Research and publications

• Research projects

  X-ray and Atomic Inner-Shell Processes
  Status: Completed

  Start	01.05.2013
  End	30.04.2017
  Funding	SNSF  Open project sheet
  Atomic inner-shell processes encompassing photon- and particle-ionization and the resulting atomic relaxation have become a growing field of both basic and applied research due to the fundamental role they play in understanding atomic structure and dynamics. To explore the many aspects of atomic excitation and decay the high-resolution x-ray spectroscopy technique represents a powerful tool. The advent of x-ray free electron lasers (XFELs) has given a boost to the field and opened new exploration routes. A survey of current research in x-ray and atomic inner-shell processes is given in [1,2]. The specific topics that will be addressed in the present proposal are the following: 1. Hollow atoms (Fribourg-SLS-SSRL collaboration) The project is divided into two parts. In the first part we plan to investigate sequential multi-photon ionization and excitation processes induced by ultra-intense XFEL pulses and the subsequent radiative decay of K-shell double vacancy states for Cu by means of high-resolution x-ray emission spectroscopy. The second part deals with the study of the Kß hypersatellite spectra of 3d transition metals and their oxides in single-photon and electron impact double ionization. 2. Probing the electronic structure of nanolayered systems with the synchrotron radiation based GEXRF-RIXS-XAS technique (Fribourg-Oulu-SLS collaboration) We plan to validate the SR-based GEXRF technique combined with high-energy resolution resonant x-ray scattering (RIXS) and x-ray absorption spectroscopy (XAS) as a powerful tool for electronic structure studies of nanoscale materials. To this end, the depth-dependent electronic properties of Si0.5Ge0.5 alloys films grown on Si and Ge, as well as MoS2 nanolayers deposited on SiO2 will be investigated. We plan to study the effect of the surface and layer thickness on the electronic structure of Si0.5Ge0.5 nano-films and of MoS2 ultra-thin layers. 3. High energy resolution off-resonant spectroscopy (HEROS) (Fribourg-SLS collaboration) This project is also divided into two parts. The first one concerns single-shot high-resolution off-resonant XES measurements of metallic Cu and complexes containing CuI and CuII that will be performed at LCLS using a pink beam of 8.96 keV. The dynamic changes in the unoccupied electronic states of the Cu atoms exposed to ultra fast and very intense XFEL pulses will be probed. The XES spectra will be measured with a multi-crystal von Hamos spectrometer. In the second part that will be carried out at the SLS, we plan to use the same high energy resolution off-resonant x-ray spectroscopy method to determine the density-of-states of Ta under in situ conditions during the formation of active complexes and activation of methane. The sample will be loaded to the quartz-capillary reactor available at the SuperXAS beamline. The reaction paths will be followed by recording continuously the x-ray spectra with a time resolution of 1-2 sec. Keywords: high-resolution x-ray spectroscopy, hollow atoms, grazing emission x-ray fluorescence, multi-photon ionization, off-resonant x-ray emission spectroscopy, x-ray absorption spectra, XFEL [1] Proc. 20th Int. Conf. on X-Ray and Inner-shell processes (Melbourne, 2006), ed. by P.M. Bergstrom, L. Wojnárovits, A. Miller, B. Krässig, J. H. Hubbel, C. T. Chantler, Radiation Physics and Chemistry, 75, 1431 (2006). [2] Proc. 21st Int. Conf. on X-Ray and Inner-shell processes (Paris, 2008), ed. by M. Simon, A. Taleb-Ibrahimi, P. Indelicato and S. Carniato, Eur. Phys. J. Special Topics 169, (2009).

