

Adsorption of H 2S, HS, S, and H on a stepped Fe(310) surface

Using periodic density functional theory we studied adsorption of H 2S, HS, S and H on the Fe(310) stepped surface, comparing our results with those on Fe(100). H 2S is predicted to weakly adsorb on all high-symmetry sites, with the bridge site at the step edge as preferred one, oriented perpendicularly to the (100) terraces with the two H atoms pointing out of the surface. Adsorption of HS, S, and H is more stable on the bridge, four-fold hollow, and three-fold hollow sites, respectively. The detailed analysis of the computed local density of states show common trends with the behavior of adsorption energies and is able to account for energy differences of all species adsorbed on Fe(100) and Fe(310). Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010