Calculations of NEXAFS Spectra of Substituted Benzenes

We examine the role of substituent strengths and positions for NEXAFS spectra of substituted benzenes by means of static exchange calculations on aniline, phenol, fluorobenzene, and terephthalaldehyde. Chemical shifts and intensities of the 1s → π* excitations are found to be closely related to the electronegative character of the substituent group. The intensities for the unconnected ring carbons exhibit a significant site dependent, alternant, behaviour, while the chemical shifts in the energy positions are small. The terephthalaldehyde molecule shows a crucial positional dependence for double substitution and gives also a nice example of the building block principle for NEXAFS. New assignments are given for the building blocks in the terephthalaldehyde spectrum and for all salient features in the discrete parts of the monosubstituted benzene spectra

Ab-initio Static Exchange Calculations of Shake-Up Spectra of Molecules and Surface Adsorbates

An ab initio approach to core electron shake spectra based on the sadden approximation and on a direct implementation of the static-exchange (STEX) method is reviewed. It employs appropriately spin-coupled two-hole potentials for the various shake-up/shake-off channels and is far extendable in the number of atoms treated,'in the dimension of the one-particle basis set and in the spectral range. Using cluster modelling it has been implemented for shake spectra of molecules adsorbed on surfaces, while considering oligomers of increasing size it can be used<to model the shake spectra of polymers. Applications of the method to shake spectra of several systems are discussed: CO chemisorbed on Cu(100), H2CO, C2H4, and aniline

Self-Consistent Field Calculations of X-Ray Emission Spectra of Surface Adsorbates and Polymers

We use modem self-consistent field methods for simulating X-ray emission spectra of surface adsorbates and polymers. Energies and intensities are computed from separately optimized wave functions with full core hole relaxation, where the surfaces and polymers axe modelled by, respectively, clusters and oligomers. X-ray intensities are evaluated both from tie relaxed wave functions and from ground state frozen orbitals in order to test the validity of the local selection- and the initial and final state rules, and, in the polymer case, to test band theory versus exciton theory interpretations of X-ray emission spectra. We review examples of calculations on surface adsorbates (CO/Cu(100) and C6H6/Cu(100)) and polymers (the polyene stquence)

X-ray absorption spectroscopy measurements of liquid water

PubMedID: 16852732Recent studies, based on X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS), have shown that the hydrogen bond network in liquid water consists mainly of water molecules with only two strong hydrogen bonds. Since this result is controversial, it is important to demonstrate the reliability of the experimental data, which is the purpose of this paper. Here we compare X-ray absorption spectra of liquid water recorded with five very different techniques sensitive to the local environment of the absorbing molecule. Overall, the spectra obtained with photon detection show a very close similarity and even the observable minor differences can be understood. The comparison demonstrates that XAS and XRS can indeed be applied reliably to study the local bonding of the water molecule and thus to reveal the hydrogen bond situation in bulk water. © 2005 American Chemical Society

The interpretation of sulfur k-edge xanes spectra: a case study on thiophenic and aliphatic sulfur compounds

Sulfur K-edge XANES has been measured for three sulfur model compounds, dibenzothiophene, dibenzothiophene sulfone, and aliphatic sulfur (dl-methionine). The spectra have been simulated with Density Functional Theory (DFT) by using a number of methods, including the half-core-hole approximation. Dipole transition elements were calculated and the transitions were convoluted with linearly increasing Gaussian functions in the first 20 eV of the near-edge region. In the case of dibenzothiophene, relaxation of the first excited states in the presence of the core-hole gave a further improvement. The theoretical results reproduce well the features of the spectra and give insight in the relation between geometric structure and molecular orbitals. Though dl-methionine and dibenzothiophene show a similar sharp rise of the white line, their molecular levels are quite different, pointing out the difficulties in finding useful “fingerprints” in the spectra for specific compounds

Sulfur-Metal orbital hybridization in sulfur-bearing compounds studied by X-ray Emission Spectroscopy

The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy (XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the Kβ emission to the cation and the local symmetry is discussed