Auger Shake-Up Assisted Electron Recapture.

The presence of doubly excited states (DESs) above the core-hole ionization threshold nontrivially modulates the x-ray absorption because the participator Auger decay couples DESs to the underlying low-energy core-hole continuum. We show that coupling also affects the high-energy continuum populated by the spectator Auger decay of DESs. For the K-L_{23}^{2} Auger decay of the 1s^{-1}3p^{-1}4s^{2}^{1}P state in argon, the competing nonresonant path is assigned to the recapture of the 1s photoelectron caused by emission of the fast electron from the shake-up K-L_{23}^{2} decay of the 1s^{-1} ion.

Fingerprinting Mean Composition of Lithium Polysulfide Standard Solutions by Applying High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy.

In a lithium/sulfur (Li/S) battery, the reduction of sulfur during discharge involves a particular mechanism, where the active material successively dissolves into the electrolyte to form lithium polysulfide intermediate species (Li2Sx), with x being a function of the state of charge. In this work, sulfur K-edge resonant inelastic X-ray scattering measurements were performed for the characterization of different Li2Sx polysulfide standard solutions. High-energy resolution fluorescence detected X-ray absorption spectroscopy allowed clear separation the pre-edge absorption peak corresponding to terminal sulfur atoms from the main absorption peak due to internal atoms and allowed quantitative evaluation of the evolution of the peak area ratio as a function of the polysulfide chain length. Results of this experimental work demonstrate that the normalized area of the pre-edge is a reliable fingerprint of the Li2Sx mean chain length in agreement with recent theoretical predictions. As a perspective, this work confirms that operando HERFD XAS can be used to differentiate mean polysulfide composition, which is key issue in the characterization of Li/S cells.

Inception of electronic damage of matter by photon-driven post-ionization mechanisms.

"Probe-before-destroy" methodology permitted diffraction and imaging measurements of intact specimens using ultrabright but highly destructive X-ray free-electron laser (XFEL) pulses. The methodology takes advantage of XFEL pulses ultrashort duration to outrun the destructive nature of the X-rays. Atomic movement, generally on the order of >50 fs, regulates the maximum pulse duration for intact specimen measurements. In this contribution, we report the electronic structure damage of a molecule with ultrashort X-ray pulses under preservation of the atoms' positions. A detailed investigation of the X-ray induced processes revealed that X-ray absorption events in the solvent produce a significant number of solvated electrons within attosecond and femtosecond timescales that are capable of coulombic interactions with the probed molecules. The presented findings show a strong influence on the experimental spectra coming from ionization of the probed atoms' surroundings leading to electronic structure modification much faster than direct absorption of photons. This work calls for consideration of this phenomenon in cases focused on samples embedded in, e.g., solutions or in matrices, which in fact concerns most of the experimental studies.

Potential Energy Surface Reconstruction and Lifetime Determination of Molecular Double-Core-Hole States in the Hard X-Ray Regime.

A combination of resonant inelastic x-ray scattering and resonant Auger spectroscopy provides complementary information on the dynamic response of resonantly excited molecules. This is exemplified for CH_{3}I, for which we reconstruct the potential energy surface of the dissociative I 3d^{-2} double-core-hole state and determine its lifetime. The proposed method holds a strong potential for monitoring the hard x-ray induced electron and nuclear dynamic response of core-excited molecules containing heavy elements, where ab initio calculations of potential energy surfaces and lifetimes remain challenging.

Structural and dynamical properties of chlorinated hydrocarbons studied with resonant inelastic x-ray scattering.

We present a theoretical and experimental study of resonant inelastic x-ray scattering on a large group of chlorinated hydrocarbons: CH3Cl, CH2Cl2, CHCl3, CCl4, CH3CH2Cl, ClCH2CH2Cl, CH3CHCl2, CH3CCl3, C2H2Cl2-iso, C2H2Cl2-cis, C2H2Cl2-trans, and C6H5Cl. Differences in structural and dynamical properties of the molecules generated by diverse chemical environments are observed in the measured Cl(K(α)) spectral maps as well as in the Cl(K) total fluorescence yield spectra. The energy position, relative intensity, and the width of the Franck-Condon distribution of low-lying σ* and π* resonances are extracted by a fitting procedure taking into account the experimental broadening. The theoretical values obtained with the transition potential and Δ Kohn-Sham methods are in good agreement with the experimental parameters indicating subtle differences due to variations in the molecular structure.

High resolution multiphoton spectroscopy by a tunable free-electron-laser light.

Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (-1,0)(+1) (1)S(e) and (0,1)(0) (1)D(e) resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.

Dissociation dynamics of simple chlorine containing molecules upon resonant Cl K-σ* excitation.

