| 1. |
- Sanchez-Gonzalez, A., et al.
(author)
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Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning
- 2017
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8
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Journal article (peer-reviewed)abstract
- Free-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.
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| 2. |
- Hartmann, G., et al.
(author)
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Circular dichroism measurements at an x-ray free-electron laser with polarization control
- 2016
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 87:8
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Journal article (peer-reviewed)abstract
- A non-destructive diagnostic method for the characterization of circularly polarized, ultraintense, short wavelength free-electron laser (FEL) light is presented. The recently installed Delta undulator at the LCLS (Linac Coherent Light Source) at SLAC National Accelerator Laboratory (USA) was used as showcase for this diagnostic scheme. By applying a combined two-color, multi-photon experiment with polarization control, the degree of circular polarization of the Delta undulator has been determined. Towards this goal, an oriented electronic state in the continuum was created by non-resonant ionization of the O2 1s core shell with circularly polarized FEL pulses at hν 700 eV. An also circularly polarized, highly intense UV laser pulse with hν 3.1 eV was temporally and spatially overlapped, causing the photoelectrons to redistribute into so-called sidebands that are energetically separated by the photon energy of the UV laser. By determining the circular dichroism of these redistributed electrons using angle resolving electron spectroscopy and modeling the results with the strong-field approximation, this scheme allows to unambiguously determine the absolute degree of circular polarization of any pulsed, ultraintense XUV or X-ray laser source. © 2016 Author(s).
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| 3. |
- Haldrup, K., et al.
(author)
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Guest-Host Interactions Investigated by Time-Resolved X-ray Spectroscopies and Scattering at MHz Rates: Solvation Dynamics and Photoinduced Spin Transition in Aqueous Fe(bipy)(3)(2+)
- 2012
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In: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 116:40, s. 9878-9887
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Journal article (peer-reviewed)abstract
- We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.
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| 4. |
- Vandelli, M., et al.
(author)
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Doping-dependent charge- and spin-density wave orderings in a monolayer of Pb adatoms on Si(111)
- 2024
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In: npj Quantum Materials. - 2397-4648. ; 9:1
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Journal article (peer-reviewed)abstract
- In this work we computed the phase diagram as a function of temperature and doping for a system of lead adatoms allocated periodically on a silicon (111) surface. This Si(111):Pb material is characterized by a strong and long-ranged Coulomb interaction, a relatively large value of the spin-orbit coupling, and a structural phase transition that occurs at low temperature. In order to describe the collective electronic behavior in the system, we perform many-body calculations consistently taking all these important features into account. We find that charge- and spin-density wave orderings coexist with each other in several regions of the phase diagram. This result is in agreement with the recent experimental observation of a chiral spin texture in the charge density wave phase in this material. We also find that the geometries of the charge and spin textures strongly depend on the doping level. The formation of such a rich phase diagram in the Si(111):Pb material can be explained by a combined effect of the lattice distortion and electronic correlations.
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| 5. |
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| 6. |
- Allum, F., et al.
(author)
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A localized view on molecular dissociation via electron-ion partial covariance
- 2022
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In: Communications Chemistry. - : Springer Science and Business Media LLC. - 2399-3669. ; 5:1
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Journal article (peer-reviewed)abstract
- Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d(3/2) and 4d(5/2) atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site. Coincidence experiments at free-electron lasers enable time resolved site-specific investigations of molecular photochemistry at high signal rates, but isolating individual dissociation processes still poses a considerable technical challenge. Here, the authors use electron-ion partial covariance imaging to isolate otherwise elusive chemical shifts in UV-induced photofragmentation pathways of the prototypical chiral molecule 1-iodo-2-methylbutane.
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| 7. |
- Bressler, C., et al.
(author)
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Solvation dynamics monitored by combined X-ray spectroscopies and scattering: photoinduced spin transition in aqueous [Fe(bpy)(3)](2+)
- 2014
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In: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 171, s. 169-178
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Journal article (peer-reviewed)abstract
- We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)(3) with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.
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| 8. |
- Biasin, Elisa, et al.
(author)
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Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated[Co(terpy)2]2$
- 2016
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In: Physical Review Letters. - : American Physical Society (APS). - 1079-7114 .- 0031-9007. ; 117:1
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Journal article (peer-reviewed)abstract
- We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.
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| 9. |
- Canton, Sophie, et al.
(author)
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Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts
- 2013
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:11, s. 1972-1976
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Journal article (peer-reviewed)abstract
- Building a detailed understanding of the structure function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 +/- 0.03 angstrom. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.
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| 10. |
- Canton, Sophie, et al.
(author)
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Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses.
- 2015
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6
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Journal article (peer-reviewed)abstract
- Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.
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| 11. |
- Naumova, Maria A., et al.
(author)
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Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum
- 2020
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In: The Journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:21
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Journal article (peer-reviewed)abstract
- Oligonuclear complexes of d4-d7 transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the FeII centers in the low-spin state are independently photoexcited. The Franck-Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)2]2+ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching.
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| 12. |
- Naumova, Maria A., et al.
(author)
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Revealing Hot and Long-Lived Metastable Spin States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies
- 2020
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:6, s. 2133-2141
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Journal article (peer-reviewed)abstract
- An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d4-d7 transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays.
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| 13. |
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| 14. |
- Vester, Peter, et al.
(author)
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Tracking structural solvent reorganization and recombination dynamics following e-photoabstraction from aqueous I-with femtosecond x-ray spectroscopy and scattering
- 2022
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 157:22
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Journal article (peer-reviewed)abstract
- We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I-(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine-electron recombination model without the need for a long-lived (I0:e-) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine-oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I-) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.
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| 15. |
- Witte, V, et al.
(author)
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HIV-1 Nef mimics an integrin receptor signal that recruits the polycomb group protein Eed to the plasma membrane
- 2004
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In: Molecular Cell. - 1097-4164. ; 13:2, s. 179-190
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Journal article (peer-reviewed)abstract
- The Nef protein of human and simian immunodeficiency virus (HIV/SIV) is believed to interfere with T cell activation signals by forming a signaling complex at the plasma membrane. Composition and function of the complex are not fully understood. Here we report that Nef recruits the Polycomb Group (PcG) protein Eed, so far known as a nuclear factor and repressor of transcription, to the membrane of cells. The Nef-induced translocation of Eed led to a potent stimulation of Tat-dependent HIV transcription, implying that Eed removal from the nucleus is required for optimal Tat function. Similar to Nef action, activation of integrin receptors recruited Eed to the plasma membrane, also leading to enhanced Tat/Nef-mediated transcription. Our results suggest a link between membrane-associated activation processes and transcriptional derepression and demonstrate how HIV exploits this mechanism.
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