  X-ray and Atomic Inner-Shell Processes
  Status: Completed

  Start	01.05.2011
  End	30.04.2013
  Funding	SNSF  Open project sheet
  Atomic inner-shell processes such as photon-atom and charged particle-atom interactions are equally important in fundamental and applied research. Experimental and theoretical advances as well as the wealth of x-ray based applications in many fields of sciences and technology have given a new boost to the domain. High-resolution x-ray spectroscopy represents a powerful and sometimes unique experimental technique. A survey of the intense current research in x-ray and atomic inner-shell processes is given in [1,2]. The specific topics that will be addressed in the present proposal are the following: 1. Hollow atoms (Fribourg-ESRF-Kielce-Ljubljana collaboration) The experimental investigation of the extremely weak radiative decay channel of the K-shell double vacancy states produced in single photon impact, namely the two-electron-one-photon (TEOP) transition Khaa (1s-2 ¿2s-12p-1) of Mg, Al and Si will be undertaken. The obtained branching ratios and energies will be compared to theoretical predictions. To the best of our knowledge, the TEOP transitions resulting from photon impact have never been observed. 2. Near-threshold evolution of the triple KKL ionization in electron impact In the proposed experiment the formation of triple KKL vacancies in Na and Si as a result of electron bombardment will be investigated. The evolution of the triple-to-double cross section ratios will be determined for electron beam energies from the onset of the double K-shell ionization to about 5 times the triple KKL threshold energy. The main aim of the project is to identify the different mechanisms leading to the multi-electron ejection, in particular the role of the two-step processes. 3. Investigation of dopant concentration profiles and nanostructure morphologies using the SR-based high-resolution grazing emission x-ray fluorescence technique (Fribourg-Kielce-SLS collaboration) The depth distributions of ions implanted at low energy in Si and Ge wafers will be determined by means of the novel SR-based high-resolution GEXRF method. Dopants such as As, P, Ga, In and Sb which are relevant for the semiconductor industry will be used. The same technique will be applied to pursue our investigations of the morphology of nanoscaled surface structures. The existing GEXRF setup will be upgraded with a x-ray polycapilary placed in front of the target holder of the von Hamos spectrometer to focus the synchrotron radiation beam on the samples. 4. X-ray spectroscopy of low-energy highly charged ions (Kielce-Fribourg Collaboration) X-ray spectroscopy investigations of recombination processes in plasma, hollow atoms and ion-surface interactions will be carried out at the highly-charged ion source EBIS-A which is presently in development at the University of Kielce. The design of the EBIS-A facility will allow to measure the x-rays emitted by the highly-charged ions either before their extraction from the source or in the experimental chamber where solid target of different materials will be installed. Depending on the project, the x-ray measurements will be performed using an energy-dispersive SDD detector or a commercially available wavelength-dispersive x-ray spectrometer. [1] Proc. 20th Int. Conf. on X-Ray and Inner-shell processes (Melbourne, 2006), ed. by P.M. Bergstrom, L. Wojnárovits, A. Miller, B. Krässig, J. H. Hubbel, C. T. Chantler, Radiation Physics and Chemistry, 75, 1431 (2006). [2] Proc. 21th Int. Conf. on X-Ray and Inner-shell processes (Paris, 2008), ed. by M. Simon, A. Taleb-Ibrahimi, P. Indelicato and S. Carniato, Eur. Phys. J. Special Topics 169, (2009).

