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Dive into the research topics where K. Kubicek is active.

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Featured researches published by K. Kubicek.


Nature | 2014

Tracking excited-state charge and spin dynamics in iron coordination complexes

Wenkai Zhang; Roberto Alonso-Mori; Uwe Bergmann; Christian Bressler; Matthieu Chollet; Andreas Galler; Wojciech Gawelda; Ryan G. Hadt; Robert W. Hartsock; Thomas Kroll; Kasper Skov Kjær; K. Kubicek; Henrik T. Lemke; Huiyang W. Liang; Drew A. Meyer; Martin Meedom Nielsen; Carola Purser; Edward I. Solomon; Zheng Sun; Dimosthenis Sokaras; Tim Brandt van Driel; Gyoergy Vanko; Tsu-Chien Weng; Diling Zhu; Kelly J. Gaffney

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.


Nature | 2012

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Sven Bernitt; G. V. Brown; Jan K. Rudolph; René Friedrich Steinbrügge; A. Graf; Marcel Leutenegger; Sascha W. Epp; Sita Eberle; K. Kubicek; V. Mäckel; M. C. Simon; E. Träbert; E. W. Magee; C. Beilmann; N. Hell; S. Schippers; A. Müller; S. M. Kahn; A. Surzhykov; Zoltan Harman; Christoph H. Keitel; J. Clementson; F. S. Porter; W. F. Schlotter; J. J. Turner; Joachim Ullrich; P. Beiersdorfer; J. R. Crespo López-Urrutia

Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 ångströms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.


Journal of Physics B | 2010

Photoionization of N3 + and Ar8 + in an electron beam ion trap by synchrotron radiation

M. C. Simon; M. Schwarz; Sascha W. Epp; C. Beilmann; B. L. Schmitt; Zoltan Harman; T. Baumann; P. H. Mokler; Sven Bernitt; R. Ginzel; Stuart G. Higgins; Christoph H. Keitel; R Klawitter; K. Kubicek; V. Mäckel; J. Ullrich; J. R. Crespo López-Urrutia

Photoionization (PI) of multiply and highly charged ions was studied using an electron beam ion trap and synchrotron radiation at the BESSY II electron storage ring. The versatile new method introduced here extends the range of ions accessible for PI investigations beyond current limitations by providing a dense target of ions in arbitrary, i.e. both low and high charge states. Data on near-threshold PI of N3 + and Ar8 + ions, species of astrophysical and fundamental interest, show high resolution and accuracy allowing various theoretical models to be distinguished, and highlight shortcomings of available PI calculations. We compare our experimental data with our new fully relativistic PI calculations within a multiconfiguration Dirac?Fock approach and with other advanced calculations and find generally good agreement; however, detailed examination reveals significant deviations, especially at the threshold region of Ar8 +.


Journal of Physical Chemistry B | 2014

Probing the Hofmeister effect with ultrafast core-hole spectroscopy.

Zhong Yin; Ivan Rajkovic; K. Kubicek; W. Quewedo; Annette Pietzsch; Philippe Wernet; A. Föhlisch; Simone Techert

In the current work, X-ray emission spectra of aqueous solutions of different inorganic salts within the Hofmeister series are presented. The results reflect the direct interaction of the ions with the water molecules and therefore, reveal general properties of the salt-water interactions. Within the experimental precision a significant effect of the ions on the water structure has been observed but no ordering according to the structure maker/structure breaker concept could be mirrored in the results indicating that the Hofmeister effect-if existent-may be caused by more complex interactions.


Review of Scientific Instruments | 2012

High-precision laser-assisted absolute determination of x-ray diffraction angles.

K. Kubicek; J. Braun; H. Bruhns; J. R. Crespo López-Urrutia; P. H. Mokler; Joachim Ullrich

A novel technique for absolute wavelength determination in high-precision crystal x-ray spectroscopy recently introduced has been upgraded reaching unprecedented accuracies. The method combines visible laser beams with the Bond method, where Bragg angles (θ and -θ) are determined without any x-ray reference lines. Using flat crystals this technique makes absolute x-ray wavelength measurements feasible even at low x-ray fluxes. The upgraded spectrometer has been used in combination with first experiments on the 1s2p(1)P(1) → 1s(2)(1)S(0) w-line in He-like argon. By resolving a minute curvature of the x-ray lines the accuracy reaches there the best ever reported value of 1.5 ppm. The result is sensitive to predicted second-order QED contributions at the level of two-electron screening and two-photon radiative diagrams and will allow for the first time to benchmark predicted binding energies for He-like ions at this level of precision.