A theoretical analysis of dissociation dynamics of chlorine K-σ(*) core-excited molecules is performed. The potential energy surfaces of HCl, Cl2, CH3Cl, CH2Cl2, CHCl3, CCl4, CFCl3, CF2Cl2, and CF3Cl are calculated along the normal vibrational modes of the ground electronic state yielding the widths of the corresponding Franck-Condon distributions. An insight into the potential energy surface of 1st σ(*) resonances shows that the initial dissociation dynamics of chloro(fluoro)methanes mainly involves the distancing of the carbon and the core-excited chlorine atom and is practically independent of other atoms in the molecule, which is in agreement with the recent experimental findings. The carbon atom pulls out the remaining three atoms shortly after piercing the three-atom plane resulting in a high vibrationally excited state of the fragment if the reconnection time is smaller than the lifetime of the L shell.

Dissociation of chloromethanes upon resonant σ* excitation studied by x-ray scattering.

The dissociation process following the Cl K-shell excitation to σ* resonances is studied by high resolution spectroscopy of resonant elastic and inelastic x-ray scattering on CH3Cl, CH2Cl2, CHCl3, and CCl4 molecules. Calculations employing the transition potential and Delta-Kohn-Sham DFT approach are in good agreement with the measured total fluorescence yield and show the presence of a second quasidegenerate group of states with σ* character above the lowest σ* unoccupied molecular orbital for molecules with more than one Cl atom. A bandwidth narrowing and a nonlinear dispersion behavior is extracted from the Kα spectral maps for both σ* resonances. The fitted data indicate that the widths of the Franck-Condon distributions for the first and second σ* resonances are comparable for all the molecules under study. In addition, an asymmetric broadening of the emission peaks is observed for resonant elastic x-ray scattering with zero detuning on both σ* resonances. This is attributed to the fast dissociation, transferring about 0.15 of the scattering probability into higher vibrational modes.

Temperature dependence of the electrical resistivity and electronic structure of amorphous Fe100-xZrx films and multilayers.

The electrical resistivity of amorphous Fe(100-x)Zr(x) metal alloy films and multilayers has been investigated in a wide temperature and composition range. The overall behavior of the resistivity is consistent with bulk measurements, exhibiting prominent semiconductor-like changes at low temperatures. The transition from positive (metallic) to negative temperature coefficient of resistivity behavior is accompanied by minute changes in magnetoresistance and we can therefore rule out magnetic phase changes as being the cause for the observed changes in the resistivity. Using x-ray absorption and emission spectroscopies we are able to probe the unoccupied and occupied electronic densities of states. The corresponding spectra are found to significantly overlap, as expected for a metallic-like electronic structure and the absence of a band gap. Besides a broadening of the x-ray emission lines expected from an amorphous material, remarkably small differences are observed in the electronic structures when changing the amount of Zr. The resistivity data were modeled and agreement with the Mott variable range hopping model was found, indicating localized electronic states due the disordered structure of the Fe(100-x)Zr(x) alloys.

Design and performance of a versatile curved-crystal spectrometer for high-resolution spectroscopy in the tender x-ray range.

A complete in-vacuum curved-crystal x-ray emission spectrometer in Johansson geometry has been constructed for a 2-6 keV energy range with sub natural line-width energy resolution. The spectrometer is designed to measure x-ray emission induced by photon and charged particle impact on solid and gaseous targets. It works with a relatively large x-ray source placed inside the Rowland circle and employs position sensitive detection of diffracted x-rays. Its compact modular design enables fast and easy installation at a synchrotron or particle accelerator beamline. The paper presents main characteristics of the spectrometer and illustrates its capabilities by showing few selected experimental examples.

First observation of two-electron one-photon transitions in single-photon K-shell double ionization.

Experimental evidence for the correlated two-electron one-photon transitions (1s(-2)→2s(-1)2p(-1)) following single-photon K-shell double ionization is reported. The double K-shell vacancy states in solid Mg, Al, and Si were produced by means of monochromatized synchrotron radiation, and the two-electron one-photon radiative transitions were observed by using a wavelength dispersive spectrometer. The two-electron one-photon transition energies and the branching ratios of the radiative one-electron to two-electron transitions were determined and compared to available perturbation theory predictions and configuration interaction calculations.

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.

Electronic state interferences in resonant x-ray emission after K-shell excitation in HCl.

We have measured a series of high-resolution x-ray spectra emitted upon resonant photoexcitation of HCl. The photon energy was tuned across the dissociative 1s→6σ* resonance and the Rydberg states converging to the Cl 1s(-1) threshold, and inelastic photon scattering was observed in the region of KL emission lines. Excellent agreement is found between fully ab initio calculated and measured spectra if interferences between different excitation-emission paths are taken into account. The effect of electronic state interferences is enhanced due to dynamical broadening of the 6σ* resonance in HCl.

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 Kbeta emission to the cation and the local symmetry is discussed.

Separation of two-electron photoexcited atomic processes near the inner-shell threshold.

By means of a high resolution resonant inelastic x-ray scattering spectroscopy, we have for the first time separated spectral features pertaining to different two-electron atomic processes in the vicinity of an inner-shell threshold. Contributions of shakeoff, shakeup, and resonant 1s3p double excitations were extracted from the Ar KM-M{2,3}M x-ray satellite line intensity measured as a function of photon energy from [1s3p] double excitation threshold to saturation. The isolated [1s3p]nln'l' excitation spectrum is critically compared to the outcome of the multiconfiguration Dirac-Fock model with relaxation.

