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.

X-ray emission spectra recorded in the off-resonant regime carry information on the density of unoccupied states. It is known that by employing the Kramers-Heisenberg formalism, the high energy resolution off-resonant spectroscopy (HEROS) is equivalent to the x-ray absorption spectroscopy (XAS) technique and provides the same electronic state information. Moreover, in the present Letter we demonstrate that the shape of HEROS spectra is not modified by self-absorption effects. Therefore, in contrast to the fluorescence-based XAS techniques, the recorded shape of the spectra is independent of the sample concentration or thickness. The HEROS may thus be used as an experimental technique when precise information about specific absorption features and their strengths is crucial for chemical speciation or theoretical evaluation. DOI: 10.1103/PhysRevLett.112.173003 PACS numbers: 32.30.Rj, 31.15.ag, 32.70.Jz, 32.80.Fb The high-intensity and highly monochromatic x-ray radiation available at synchrotron sources allow researchers from different disciplines to study the local electronic and geometric structure of materials by means of x-ray absorption spectroscopy (XAS). To record a XAS spectrum the x-ray energy is tuned around the electron binding energy of the element of interest, thereby exciting electrons from one of the core levels to unoccupied states or into the continuum. The photo-absorption process depends on the photon energy and is a measure of the material's absorption coefficient which in turn provides information on the material properties. The energy region around the absorption edge, namely the x-ray absorption near edge structure (XANES), reflects the density of empty states [1]. The region starting about 100 eV above the absorption edge is called the extended x-ray absorption fine structure (EXAFS) region, and the analysis of this region yields information on the local geometric structure [2].X-ray absorption spectra can be measured in the transmission, fluorescence, and electron-yield mode [3,4]. In the transmission mode, the XAS spectra are measured by recording the intensity of the photon beam before and after the sample. The fluorescence mode involves the use of x-ray emission spectroscopy (XES) techniques allowing detection of the fluorescence yield as a function of the incident photon energy. The XAS spectrum can be obtained from either the total fluorescence yield (TFY) or the partial fluorescence yield (PFY) measurements. In TFY the fluorescence intensity is integrated over a broad emission energy range, while in the PFY it is integrated over a selected emission energy range. In particular, the fluorescence yield integrated over an energy band centered at a given fluorescence line and narrower than the natural linewidth of the latter is referred to as the high energy resolution XAS [5,6]. It provides more detailed information about absorption features and allows precise chemical speciation [7].The field of x-ray absorption spectroscopy is a very important tool in the fields of physics, chemist...

Stochastic processes are highly relevant in research fields as different as neuroscience, economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic unpredictability, stochastic mechanisms are very challenging for any kind of investigations and practical applications. Here we report the deliberate use of stochastic X-ray pulses in two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied electronic states of atoms in a regime where the opacity and transparency properties of matter are subject to the incident intensity and photon energy. A readily transferable matrix formalism is presented to extract the electronic states from a dataset measured with the monitored input from a stochastic excitation source. The presented formalism enables investigations of the response of the electronic structure to irradiation with intense X-ray pulses while the time structure of the incident pulses is preserved.

A newly developed laboratory-based double X-ray spectrometer enables systematic and simultaneous X-ray emission (XES) and X-ray absorption (XAS) measurements.

Physical, biological, and chemical transformations are initiated by changes in the electronic configuration of the species involved. These electronic changes occur on the timescales of attoseconds (10−18 s) to femtoseconds (10−15 s) and drive all subsequent electronic reorganization as the system moves to a new equilibrium or quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is crucial for understanding the potential energy surfaces upon which chemical and biological reactions take place. Here, we report on the determination of the electronic structure of matter using a single self-seeded femtosecond x-ray pulse from the Linac Coherent Light Source hard x-ray free electron laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we were able to obtain information about the electronic density of states with a single femtosecond x-ray pulse. We show that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis and that the measured spectral shape is independent of the large intensity fluctuations of the incoming x-ray beam. Moreover, we demonstrate the chemical sensitivity and single-shot capability and limitations of HEROS, which enables the technique to track the electronic structural dynamics in matter on femtosecond time scales, making it an ideal probe technique for time-resolved X-ray experiments.

