Jean-Claude Dousse

High energy resolution off-resonant spectroscopy at sub-second time resolution: (Pt(acac)2) decomposition.

We report on the decomposition of platinum acetylacetonate (Pt(acac)(2)) in hydrogen induced by flash heating. The changes in the local Pt structure were followed by high energy resolution off-resonant spectroscopy uniquely performed with sub-second time resolution. The decomposition consists of a two-step reduction process of the Pt(II) species.

Wavelength-dispersive spectrometer for X-ray microfluorescence analysis at the X-ray microscopy beamline ID21 (ESRF)

A polycapillary-based wavelength-dispersive spectrometer is reported. The design consideration as well as operation characteristics are presented.

Real Time Determination of the Electronic Structure of Unstable Reaction Intermediates during Au2O3 Reduction.

Chemical reactions are always associated with electronic structure changes of the involved chemical species. Determining the electronic configuration of an atom allows probing its chemical state and gives understanding of the reaction pathways. However, often the reactions are too complex and too fast to be measured at in situ conditions due to slow and/or insensitive experimental techniques. A short-lived Au2O compound has been detected for the first time under in situ conditions during the temperature-programmed reduction of Au2O3. A time-resolved resonant inelastic X-ray scattering experiment (RIXS) allowed the determination of changes in the Au electronic structure, enabling a better understanding of the reaction mechanism of Au(III) reduction. On the basis of time-resolved RIXS data analysis combined with genetic algorithm methodology, we determined the electronic structure of the metastable Au2O intermediate species. The data analysis showed a notably larger value for the lattice constant of the intermediate Au as compared to the theoretical predictions. With support of DFT calculations, we found that such a structure may indeed be formed and that the expanded lattice constant is due to the termination of Au2O on the Au2O3 structure.

Establishing nonlinearity thresholds with ultraintense X-ray pulses

X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state.

Geometrical optics modelling of grazing incidence X-ray fluorescence of nanoscaled objects

X-ray Standing Wave (XSW) is a well established formalism for modelling Grazing Incidence X-ray Fluorescence (GIXRF) experiments. However, when probing nanostructured surfaces with complex morphology the effects of the interaction of the XSW with structure elements need to be investigated. This is not always easy and sometimes even not possible. In the present work a novel approach employing Geometrical Optics (GO) calculations is proposed. The model is employed for simulations of two different types of nano-particles distributed on a flat surface. It is shown that GO simulation yields results with good agreement when compared to absolute measurements even when XSW deteriorates.

Depth profiling of low energy ion implantations in Si and Ge by means of micro-focused grazing emission X-ray fluorescence and grazing incidence X-ray fluorescence

Depth-profiling measurements by means of synchrotron radiation based grazing XRF techniques, i.e., grazing emission X-ray fluorescence (GEXRF) and grazing incidence X-ray fluorescence (GIXRF), present a promising approach for the non-destructive, sub-nanometer scale precision characterization of ultra shallow ion-implantations. The nanometer resolution is of importance with respect to actual semiconductor applications where the down-scaling of the device dimensions requires the doping of shallower depth ranges. The depth distributions of implanted ions can be deduced from the intensity dependence of the detected X-ray fluorescence (XRF) signal from the dopant atoms on either the grazing emission angle of the emitted X-rays (GEXRF), or the grazing incidence angle of the incident X-rays (GIXRF). The investigated sample depth depends on the grazing angle and can be varied from a few to several hundred nanometers. The GEXRF setup was equipped with a focusing polycapillary half-lens to allow for laterally resolved studies. The dopant depth distribution of the investigated low-energy (energy range from 1 keV up to 8 keV) P, In and Sb ion-implantations in Si or Ge wafers were reconstructed from the GEXRF data by using two different approaches, one with and one without a priori knowledge about the bell-shaped dopant depth distribution function. The results were compared to simulations and the trends predicted by theory were found to be well reproduced. The experimental GEXRF findings were moreover verified for selected samples by GIXRF.

Grazing incidence X-ray fluorescence of periodic structures – a comparison between X-ray standing waves and geometrical optics calculations

Grazing incidence X-ray fluorescence spectra of nano-scaled periodic line structures were recorded at the four crystal monochromator beamline in the laboratory of the Physikalisch-Technische Bundesanstalt at the synchrotron radiation facility BESSY II. For different tilt angles between the lines and the plane of incidence of the monochromatic synchrotron radiation, spectral features are observed which can be understood and explained with calculations of the emerging X-ray standing wave (XSW) field. On the other hand, there are structures, i.e., pronounced modulations above the substrates critical angle of external total reflection, which are not included in the XSW concept. Novel geometrical optics calculations can reproduce these structures taking the samples specific geometric conditions into account.

X-ray two-photon absorption with high fluence XFEL pulses

We report on nonlinear interaction of solid Fe with intense femtosecond hard x-ray free-electron laser (XFEL) pulses. The experiment was performed at the CXI end-station of the Linac Coherent Light Source (LCLS) by means of high- resolution x-ray emission spectroscopy. The focused x-ray beam provided extreme fluence of ~105 photons/A2. Two-photon absorption leading to K-shell hollow atom formation and to single K-shell ionization of solid Fe was investigated.

Single-photon double K-shell ionization of low-Z atoms

The photon energy dependence of the double K-shell ionization of light atoms is reported. Experimental double-to-single photoionization cross section ratios for Mg, Al, Si and Ca were obtained from measurements of high-resolution x-ray emission spectra. The double photoionization (DPI) cross-sections for K-shell hollow atom production are compared to convergent close-coupling calculations (CCC) for neutral atoms and He-like ions. The relative importance of the initial-state and final-state electron-electron interactions to the K-shell DPI in many-electron atoms and two-electron ions is addressed. Physical mechanisms and scaling laws of the K-shell double photoionization are examined. A semiempirical universal scaling of the DPI cross sections with the effective nuclear charge for neutral atoms 2≤Z≤47 is established.

High energy resolution measurements of the radiative decay of double K-shell vacancies in 20 ≤ Z ≤ 29 elements bombarded by fast C and Ne ions

We report on high energy resolution measurements of the Kα hypersatellite x-ray spectra of Ca, V, Fe and Cu induced by impact with 144 MeV C and 180 MeV Ne ions.