Physical mechanisms and scaling laws of K-shell double photoionization.

We report on the photon energy dependence of the K-shell double photoionization (DPI) of Mg, Al, and Si. The DPI cross sections were derived from high-resolution measurements of x-ray spectra following the radiative decay of the K-shell double vacancy states. Our data evince the relative importance of the final-state electron-electron interaction to the DPI. By comparing the double-to-single K-shell photoionization cross-section ratios for neutral atoms with convergent close-coupling calculations for He-like ions, the effect of outer shell electrons on the K-shell DPI process is assessed. Universal scaling of the DPI cross sections with the effective nuclear charge for neutral atoms is revealed.

Relative detection efficiency of back- and front-illuminated charge-coupled device cameras for X-rays between 1 keV and 18 keV.

High-resolution x-ray measurements were performed with a von Hamos-type bent crystal spectrometer using for the detection of the diffracted photons either a back-illuminated charge-coupled device (CCD) camera or a front-illuminated one. For each CCD the main x-ray emission lines (e.g., Kalpha, Kbeta, Lalpha, and Lbeta) of a variety of elements were measured in order to probe the performances of the two detectors between 1 and 18 keV. From the observed x-ray lines the linearity of the energy response, the noise level, the energy resolution, and the quantum efficiency ratio of the two CCDs were determined.

The change of DMFT counts in Slovenia.

This longitudinal study was designed to assess the change in caries prevalence and DMFT counts in the adult population of Ljubljana, Slovenia, over a 10-year period. All data in the present study are derived from two surveys conducted in 1983-1987 and 1993-1997. In the first survey, 1,609 dentate subjects in the range from 15 to 65 years were examined. Ten years later, 555 (34.5%) of these subjects participated in the second oral survey. In the 538 dentate 25- to 75-year-old subjects examined in the second survey, a decline in the mean number of decayed teeth (1.1 vs. 0.5) was found, and an increase in DMFT counts (15.9 vs. 19.1) due to a higher number of missing (5.2 vs. 7.5) and filled teeth (9.7 vs. 11.1) was observed. However, a comparison with the previous generation of the same age showed a decline in the average number of decayed and missing teeth in all age groups and an increase in the mean number of filled teeth in all age groups as well. The population of Ljubljana from the 1993-1997 survey had retained in most of the age groups about 2 teeth more than had the generation 10 years before.

The change of periodontal treatment needs in a population of Ljubljana, Slovenia, over a ten year period.

A follow-up study was performed to assess the change of periodontal treatment needs in the adult population of Ljubljana, Slovenia, over a ten year period. In the initial survey, conducted from 1983-1987, 1,609 dentate subjects in the range from 15-65 years of age were clinically examined. Ten years later of the 1,609 invited subjects a total 555 (34.5%) came to the oral re-examination. During the intervening period 3.1% of them had become edentulous. Periodontal treatment needs in both surveys were assessed by the Community Periodontal Index of Treatment Needs methodology. At the initial examination 2.4% of 1,609 subjects were found to be without any signs of periodontal disease, 97.6% needed oral hygiene instruction, 77.71% needed scaling and 20.3% of them needed complex periodontal treatment. Ten years later, of the 538 dentate subjects aged 25-75 years, only 0.7% of presented with healthy periodontal tissues, 99.3% needed oral hygiene instruction, 93.9% needed scaling and 27.3% needed periodontal surgery. In both surveys advanced periodontal disease in the Ljubljana population was limited to 6.0% and 8.3% of all sextants respectively. After evaluation of the population of the same age in both surveys (25-65-year-old groups) a slight decrease in subjects with healthy periodontal tissues (1.21% vs. 0.84%) was observed. In comparison, there was an increase of subjects with shallow pockets (24.8% vs. 31.1%) and a slight decrease of subjects with deep periodontal pockets (27.7% vs. 24.1%). Comparing the results we concluded that the prevalence and severity of periodontal disease in the Ljubljana population remained similar in both surveys. The findings of the present follow-up study suggest a need for more efficient primary and secondary preventive measures in order to decrease the prevalence and severity of periodontal disease in Ljubljana and Slovenia.

Some risk factors for the progression of periodontal disease.

Inflammatory periodontal disease is one of the most common diseases of mankind. Gingival inflammation is widespread, but advanced periodontitis is limited to relatively small subgroups of the population. Gingivitis is initiated by microbial plaque deposits on the dento-gingival interface but progression to periodontitis is modified by several environmental, behavioural, biological and health care variables. This paper reviews the reports dealing with some risk factors for periodontal disease published in recent years and compares the data with findings in a Ljubljana population. It is concluded that male smokers with lower education and low frequency of tooth brushing represent a risk population for progression of periodontal disease. Marital status and body mass need further study to be proved as risk factors for periodontitis. A socioecological model proposed by Hansen et al. (1993) should be used for understanding the interplay of different risk factors for progression of periodontal disease.