Uwe Bergmann - Academia.edu (original) (raw)
Papers by Uwe Bergmann
Nature communications, Nov 23, 2017
Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase ... more Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is inv...
Physical Review Letters
Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transi... more Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of Kα amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the 1s core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)]. To apply this new approach to the chemically more sensitive but much weaker Kβ x-ray emission lines requires a mechanism to outcompete the dominant amplification of the Kα emission. Here we report the observation of seeded amplified Kβ x-ray emission from a NaMnO 4 solution using two colors of x-ray free-electron laser pulses, one to create the 1s core-hole population inversion and the other to seed the amplified Kβ emission. Comparing the observed seeded amplified Kβ emission signal with that from conventional Kβ emission into the same solid angle, we obtain a signal enhancement of more than 10 5. Our findings are the first important step of enhancing and controlling the emission of selected final states of the Kβ spectrum with applications in chemical and materials science.
Structural Dynamics
Femtosecond carrier dynamics in layered 2H-MoTe 2 semiconductor crystals have been investigated u... more Femtosecond carrier dynamics in layered 2H-MoTe 2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe 2 , the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d 5/2 core level (M 5-edge, 572-577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M 5-edge involving partially occupied states of the conduction band. A 400 6 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M 5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1-2 ps timescale, which is interpreted as electron-hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.
In situ Structural Investigations of LiNi1/2Mn1/2O2 During Cycling by X-ray Absorption Spectroscopy
ECS Meeting Abstracts
Characterization of LiCoO2 Coating and In-situ XAS on Li1.05Ni0.35Co0.25Mn0.4O2 Cathode Material
ECS Meeting Abstracts
On the Nature of Pt-O Species Observed with XANES and Implications for the Oxygen Reduction Reaction
ECS Meeting Abstracts
Reflets de la physique
L’identification des composés à base de carbone, bien que difficile, est une source d’information... more L’identification des composés à base de carbone, bien que difficile, est une source d’information essentielle dans de nombreuses études archéologiques et paléontologiques. La diffusion Raman de rayons X est une méthode de spectroscopie sur synchrotron qui permet d’identifier des signatures organiques, de retracer l’origine chimique des systèmes étudiés et de comprendre l’altération des composés organiques dans le temps. Cette technique, conduite de manière non destructive, dans l’air, avec une sensibilité en profondeur afin de fournir des informations non compromises par la contamination superficielle, surmonte ainsi plusieurs contraintes fondamentales à la caractérisation des matériaux organiques anciens.
Journal of Synchrotron Radiation
This paper describes a new large-range rapid-scan X-ray fluorescence (XRF) imaging station at bea... more This paper describes a new large-range rapid-scan X-ray fluorescence (XRF) imaging station at beamline 6-2 at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory. This station uses a continuous rapid-scan system with a scan range of 1000 × 600 mm and a load capacity of up to 25 kg, capable of 25–100 µm resolution elemental XRF mapping and X-ray absorption spectroscopy (XAS) of a wide range of objects. XRF is measured using a four-element Hitachi Vortex ME4 silicon drift detector coupled to a Quantum Detectors Xspress3 multi-channel analyzer system. A custom system allows the X-ray spot size to be changed quickly and easily via pinholes ranging from 25 to 100 µm, and the use of a poly-capillary or axially symmetric achromatic optic may achieve a <10 µm resolution in the future. The instrument is located at wiggler beamline 6-2 which has an energy range of 2.1–17 keV, creating K emission for elements up to strontium, and L or M emission for all ot...