  X-ray and Atomic Inner-Shell Processes
  Status: Completed

  Start	01.05.2009
  End	30.04.2011
  Funding	SNSF  Open project sheet
  The potential of x-ray and atomic inner-shell processes as a probe of atomic structure and dynamics, as well as in applied research is well established. Photon-atom and charged particle-atom interactions continue to play an important role in the field of modern atomic physics. High-resolution x-ray emission spectroscopy represents a powerful tool to investigate x-ray inner-shell processes. Indeed, many aspects of atomic excitation and decay channels can be studied by this technique.Experimental results help to improve the atomic structure calculations that are also used as a basis in other fields of sciences and technology. The development of synchrotron radiation facilities and numerous x-ray based applications have given boost to the field. The specific topics that will be addressed in the present project include the double ionization of two innermost atomic electrons upon single-photon absorption in neutral atoms. This process is one of the most sensitive probes of atomic electron correlation effects that lie at the heart of understanding atomic and molecular structure. Further, the L-subshell Coster-Kronig yields of mid-Z elements via a new technique of synchrotron radiation based high-resolution x-ray spectroscopy will be determined. The Coster-Kronig transitions are sensitive to many-body and solid-state effects and are also of practical importance. The diffraction x-ray spectroscopy of high-Z few-electron ions investigating the K-shell radiative recombination effects will be also investigated. An applied research topic concerns the application of the novel technique of synchrotron radiation based high-resolution grazing-emission x-ray fluorescence (GEXRF) technique for the detection of ultra-low level impurities in Si. The technique offers also important aspects for x-ray microanalysis, namely the elemental 2D-mapping and depth profiling capabilities. New possibilities offered by the high-resolution GEXRF technique will be also tested by studying different materials related to the fields of microelectronics, archaeology and geology.The high-resolution x-ray spectroscopy experiments will be performed in-house using x-ray tubes and electron guns, with x-ray synchrotron radiation at the ESRF, Grenoble, France, SLS, Villigen, Switzerland, and ELETTRA, Trieste, Italy, and also with heavy ions at GSI, Darmstadt, Germany.

  X-Ray and Atomic Inner-Shell Processes
  Status: Completed

  Start	01.05.2007
  End	30.04.2009
  Funding	SNSF  Open project sheet
  1. Hollow atoms The excitation energy dependence of the double-to-single K-shell photoionization cross section for 20 = Z = 24 will be investigated. 2. KMM Radiative Auger emission in low Z atoms following near threshold photoexcitation (Fribourg-Ljubljana collaboration) We plan to study the KMM Radiative Auger process for Ca and Ti in the near-threshold regime. 3. Energy dependence of photoinduced doubly KL and triply KLL excited states in Mg and Al (Fribourg-Kielce collaboration) We propose to carry out a high resolution study of the photoinduced double (KL) and triple (KLL) excited states for Mg and Al. 4. Probing the detection threshold of ultra-trace level Al impurities on Si wafer surfaces by means of high-resolution GEXRF (Fribourg-Kielce-ESRF collaboration) We propose to probe the possibilities of improving the detection threshold of ultra-trace aluminium impurities on the surface of silicon wafers by combining the advantages brought by the use of intense, monochromatic and polarized x-ray beams for the sample excitation, grazing emission x-ray fluorescence and high-resolution x-ray detection. 5. High-resolution investigation of the REXS and XRRS processes in free atoms and molecules (Ljubljana-Fribourg collaboration) It is proposed to study the resonant elastic x-ray scattering (REXS) and x-ray resonant Raman scattering (XRRS) around the K-edges of Ne, gaseous K and Kr as well as around the L-edge of Xe. The measurements will be then extended to some simple molecules such as SF6 at the K-edge of fluorine and CO and CO2 at the K-edge of oxygen. The measurements will be performed at the synchrotron radiation facility ELETTRA by means of high-resolution using a Johansson-type bent crystal spectrometer. 6. X-ray studies of the radiative recombination (Kielce-GSI-Fribourg collaboration) As a continuation of a former experiment performed at GSI in which the radiative recombination (RR) of bare heavy ions was found to depend on the strength of the guiding magnetic field in the electron cooler, we plan to pursue this project with the specific aim of investigating in details the interaction between the RR process and external magnefic fields, an effect which is new and not understood. 7. Diffraction x-ray spectroscopy of few-electron ions (SPARC collaboration) One of the main activities of the SPARC collaboration in the coming years will concern the investigation of QED effects in high-Z few electron ions. For these studies, the existing FOCAL crystal spectrometer which was developed for high energy photons (30-120 keV) should be improved and a new diffraction spectrometer has to be developed for lower energy x-rays (1-10 keV).

  X-ray Atomic and Inner-Shell Processes
  Status: Completed

  Start	01.05.2007
  End	30.04.2009
  Funding	SNSF

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