S. Carniato

Accurate core electron binding energy calculations using small 6-31G and TZV core hole optimized basis sets

A procedure for optimizing basis sets for core hole binding energies is described. Contracted Gaussian basis sets are optimized for ground state and core hole state atomic configurations, exponents and contraction coefficients being determined by a minimization of the atomic self-consistent field state within a simulated annealing procedure. The basis sets are used in connection with Δself-consistent field, ΔMoller–Plesset and ΔKohn–Sham theory calculations of core electron binding energies and chemical shifts of high accuracy. Whatever the method, the small basis sets optimized in this way give results with an accuracy comparable to that obtained using very extended normal basis sets close to the complete basis set limit. They provide an excellent alternative to treat large molecular systems and push the accuracy of the ΔKohn–Sham technique for binding energy computations even further, exhibiting only small (a few tenths of an electron volt) deviations from experimental data.

Resonant inelastic x-ray scattering at the limit of subfemtosecond natural lifetime

We present measurements of the resonant inelastic x-ray scattering (RIXS) spectra of the CH(3)I molecule in the hard-x-ray region near the iodine L(2) and L(3) absorption edges. We show that dispersive RIXS spectral features that were recognized as a fingerprint of dissociative molecular states can be interpreted in terms of ultrashort natural lifetime of ∼200 attoseconds in the case of the iodine L-shell core-hole. Our results demonstrate the capacity of the RIXS technique to reveal subtle dynamical effects in molecules with sensitivity to nuclear rearrangement on a subfemtosecond time scale.

A new method to derive electronegativity from resonant inelastic x-ray scattering

Electronegativity is a well-known property of atoms and substituent groups. Because there is no direct way to measure it, establishing a useful scale for electronegativity often entails correlating it to another chemical parameter; a wide variety of methods have been proposed over the past 80 years to do just that. This work reports a new approach that connects electronegativity to a spectroscopic parameter derived from resonant inelastic x-ray scattering. The new method is demonstrated using a series of chlorine-containing compounds, focusing on the Cl 2p(-1)LUMO(1) electronic states reached after Cl 1s → LUMO core excitation and subsequent KL radiative decay. Based on an electron-density analysis of the LUMOs, the relative weights of the Cl 2p(z) atomic orbital contributing to the Cl 2p(3/2) molecular spin-orbit components are shown to yield a linear electronegativity scale consistent with previous approaches.

Functional and basis set dependence of K-edge shake-up spectra of molecules

A straightforward approach for computing the K-edge shake-up spectra of molecules based on equivalent core-hole linear response theory at both Hartree-Fock and density functional theory levels is proposed. Benchmark calculations have been performed to explore its sensitivity to different types of functionals and basis sets for the carbon 1s shake-up spectra of benzene and metal-free phthalocyanine (H2Pc). A very good agreement with previous theoretical and experimental works for the benzene molecule has been obtained for all the functionals and basis sets tested. Electron correlation is found to be essential for a good description of the H2Pc system, whose experimental C 1s shake-up spectrum is best reproduced by the hybrid density functional.

Single photon simultaneous K-shell ionization and K-shell excitation. I. Theoretical model applied to the interpretation of experimental results on H2O

We present in detail a theoretical model that provides absolute cross sections for simultaneous core-ionization core-excitation (K(-2)V) and compare its predictions with experimental results obtained on the water molecule after photoionization by synchrotron radiation. Two resonances of different symmetries are assigned in the main K(-2)V peak and comparable contributions from monopolar (direct shake-up) and dipolar (conjugate shake-up) core-valence excitations are identified. The main peak is observed with a much greater width than the total experimental resolution. This broadening is the signature of nuclear dynamics.

Resonant inelastic x-ray scattering of methyl chloride at the chlorine K edge

We present a combined experimental and theoretical study of isolated CH(3)Cl molecules using resonant inelastic x-ray scattering (RIXS). The high-resolution spectra allow extraction of information about nuclear dynamics in the core-excited molecule. Polarization-resolved RIXS spectra exhibit linear dichroism in the spin-orbit intensities, a result interpreted as due to chemical environment and singlet-triplet exchange in the molecular core levels. From analysis of the polarization-resolved data, Cl 2p(x, y) and 2p(z) electronic populations can be determined.

Performances of a bent-crystal spectrometer adapted to resonant x-ray emission measurements on gas-phase samples.

We describe a bent-crystal spectrometer adapted to measure x-ray emission resulting from core-level excitation of gas-phase molecules in the 0.8-8 keV energy range. The spectrometer is based on the Johann principle, and uses a microfocused photon beam to provide high-resolution (resolving power of approximately 7500). A gas cell was designed to hold a high-pressure (300 mbar) sample of gas while maintaining a high vacuum (10(-9) mbar) in the chamber. The cell was designed to optimize the counting rate (2000 cts/s at the maximum of the Cl Kalpha emission line), while minimizing self-absorption. Example of the Kalpha emission lines of CH(3)Cl molecules is presented to illustrate the capabilities of this new instrument.

Local density approximation and generalized gradient approximation calculations for oxygen and silicon vacancies in silica

Neutral oxygen and silicon vacancies’ energies in silica polytypes (α-quartz, β-cristobalite, and stishovite) have been studied using the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation. While the energies of formation of unrelaxed oxygen vacancies are remarkably constant (9.9 ±0.1 eV) in the three studied systems in GGA, the relaxation behavior is quite different: A strong Si–Si bond is formed in α-quartz, a weak one in β-cristobalite, while no bond at all is formed in stishovite. In α-quartz, peroxyl bridges are formed as the consequences of the relaxation of silicon vacancies. Their energy of formation from O2 (gas) is low (about 2 eV).

Theory assisted interpretation of copper phthalocyanine core levels XPS spectra

Abstract X-ray photoemission spectroscopy (XPS) and ab initio quantum mechanical calculations (SCF, CI, MCSCF) are used in combination to study the electronic structure of copper phthalocyanine (CuPc). The XPS core level spectra as well as the molecular orbital calculations suggest that in this molecule, the CuN bonds are ionic to a large extent. In the Cu2p spectrum, the final state corresponding to the main line, at the lower binding energy side, appears as a large mixing of two configurations; in one of them the core hole is fully screened by a charge transfer from the ligand 2p orbitals to the Cu3d x 2 − y 2 orbital. For the Cu3s and Cu3p core holes the screening by the ligand to metal (L → M) charge transfer is less important. For the Cu2p and Cu3p core levels, the higher binding energy states (satellites) correspond to shake-up processes exhibiting L → M charge transfer as well as M → M transitions, whereas in the case of Cu3s these are purely L → M.

Core-hole-clock spectroscopies in the tender x-ray domain

The core-hole-clock method to observe dynamical phenomena in molecular photoexcitation on the 1 fs-hundreds-of-attoseconds time scale is illustrated with examples from resonant inelastic x-ray scat ...