Nature
Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II (PS I... more Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II (PS II) discovered by Pierre Joliot in 1969, Bessel Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok's S-state clock or cycle 1,2. The model comprises four (meta)stable intermediates (S 0 , S 1 , S 2 and S 3) and one transient S 4 state, which precedes dioxygen formation occurring in a concerted reaction from two waterderived oxygens bound at an oxo-bridged tetra manganese calcium (Mn 4 CaO 5) cluster in the oxygen evolving complex (OEC) (reviewed in 3-7). This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q B at the acceptor side of PS II (Fig. 1a, Extended Data Fig. 1). Using serial femtosecond X-ray crystallography (SFX) and simultaneous X-ray emission spectroscopy (XES) with multi-flash visible laser excitation at room temperature (RT), we visualize here, for the first time, all (meta)stable states of Kok's cycle by high-resolution structures (2.04-2.08 Å, Extended Data Fig. 1, Extended Data Table 1). In addition, we report structures of two transient states at 150 and 400 µs, revealing important structural changes including the binding of one additional 'water', Ox, during the S 2 →S 3 state transition (2.20-2.50 Å). Our results suggest direct involvement of one water ligand to calcium (W3) in substrate delivery. The binding of the additional oxygen Ox in the S 3 state between Ca and Mn1 supports O-O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O 2 release. Thus, our results exclude peroxo-bond formation in the S 3 state, and the nucleophilic attack of W3 onto W2 is unlikely. Nature (2018), In Press 4 All four (meta)stable S-states of PS II were populated by illumination of darkadapted PS II crystals with 0, 1, 2 or 3 flashes (0F-3F; Fig 1b). The ~2 Å resolution was sufficient for determining the positions of the oxygens bridging the metal atoms, in addition to the terminal water positions of the Mn 4 CaO 5 cluster, critical for discriminating between proposed structures of the S-states, unlike previous structures 8,9. Pivotal for a correct analysis of higher S-state structures is the reliable determination of the S-state composition of the PS II crystals obtained by each flash. We therefore collected the Mn Kβ 1,3 emission spectra in situ (see Methods) 10 , simultaneously with diffraction data. The first moment of the Kβ 1,3 peak shifts toward lower energy in response to the first two flashes (0F→1F→2F) and to higher energy in the 3F sample (Fig. 1c) as expected based on reported Mn redox states (Fig. 1a) 4,11-14. These data, in combination with ex situ O 2 evolution measurements, were used to determine the S-state distribution in each illuminated state (see Methods, Extended Data Fig. 2). Isomorphous difference maps (F obs-F obs) between the dark and flash-illuminated states at the acceptor side (Fig. 1d-f) indicate a clear period two oscillation of Q B between its fully oxidized (0F, 2F) and semiquinone Q B •-(1F) forms. Decrease of the B-factor of the Q B site after 1F suggests a more tightly bound quinone in the 1F sample, due to the formation of the semiquinone Q B •-. The observation of Q B •in the 3F data implies that three electrons were successfully transferred from the Mn 4 CaO 5 cluster to the acceptor side, confirming S-state advancement in both PS II monomers ('a' and 'A', Fig. 1d-f, Extended Data Fig. 3). In the S 1 state, the cluster is in a distinct 'right-open' structure with no bond between Mn1 and O5;
Biochemistry, Jan 28, 2018
Serial femtosecond crystallography (SFX) using the ultrashort X-ray pulses from a X-ray free-elec... more Serial femtosecond crystallography (SFX) using the ultrashort X-ray pulses from a X-ray free-electron laser (XFEL) provides a new way of collecting structural data at room temperature that allows for following the reaction in real time after initiation. XFEL experiments are conducted in a shot-by-shot mode as the sample is destroyed and replenished after each X-ray pulse, and therefore, monitoring and controlling the data quality by using in situ diagnostic tools is critical. To study metalloenzymes, we developed the use of simultaneous collection of X-ray diffraction of crystals along with X-ray emission spectroscopy (XES) data that is used as a diagnostic tool for crystallography, by monitoring the chemical state of the metal catalytic center. We have optimized data analysis methods and sample delivery techniques for fast and active feedback to ensure the quality of each batch of samples and the turnover of the catalytic reaction caused by reaction triggering methods. Here, we des...
Physical review letters, Jan 30, 2018
We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solu... more We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ∼1.7 eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ∼1 fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ∼10^{20} W/cm^{2} for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous...
Physical review letters, Jan 5, 2018
X-ray free-electron lasers combine a high pulse power, short pulse length, narrow bandwidth, and ... more X-ray free-electron lasers combine a high pulse power, short pulse length, narrow bandwidth, and high degree of transverse coherence. Any increase in the photon pulse power, while shortening the pulse length, will further push the frontier on several key x-ray free-electron laser applications including single-molecule imaging and novel nonlinear x-ray methods. This Letter shows experimental results at the Linac Coherent Light Source raising its maximum power to more than 300% of the current limit while reducing the photon pulse length to 10 fs. This was achieved by minimizing residual transverse-longitudinal centroid beam offsets and beam yaw and by correcting the dispersion when operating over 6 kA peak current with a longitudinally shaped beam.
