TY - JOUR A1 - Send, Sebastian A1 - Abboud, Ali A1 - Hartmann, Robert A1 - Huth, M. A1 - Leitenberger, Wolfram A1 - Pashniak, N. A1 - Schmidt, J. A1 - Strüder, Lothar A1 - Pietsch, Ullrich T1 - Characterization of a pnCCD for applications with synchrotron radiation JF - Nuclear instruments & methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics ; A, Accelerators, spectrometers, detectors and associated equipment N2 - In this work we study the response of a pnCCD by means of X-ray spectroscopy in the energy range between 6 key and 20 key and by Laue diffraction techniques. The analyses include measurements of characteristic detector parameters like energy resolution, count rate capability and effects of different gain settings. The limit of a single photon counting operation in white beam X-ray diffraction experiments is discussed with regard to the occurrence of pile-up events, for which the energy information about individual photons is lost. In case of monochromatic illumination the pnCCD can be used as a fast conventional CCD with a charge handling capacity (CHC) of about 300,000 electrons per pixel. If the CHC is exceeded, any surplus charge will spill to neighboring pixels perpendicular to the transfer direction due to electrostatic repulsion. The possibilities of increasing the number of storable electrons are investigated for different voltage settings by exposing a single pixel with X-rays generated by a microfocus X-ray source. The pixel binning mode is tested as an alternative approach that enables a pnCCD operation with significantly shorter readout times. KW - pnCCD KW - X-ray spectroscopy KW - X-ray imaging KW - Energy-dispersive Laue diffraction Y1 - 2013 U6 - https://doi.org/10.1016/j.nima.2013.01.044 SN - 0168-9002 VL - 711 IS - 5 SP - 132 EP - 142 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Oberg, H. A1 - Gladh, Jörgen A1 - Anniyev, Toyli A1 - Beye, Martin A1 - Coffee, Ryan A1 - Föhlisch, Alexander A1 - Katayama, T. A1 - Kaya, Sarp A1 - LaRue, Jerry A1 - Mogelhoj, Andreas A1 - Nordlund, Dennis A1 - Ogasawara, Hirohito A1 - Schlotter, William F. A1 - Sellberg, Jonas A. A1 - Sorgenfrei, Florian A1 - Turner, Joshua J. A1 - Wolf, Martin A1 - Wurth, W. A1 - Ostrom, Henrik A1 - Nilsson, Anders A1 - Norskov, Jens K. A1 - Pettersson, Lars G. M. T1 - Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state JF - Surface science N2 - We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate. (C) 2015 Elsevier B.V. All rights reserved. KW - CO desorption KW - Potential of mean force KW - Two-temperature model KW - Pump-probe KW - X-ray spectroscopy KW - Density functional theory Y1 - 2015 U6 - https://doi.org/10.1016/j.susc.2015.03.011 SN - 0039-6028 SN - 1879-2758 VL - 640 SP - 80 EP - 88 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Niskanen, Johannes A1 - Fondell, Mattis A1 - Sahle, Christoph J. A1 - Eckert, Sebastian A1 - Jay, Raphael Martin A1 - Gilmore, Keith A1 - Pietzsch, Annette A1 - Dantz, Marcus A1 - Lu, Xingye A1 - McNally, Daniel E. A1 - Schmitt, Thorsten A1 - Vaz da Cruz, Vinicius A1 - Kimberg, Victor A1 - Föhlisch, Alexander A1 - Gel’mukhanov, Faris T1 - Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions JF - Proceedings of the National Academy of Sciences of the United States of America N2 - The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 +/- 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuous-distribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray-matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscale-length correlations within the continuous distribution remains open. KW - structure of water KW - X-ray spectroscopy KW - continuous distribution model Y1 - 2019 U6 - https://doi.org/10.1073/pnas.1815701116 SN - 0027-8424 VL - 116 IS - 10 SP - 4058 EP - 4063 PB - National Acad. of Sciences CY - Washington ER -