Ultrafast core-hole-induced dynamics in water probed by x-ray emission spectroscopy

The isotope effect and excitation-energy dependence have been measured in the oxygen K-edge x-ray emission spectrum (XES). The use of XES to monitor core decay processes provides information about molecular dynamics (MD) on an ultrafast time scale through the O1s lifetime of a few femtoseconds. Different nuclear masses give rise to differences in the dynamics and the observed isotope effect in XES is direct evidence of the importance of such processes. MD simulations show that even the excitation-energy dependence in the XES is mainly related to differences in core-excited-state dynamics.     

Isotope and temperature effects in liquid water probed by x-ray absorption and resonant x-ray emission spectroscopy

High-resolution x-ray absorption and emission spectra of liquid water exhibit a strong isotope effect. Further, the emission spectra show a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced excitation-energy dependence. They can be described as a superposition of two independent contributions. By comparing with gas phase, ice, and NaOH/NaOD, we propose that the two components are governed by the initial state hydrogen bonding configuration and ultrafast dissociation on the time scale of the O 1s core hole decay.     

Laser spectroscopy of free molecular oxygen dispersed in wood materials

The recently introduced Gas in Scattering Media Absorption Spectroscopy (GASMAS) technique is applied to the study of various wood samples. Molecular oxygen in the pores of the strongly scattering material is detected using diode laser spectroscopy around 760 nm. Diffuse light propagation in these media is studied by time-dispersion measurements. Furthermore, anisotropy related to the fibre structure of wood and gas diffusion properties are studied. Promising extensions of the experiments are discussed. PACS 42.55.Px; 42.62.Fi; 81.05.Rm; 81.70.Fy; 87.64.Cc     

Resonant inelastic x-ray scattering at the oxygen K resonance of NiO: nonlocal charge transfer and double-singlet excitations

We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O K resonance of NiO showing a rich excitation spectrum. We perform multisite Ni6O19 cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4-5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also dd excitations at 1 eV are observed. Finally, we point out that O K RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy delta and the Hund coupling energy J(H).     

Investigation of electronic structure of ternary molybdenum sulphides by means of x-ray emission and photoelectron spectroscopy

The electronic structure of superconducting Chevrel phases (SnMo₆S₈, PbMo₆S₈, Cu_2.8Mo₆S₈) and nonsuperconducting spinel (GaMo₄S₈) was investigated by X-ray emission (XES) and X-ray photoelectron (XPS) spectroscopy. The comparison of the obtained results with band structure and cluster calculations of given compounds was performed.     

K-shell x-ray spectroscopy of atomic nitrogen

Absolute K-shell photoionization cross sections for atomic nitrogen have been obtained from both experiment and state-of-the-art theoretical techniques. Because of the difficulty of creating a target of neutral atomic nitrogen, no high-resolution K-edge spectroscopy measurements have been reported for this important atom. Interplay between theory and experiment enabled identification and characterization of the strong 1s → np resonance features throughout the threshold region. An experimental value of 409.64±0.02 eV was determined for the K-shell binding energy.     

Analysis of plume-buffer gas interaction through molecular and atomic oxygen absorption spectroscopy

We present results acquired by using in-situ diagnostics based on atomic and molecular oxygen absorption spectroscopy during laser ablation in the presence of an O₂ environment. Our investigation provides detailed space- and time-resolved information on the relative density of atomic and molecular oxygen in the plume, and demonstrates a strong correlation between their behavior. The results shed light on the occurrence of reactive-collisional processes involving the ablated material and the environment, leading to O₂ dissociation.     

Resonant soft x-ray scattering from stepped surfaces of SrTiO3

We studied the resonant diffraction signal from stepped surfaces of SrTiO(3) at the Ti 2p → 3d (L(2,3)) resonance in comparison with x-ray absorption (XAS) and specular reflectivity data. The steps on the surface form an artificial superstructure suitable as a model system for resonant soft x-ray diffraction. A small step density on the surface is sufficient to produce a well defined diffraction peak. We determined the optical parameters of the sample across the resonance and found that the differences between the energy dependence of the x-ray absorption signal, the specular reflectivity and the step-related peak reflect the different quantities probed in these signals. When recorded at low incidence or detection angles, XAS and specular reflectivity spectra are strongly distorted by the changes of the angle of total reflection with energy. The resonant diffraction spectrum is less affected and can be used as a spectroscopic probe even in less favorable geometries.     

Optical pathlength saturation signatures in wavelength modulation spectroscopy signals of atmospheric molecular oxygen

We discuss experimental and theoretical results of absorption features of the oxygen A-band transitions in the optically thick regime when synchronous detection at higher harmonics (N≥2), using wavelength modulation spectroscopy (WMS), is performed. We show that the absorption saturation effects demonstrate a distinctive feature which results in suppression of the line-center lobes of the harmonic signals. These effects depend on the optical pathlength as well as on the modulation index. The rich structure of WMS signals, especially at higher detection orders, is central to the technique’s advantages in resolving spectra congested with highly disparate oscillator-strength lines. The effect of pathlength saturation on this structure is investigated.     

Resonant ultrasound spectroscopy and homogeneity in polycrystals

Resonant ultrasound spectroscopy (RUS) is capable of determining the bulk elastic properties of a solid from its characteristic vibration frequencies, given the dimensions, density and shape of the sample. The model used for extracting values of the elastic constants assumes perfect homogeneity, which can be approximated by average-isotropic polycrystals. This approximation is excellent in the small grain regime assumed for most averaging procedures, but for real samples with indeterminate grain size distributions, it is not clear where the approximation breaks down. RUS measurements were made on pure copper samples where the grain size distribution was changed by progressive heat treatments in order to find a quantitative limit for the loss of homogeneity. It is found that when a measure of the largest grains is 15% of the sample’s smallest dimension, the deviation in RUS fits indicates elastic inhomogeneity.     

Resonant x-ray diffraction studies on the charge ordering in magnetite

Here we show that the low temperature phase of magnetite is associated with an effective, although fractional, ordering of the charge. Evidence and a quantitative evaluation of the atomic charges are achieved by using resonant x-ray diffraction (RXD) experiments whose results are further analyzed with the help of ab initio calculations of the scattering factors involved. By confirming the results obtained from x-ray crystallography we have shown that RXD is able to probe quantitatively the electronic structure in very complex oxides, whose importance covers a wide domain of applications.