AIP Conference Proceedings
The creation and the decay of a 1s vacancy can result in the excitation of a second electron. In ... more The creation and the decay of a 1s vacancy can result in the excitation of a second electron. In this paper, two different modes of 1s core hole creation as a diagnostic tool to study multi-electron excitations in the K fluorescence emission are compared. The 1s core hole excited state can be created either by photoionization or by radioactive K capture decay. In the latter case, a 1s electron reacts with a proton in the nucleus to yield a neutron and an escaping electron neutrino. We report a comparison of Kβ spectra obtained from x-ray excitation in Mn and K capture in 55-Fe in various chemical environments. The Kβ 1,3 main lines of the photoexcited spectra are broader than the corresponding lines obtained after K capture and the weak satellite lines at higher energies (Kβ 2,5) differ in shape. A theoretical model on the basis of different electron relaxation depending on the mode of core-hole creation is presented.
Physical Review B
The Mn K fluorescence emission in MnO after photoionization and in 55 Fe 2 O 3 after radioactive... more The Mn K fluorescence emission in MnO after photoionization and in 55 Fe 2 O 3 after radioactive electron capture decay from the K shell have been measured using a crystal array spectrometer with an instrumental energy bandwidth of 0.7 eV ͑full width at half maximum͒. Both compounds have a 3d 5 valence electron configuration in the ionic approximation. It is found that the spectral features after K capture in 55 Fe 2 O 3 are shifted in emission energy and are sharper, compared to the spectra following photoionization in MnO, i.e., the spectra exhibit a dependence on the mode of excitation. Crystal-field multiplet calculations including ligandto-metal charge transfer have been carried out for the 1s intermediate states as well as for the 3p to 1s (K) radiative transition. The populated 1s intermediate states after photoionization are found to be spread over several eV. In comparison, only the lowest-lying 1s intermediate states split by the weak (1s,3d) exchange interaction are populated after K capture. It is proposed that the differences in population of the 1s intermediate states together with a term-dependent final-state lifetime broadening can account for the changes in the spectral shapes due to the different modes of excitation.
Surface Roughness And Oxide Layers Of Sputtered Polycrystalline Films
X-Ray/EUV Optics for Astronomy and Microscopy
ABSTRACT
Mössbauer spectroscopy using synchrotron radiation: overcoming detector limitations
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
ABSTRACT
Non-invasive synchrotron-based X-Ray Raman scattering discriminates carbonaceous compounds in ancient and historical materials
Analytical Chemistry
Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems includ... more Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems including ancient and historic materials ranging from cultural heritage to paleontology. Determining their speciation cannot only provide unique information on their origin but may also elucidate degradation processes. Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge (280-350 eV) is a very powerful method to probe carbon speciation. However, the short penetration depth of soft X-rays imposes stringent constraints on sample type, preparation, and analytical environment. A hard X-ray probe such as X-ray Raman scattering (XRS) can overcome many of these difficulties. Here we report the use of XRS at ∼6 keV incident energy to collect carbon K-edge XANES data and probe the speciation of organic carbon in several specimens relevant to cultural heritage and natural history. This methodology enables the measurement to be done in a nondestructive way, in air, and provides information that is not compromised by surface contamination by ensuring that the dominant signal contribution is from the bulk of the probed material. Using the backscattering geometry at large photon momentum transfer maximizes the XRS signal at the given X-ray energy and enhances nondipole contributions compared to conventional XANES, thereby augmenting the speciation sensitivity. The capabilities and limitations of the technique are discussed. We show that despite its small cross section, for a range of systems the XRS method can provide satisfactory signals at realistic experimental conditions. XRS constitutes a powerful complement to FT-IR, Raman, and conventional XANES spectroscopy, overcoming some of the limitations of these techniques.
Acta Crystallographica Section A Foundations of Crystallography
Structural Dynamics
Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetitio... more Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates Structural Dynamics 4, 054902 (2017); 10.1063/1.4993755 Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer Structural Dynamics 4, 044021 (2017); 10.1063/1.4983157 Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO) 5 via a singlet pathway upon excitation at 266 nm
Science (New York, N.Y.), Jun 23, 2017
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apopto... more The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.