The high-resolution von Hamos bent crystal spectrometer of the University of Fribourg was upgraded with a focused X-ray beam source with the aim of performing micro-sized X-ray fluorescence (XRF) measurements in the laboratory. The focused X-ray beam source integrates a collimating optics mounted on a low-power micro-spot X-ray tube and a focusing polycapillary half-lens placed in front of the sample. The performances of the setup were probed in terms of spatial and energy resolution. In particular, the fluorescence intensity and energy resolution of the von Hamos spectrometer equipped with the novel micro-focused X-ray source and a standard high-power water-cooled X-ray tube were compared. The XRF analysis capability of the new setup was assessed by measuring the dopant distribution within the core of Er-doped SiO 2 optical fibers.

The potential of valence to core Al X-ray emission spectroscopy to determine aluminum distribution in ferrierite zeolites was investigated. The recorded emission spectra of four samples prepared with different structure directing agents exhibit slight variations in the position of the main emission peak and the intensity of its low energy shoulder. Theoretical calculations indicate that an increased intensity of the Kb x shoulder in the Al emission spectra can be linked to a predominant occupation of the T3 site by a single aluminum atom. This study thus suggests that valence to core X-ray emission spectroscopy can be applied to help determine the occupation of aluminum at crystallographic T-sites in zeolites.Zeolites are microporous crystalline aluminosilicates whose frameworks are composed of corner sharing TO 4 tetrahedra (T = either silicon or aluminum). The mutual arrangement of these subunits defines the framework structure which is characterized by periodical pores and channels. Replacing silicon atoms with aluminum atoms at the T-sites introduces a local negative charge to the framework which can be stabilized by protons or cations, which confer the catalytic activity to the zeolite. The catalytic activity is therefore directly related to the positioning of aluminum atoms within the zeolite framework.The determination of both the location of aluminum atoms at individual T-sites (aluminum sitting) and the mutual arrangement of several aluminum atoms (aluminum distribution) remains an open question in zeolite structural analysis.1 The Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) spectroscopy of 29Si represents a standard method to address the Si-Al connectivity, however, not providing information about the absolute position of aluminum and silicon atoms inside the framework.27 Al MAS NMR is sensitive to the averaged Al-O-Si angle and therefore partially yields aluminum occupation. sequences, where at least one aluminum atom is located in the second coordination shell of another aluminum atom, has been determined. Due to a large penetration depth of X-rays, X-ray-based techniques are suited for the characterization of catalysts under reaction conditions. X-ray absorption fine structure (EXAFS) analysis combined with DFT simulations showed a preference of aluminum for the T sites in the 4-ring of zeolite beta. 7 X-ray absorption near-edge structure spectroscopy (XANES) was successfully applied to monitor aluminum coordination as a function of temperature and gas composition. 8 X-ray diffraction (XRD) techniques are, in general, not suitable for distinguishing silicon and aluminum atoms because of their similar scattering power. The structural changes induced by introducing aluminum, mainly an elongation of the T-O distance by ca. 0.1 Å, can in some cases indicate the position of aluminum. 9 However, this difference is obscured by the low Al/Si proportion at that specific T-site. Using the X-ray standing waves, the location of aluminum in scolecite (NAT framework type) has been determined. 10 Alt...

X-ray atomic properties of nickel were investigated in a singular approach that combines different experimental techniques to obtain new and useful reliable values of atomic fundamental parameters for X-ray spectrometric purposes and for comparison to theoretical predictions. We determined the mass attenuation coefficients in an energy range covering the L-and K-absorption edges, the K-shell fluorescence yield and the Kβ/Kα and Kβ 1,3/K α1,2 transition probability ratios. The obtained line profiles and linewidths of the Kα and Kβ transitions in Ni can be considered as the contribution of the satellite lines arising from the [KM] shake processes suggested by Deutsch et al. [1] and Ito et al. [2]. Comparison of the new data with several databases showed a good agreement but also discrepancies were found with existing tabulated values.