The Astrophysical Journal | 2009

On the Transition Rate of the Fe x Red Coronal Line

G. Brenner; J. R. Crespo López-Urrutia; Sven Bernitt; D. Fischer; R. Ginzel; K. Kubicek; V. Mäckel; P. H. Mokler; M. C. Simon; J. Ullrich

We present a lifetime measurement of the 3s 23p 5 2 Po 1/2 first excited fine-structure level of the ground state configuration in chlorine-like Fe X, which relaxes to the ground state through a magnetic dipole (M1) transition (the so-called red coronal line) with a wavelength accurately determined to 637.454(1) nm. Moreover, the Zeeman splitting of line was observed. The lifetime of 14.2(2) ms is the most precise one measured in the red wavelength region and agrees well with advanced theoretical predictions and an empirically scaled interpolation based on experimental values from the same isoelectronic sequence.


Structural Dynamics | 2017

Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2′-bipyridine)2(CN)2]

Kasper Skov Kjær; Wenkai Zhang; Roberto Alonso-Mori; Uwe Bergmann; Matthieu Chollet; Ryan G. Hadt; Robert W. Hartsock; Tobias Harlang; Thomas Kroll; K. Kubicek; Henrik T. Lemke; Huiyang W. Liang; Yizhu Liu; Martin Meedom Nielsen; Edward I. Solomon; Dimosthenis Sokaras; Tim Brandt van Driel; Tsu Chien Weng; Diling Zhu; Petter Persson; Kenneth Wärnmark; Villy Sundström; Kelly J. Gaffney

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2− in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6–2N]2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.


XXVI International Conference on Photonic, Electronic and Atomic Collisions | 2009

Photoionization of ions in arbitrary charge states by synchrotron radiation in an electron beam ion trap

M. C. Simon; M. Schwarz; B. L. Schmitt; C. Beilmann; Sascha W. Epp; T. Baumann; K. Kubicek; R. Ginzel; Stuart G. Higgins; R. Klawitter; V. Mäckel; Sven Bernitt; P. H. Mokler; J. Ullrich; J. R. Crespo López-Urrutia

Photoionization of ions in various charge states is studied with an electron beam ion trap at the synchrotron BESSY II. The ion target density achieved by this method, representing an increase of up to four orders of magnitude with respect to conventional techniques, gives unprecedented access to photoionization of highly charged ions at photon energies reaching the keV range. Data on near-threshold photoionization of N3+, Ar12+, Fe12+ combined with measurements on neutral gas targets in the same setup demonstrate the versatility of this technique and show both very good resolution and accuracy.


14th International Conference on the Physics of Highly Charged Ions (HCI 2008) | 2009

Two-loop QED contributions tests with mid-Z He-like ions

K. Kubicek; H. Bruhns; J. Braun; J. R. Crespo López-Urrutia; J. Ullrich

We report about high-precision wavelength determination of H-like and He-like ions at the Heidelberg Electron Beam Ion Trap (HD-EBIT). The experiment was carried out with a novel flat crystal (Si-111) spectrometer applying a reference technique without collimation. The result for the transition energy of the 1s2p 1P1→ 1s2 1S0 resonance line in He-like S14+ agrees with theoretical predictions with an experimental accuracy five times higher than earlier experiments. The same line in Ar16+ was measured with a relative uncertainty of δλ/λ = 2 × 10-6, a factor of 2.5 more accurate than any X-ray wavelength in highly charged ions ever reported, and for He-like ions probes QED two-electron and two-photon radiative corrections. Beside relative wavelength measurements absolute ones were carried out using the Bond method. The results point at the possibility of establishing absolute Lyman-α1 transition X-ray wavelength standards in the future.


Scientific Reports | 2017

Transmission zone plates as analyzers for efficient parallel 2D RIXS-mapping

Felix Marschall; Zhong Yin; Jens Rehanek; Martin Beye; Florian Döring; K. Kubicek; Dirk Raiser; Sreevidya Thekku Veedu; Jens Buck; André Rothkirch; Benedikt Rösner; Vitaliy A. Guzenko; Jens Viefhaus; Christian David; Simone Techert

We have implemented and successfully tested an off-axis transmission Fresnel zone plate as spectral analyzer for resonant inelastic X-ray scattering (RIXS). The imaging capabilities of zone plates allow for advanced two-dimensional (2D) mapping applications. By varying the photon energy along a line focus on the sample, we were able to simultaneously record the emission spectra over a range of excitation energies. Moreover, by scanning a line focus across the sample in one dimension, we efficiently recorded RIXS spectra spatially resolved in 2D, increasing the throughput by two orders of magnitude. The presented scheme opens up a variety of novel measurements and efficient, ultra-fast time resolved investigations at X-ray Free-Electron Laser sources.

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