Nature communications, Nov 23, 2017
Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase ... more Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is inv...
Physical Review Letters
Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transi... more Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of Kα amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the 1s core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)]. To apply this new approach to the chemically more sensitive but much weaker Kβ x-ray emission lines requires a mechanism to outcompete the dominant amplification of the Kα emission. Here we report the observation of seeded amplified Kβ x-ray emission from a NaMnO 4 solution using two colors of x-ray free-electron laser pulses, one to create the 1s core-hole population inversion and the other to seed the amplified Kβ emission. Comparing the observed seeded amplified Kβ emission signal with that from conventional Kβ emission into the same solid angle, we obtain a signal enhancement of more than 10 5. Our findings are the first important step of enhancing and controlling the emission of selected final states of the Kβ spectrum with applications in chemical and materials science.
Structural Dynamics
Femtosecond carrier dynamics in layered 2H-MoTe 2 semiconductor crystals have been investigated u... more Femtosecond carrier dynamics in layered 2H-MoTe 2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe 2 , the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d 5/2 core level (M 5-edge, 572-577 eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M 5-edge involving partially occupied states of the conduction band. A 400 6 110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M 5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1-2 ps timescale, which is interpreted as electron-hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.
In situ Structural Investigations of LiNi1/2Mn1/2O2 During Cycling by X-ray Absorption Spectroscopy
ECS Meeting Abstracts
Characterization of LiCoO2 Coating and In-situ XAS on Li1.05Ni0.35Co0.25Mn0.4O2 Cathode Material
ECS Meeting Abstracts
On the Nature of Pt-O Species Observed with XANES and Implications for the Oxygen Reduction Reaction
ECS Meeting Abstracts
Reflets de la physique
L’identification des composés à base de carbone, bien que difficile, est une source d’information... more L’identification des composés à base de carbone, bien que difficile, est une source d’information essentielle dans de nombreuses études archéologiques et paléontologiques. La diffusion Raman de rayons X est une méthode de spectroscopie sur synchrotron qui permet d’identifier des signatures organiques, de retracer l’origine chimique des systèmes étudiés et de comprendre l’altération des composés organiques dans le temps. Cette technique, conduite de manière non destructive, dans l’air, avec une sensibilité en profondeur afin de fournir des informations non compromises par la contamination superficielle, surmonte ainsi plusieurs contraintes fondamentales à la caractérisation des matériaux organiques anciens.
Journal of Synchrotron Radiation
This paper describes a new large-range rapid-scan X-ray fluorescence (XRF) imaging station at bea... more This paper describes a new large-range rapid-scan X-ray fluorescence (XRF) imaging station at beamline 6-2 at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory. This station uses a continuous rapid-scan system with a scan range of 1000 × 600 mm and a load capacity of up to 25 kg, capable of 25–100 µm resolution elemental XRF mapping and X-ray absorption spectroscopy (XAS) of a wide range of objects. XRF is measured using a four-element Hitachi Vortex ME4 silicon drift detector coupled to a Quantum Detectors Xspress3 multi-channel analyzer system. A custom system allows the X-ray spot size to be changed quickly and easily via pinholes ranging from 25 to 100 µm, and the use of a poly-capillary or axially symmetric achromatic optic may achieve a <10 µm resolution in the future. The instrument is located at wiggler beamline 6-2 which has an energy range of 2.1–17 keV, creating K emission for elements up to strontium, and L or M emission for all ot...
Nature
Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II (PS I... more Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II (PS II) discovered by Pierre Joliot in 1969, Bessel Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok's S-state clock or cycle 1,2. The model comprises four (meta)stable intermediates (S 0 , S 1 , S 2 and S 3) and one transient S 4 state, which precedes dioxygen formation occurring in a concerted reaction from two waterderived oxygens bound at an oxo-bridged tetra manganese calcium (Mn 4 CaO 5) cluster in the oxygen evolving complex (OEC) (reviewed in 3-7). This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q B at the acceptor side of PS II (Fig. 1a, Extended Data Fig. 1). Using serial femtosecond X-ray crystallography (SFX) and simultaneous X-ray emission spectroscopy (XES) with multi-flash visible laser excitation at room temperature (RT), we visualize here, for the first time, all (meta)stable states of Kok's cycle by high-resolution structures (2.04-2.08 Å, Extended Data Fig. 1, Extended Data Table 1). In addition, we report structures of two transient states at 150 and 400 µs, revealing important structural changes including the binding of one additional 'water', Ox, during the S 2 →S 3 state transition (2.20-2.50 Å). Our results suggest direct involvement of one water ligand to calcium (W3) in substrate delivery. The binding of the additional oxygen Ox in the S 3 state between Ca and Mn1 supports O-O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O 2 release. Thus, our results exclude peroxo-bond formation in the S 3 state, and the nucleophilic attack of W3 onto W2 is unlikely. Nature (2018), In Press 4 All four (meta)stable S-states of PS II were populated by illumination of darkadapted PS II crystals with 0, 1, 2 or 3 flashes (0F-3F; Fig 1b). The ~2 Å resolution was sufficient for determining the positions of the oxygens bridging the metal atoms, in addition to the terminal water positions of the Mn 4 CaO 5 cluster, critical for discriminating between proposed structures of the S-states, unlike previous structures 8,9. Pivotal for a correct analysis of higher S-state structures is the reliable determination of the S-state composition of the PS II crystals obtained by each flash. We therefore collected the Mn Kβ 1,3 emission spectra in situ (see Methods) 10 , simultaneously with diffraction data. The first moment of the Kβ 1,3 peak shifts toward lower energy in response to the first two flashes (0F→1F→2F) and to higher energy in the 3F sample (Fig. 1c) as expected based on reported Mn redox states (Fig. 1a) 4,11-14. These data, in combination with ex situ O 2 evolution measurements, were used to determine the S-state distribution in each illuminated state (see Methods, Extended Data Fig. 2). Isomorphous difference maps (F obs-F obs) between the dark and flash-illuminated states at the acceptor side (Fig. 1d-f) indicate a clear period two oscillation of Q B between its fully oxidized (0F, 2F) and semiquinone Q B •-(1F) forms. Decrease of the B-factor of the Q B site after 1F suggests a more tightly bound quinone in the 1F sample, due to the formation of the semiquinone Q B •-. The observation of Q B •in the 3F data implies that three electrons were successfully transferred from the Mn 4 CaO 5 cluster to the acceptor side, confirming S-state advancement in both PS II monomers ('a' and 'A', Fig. 1d-f, Extended Data Fig. 3). In the S 1 state, the cluster is in a distinct 'right-open' structure with no bond between Mn1 and O5;
Biochemistry, Jan 28, 2018
Serial femtosecond crystallography (SFX) using the ultrashort X-ray pulses from a X-ray free-elec... more Serial femtosecond crystallography (SFX) using the ultrashort X-ray pulses from a X-ray free-electron laser (XFEL) provides a new way of collecting structural data at room temperature that allows for following the reaction in real time after initiation. XFEL experiments are conducted in a shot-by-shot mode as the sample is destroyed and replenished after each X-ray pulse, and therefore, monitoring and controlling the data quality by using in situ diagnostic tools is critical. To study metalloenzymes, we developed the use of simultaneous collection of X-ray diffraction of crystals along with X-ray emission spectroscopy (XES) data that is used as a diagnostic tool for crystallography, by monitoring the chemical state of the metal catalytic center. We have optimized data analysis methods and sample delivery techniques for fast and active feedback to ensure the quality of each batch of samples and the turnover of the catalytic reaction caused by reaction triggering methods. Here, we des...
Physical review letters, Jan 30, 2018
We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solu... more We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ∼1.7 eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ∼1 fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ∼10^{20} W/cm^{2} for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous...
Physical review letters, Jan 5, 2018
X-ray free-electron lasers combine a high pulse power, short pulse length, narrow bandwidth, and ... more X-ray free-electron lasers combine a high pulse power, short pulse length, narrow bandwidth, and high degree of transverse coherence. Any increase in the photon pulse power, while shortening the pulse length, will further push the frontier on several key x-ray free-electron laser applications including single-molecule imaging and novel nonlinear x-ray methods. This Letter shows experimental results at the Linac Coherent Light Source raising its maximum power to more than 300% of the current limit while reducing the photon pulse length to 10 fs. This was achieved by minimizing residual transverse-longitudinal centroid beam offsets and beam yaw and by correcting the dispersion when operating over 6 kA peak current with a longitudinally shaped beam.
AIP Conference Proceedings
The creation and the decay of a 1s vacancy can result in the excitation of a second electron. In ... more The creation and the decay of a 1s vacancy can result in the excitation of a second electron. In this paper, two different modes of 1s core hole creation as a diagnostic tool to study multi-electron excitations in the K fluorescence emission are compared. The 1s core hole excited state can be created either by photoionization or by radioactive K capture decay. In the latter case, a 1s electron reacts with a proton in the nucleus to yield a neutron and an escaping electron neutrino. We report a comparison of Kβ spectra obtained from x-ray excitation in Mn and K capture in 55-Fe in various chemical environments. The Kβ 1,3 main lines of the photoexcited spectra are broader than the corresponding lines obtained after K capture and the weak satellite lines at higher energies (Kβ 2,5) differ in shape. A theoretical model on the basis of different electron relaxation depending on the mode of core-hole creation is presented.
Physical Review B
The Mn K fluorescence emission in MnO after photoionization and in 55 Fe 2 O 3 after radioactive... more The Mn K fluorescence emission in MnO after photoionization and in 55 Fe 2 O 3 after radioactive electron capture decay from the K shell have been measured using a crystal array spectrometer with an instrumental energy bandwidth of 0.7 eV ͑full width at half maximum͒. Both compounds have a 3d 5 valence electron configuration in the ionic approximation. It is found that the spectral features after K capture in 55 Fe 2 O 3 are shifted in emission energy and are sharper, compared to the spectra following photoionization in MnO, i.e., the spectra exhibit a dependence on the mode of excitation. Crystal-field multiplet calculations including ligandto-metal charge transfer have been carried out for the 1s intermediate states as well as for the 3p to 1s (K) radiative transition. The populated 1s intermediate states after photoionization are found to be spread over several eV. In comparison, only the lowest-lying 1s intermediate states split by the weak (1s,3d) exchange interaction are populated after K capture. It is proposed that the differences in population of the 1s intermediate states together with a term-dependent final-state lifetime broadening can account for the changes in the spectral shapes due to the different modes of excitation.
Surface Roughness And Oxide Layers Of Sputtered Polycrystalline Films
X-Ray/EUV Optics for Astronomy and Microscopy
ABSTRACT
Mössbauer spectroscopy using synchrotron radiation: overcoming detector limitations
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
ABSTRACT
Non-invasive synchrotron-based X-Ray Raman scattering discriminates carbonaceous compounds in ancient and historical materials
Analytical Chemistry
Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems includ... more Carbon compounds are ubiquitous and occur in a diversity of chemical forms in many systems including ancient and historic materials ranging from cultural heritage to paleontology. Determining their speciation cannot only provide unique information on their origin but may also elucidate degradation processes. Synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge (280-350 eV) is a very powerful method to probe carbon speciation. However, the short penetration depth of soft X-rays imposes stringent constraints on sample type, preparation, and analytical environment. A hard X-ray probe such as X-ray Raman scattering (XRS) can overcome many of these difficulties. Here we report the use of XRS at ∼6 keV incident energy to collect carbon K-edge XANES data and probe the speciation of organic carbon in several specimens relevant to cultural heritage and natural history. This methodology enables the measurement to be done in a nondestructive way, in air, and provides information that is not compromised by surface contamination by ensuring that the dominant signal contribution is from the bulk of the probed material. Using the backscattering geometry at large photon momentum transfer maximizes the XRS signal at the given X-ray energy and enhances nondipole contributions compared to conventional XANES, thereby augmenting the speciation sensitivity. The capabilities and limitations of the technique are discussed. We show that despite its small cross section, for a range of systems the XRS method can provide satisfactory signals at realistic experimental conditions. XRS constitutes a powerful complement to FT-IR, Raman, and conventional XANES spectroscopy, overcoming some of the limitations of these techniques.
Acta Crystallographica Section A Foundations of Crystallography
Structural Dynamics
Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetitio... more Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates Structural Dynamics 4, 054902 (2017); 10.1063/1.4993755 Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer Structural Dynamics 4, 044021 (2017); 10.1063/1.4983157 Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO) 5 via a singlet pathway upon excitation at 266 nm
Science (New York, N.Y.), Jun 23, 2017
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apopto... more The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.