Publications
- Antolini et al., "The liquid jet endstation for hard X-ray scattering and spectroscopy at the linac coherent light source", Molecules, 29, 2323 (2024).
This paper describes in detail our standard configuration setup for hard X-ray liquid phase scattering and spectroscopy experiments. - Reinhard et al., “Time-resolved X-ray emission spectroscopy and synthetic high-spin model complexes resolve ambiguities in excited-state assignments of transition-metal chromophores: A case study of Fe-amido complexes”, J. Am. Chem. Soc., 146, 17908-17916 (2024).
First results from the Scientific Campaign focused on the role of covalency in photoinduced dynamics of transition metal compounds.
- Shim et al., "Ultrafast x-ray detection of low-spin iron in molten silicate under deep planetary interior conditions", Sci. Adv. 9, eadi615 (2023). In the present-day Earth’s mantle, recent seismic imaging studies have unveiled large mountain to continent-sized enigmatic structures in the lowermost mantle. Some models propose that these dense structures may be the last vestiges of the ancient global magma ocean. A critical experimental observation supporting these models is that iron has a tendency to concentrate more within the magma than crystals at high pressures relevant to the lowermost mantle – making the magma denser than the crystallized rocky layer. Yet, a fundamental question persists: why does iron preferentially dissolve into the magma rather than the crystals? In this study we probe the lattice and electronic structures of iron atoms in silicate magma to find the first direct evidence of iron rearrangement into low spin, resulting in a denser state. This denser state is more stable for iron atoms under extreme pressures, making the magma a more favorable environment for iron atoms to be in, and providing an explanation for why the magma can become denser than crystals as the magma ocean was solidifying. These results suggest that magma densification could occur to a much larger extent in the magma oceans of super-Earths and encouraging the generation of dynamos at high pressures. This potentially shielding of the thin atmospheres of these planets from the intense radiation emitted by their host stars could enable habitable conditions on the super-Earth planetary surface.
- Liu et al., "Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time." Nat. Commun. 14, 2795 (2023). In this study, we used a combination of megaelectronvolt ultrafast electron diffraction to reveal new mechanistic insights into electrocyclic reactions. These reactions are characterized by the simultaneous formation and dissociation of multiple bonds and by their stereospecificity, the ability to chemically lock specific reactant stereoconfigurations in the product. The results show that the stereospecificity is achieved by the multiple bond reformation steps not happening exactly at the same time.
- Reinhard et al., "Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy." Nat. Commun. 14, 2443 (2023). In this study we utilized fs Fe Kβ main line and valence to core X-ray emission spectroscopy, along with x-ray solution scattering, to investigate the photo-aquation reaction of ferricyanide under UV excitation. By analyzing both the Fe Kβ main line and the valence-to-core regions, we were able to identify short-lived intermediates crucial for understanding the reaction mechanism. In the proposed reaction process, the photoexcited ligand-to-metal charge transfer excited state undergoes sub-ps deactivation through dissociative metal-centered excited states, resulting in the formation of a penta-coordinate intermediate. This intermediate decays within 5 ps either by rebinding the dissociated cyanide or via uptake of a water molecule. The methodology employed in this study holds broad applicability for studying reactions in more intricate systems, including heme proteins.
- Hoffman et al., "Liquid Heterostructures: Generation of Liquid–Liquid Interfaces in Free-Flowing Liquid SheetsA microfluidic nozzle was used to collide two jets of oil on either side of a jet of water." Langmuir 38, 12822 (2022)
The hydrodynamic forces of the colliding oil jets cause them to spread out to a micron-thick sheet, while also causing the water to spread out into an even thinner layer inside the oil sheet. The buried oil-water interfaces were identified with ellipsometry, showing the sheets contain large-area liquid-liquid interfaces with minimized bulk liquid background contribution. - Montoya-Castillo et al., "Optically Induced Anisotropy in Time-Resolved Scattering: Imaging Molecular-Scale Structure and Dynamics in Disordered Media with Experiment and Theory", Phys. Rev. Lett. 129, 056001 (2022)
This paper presents the introduction of a rigorous general framework for simulating and interpreting impulsive nuclear and Raman scattering (INXS) data, exemplified by experimental INXS measurements of chloroform performed at LCLS. - Gleason et al., “Ultrafast structural response of shock-compressed plagioclase”. Meteorit. Planet. Sci. 57, 635 (2022)
This paper presents time-resolved in situ XRD measurements probing the phase transformation pathway of plagioclase during shock compression at a sub-nanosecond timescale. These results have implications for the processes induced by meteor impacts on minerals due to the shock waves. We observe how direct amorphization begins at pressures much lower than what was previously assumed, just above the Hugoniot elastic limit of 5 GPa, with full amorphization to a high-density amorphous phase, observed at 32(10) GPa and 20 ns. Upon release, the material partially recrystallizes back into the original structure, demonstrating a memory effect. - Centurion et al, "Ultrafast Imaging of Molecules with Electron Diffraction" , Annu. Rev. Phys. Chem., 73, 21 (2022)
This is a review paper on sub-picosecond ultrafast electron diffraction of structural dynamics in the gas phase.
- Champenois et al., Conformer-specific Photochemistry Imaged in Real Space and Time, Science, 374, 178 (2021)
This paper demonstrates the sensitivity of ultrafast electron diffraction to nuclear structure changes down to the level of conformers. It demonstrates in real-time how a specific conformer of the molecule α-phellandrene transforms into the specific photoproduct isomer, which is predicted by the Woodward-Hoffmann rules. - Fuller et al. " Resonant X-ray emission spectroscopy from broadband stochastic pulses at an X-ray free electron laser”. Commun. Chem. 4, 84 (2021)
This paper demonstrates that a resonant X-ray emission (RXES) spectrum can be inferred by correlating for each shot the fluorescence intensity from the sample with spectra of the LCLS SASE beam. We obtain resolved narrow and chemically rich information in core-to-valence transitions of the pre-edge region at the Fe K-edge. Our approach avoids monochromatization, provides higher photon flux to the sample, and allows non-resonant signals like elastic scattering to be simultaneously recorded. - Ledbetter et al., "Photodissociation of aqueous I3- observed with liquid-phase ultrafast mega-electron-volt electron diffraction", Struct. Dyn., 7, 064901 (2020)
This paper presents results from an investigation into the structural dynamics of I3- after photoexcitation. - Lin et al., "Imaging the short-lived hydroxyl-hydronium pair in ionized liquid water", Science, 374, 92-95 (2021)
This paper represents the first observation of the structural signature of the hydroxyl-hydronium pair in water created by 800 nm strong field ionization. - Wolf et al., "Transient Resonant Auger-Meitner Spectra of Photoexcited Thymine", Faraday Discuss., 228, 555 (2021)
This study explores effects in excited state resonant Auger-Meitner electron spectra of the nucleobase thymine. - Yang et al., "Direct Observation of Ultrafast Hydrogen Bond Strengthening in Liquid Water", Nature, 596, 531 (2021).
This paper reports on a time-resolved study of structural relaxation of water after vibrational excitation. It marks the first time-resolved study with direct sensitivity to hydrogen structural dynamics.
- van Driel et al., “The epix10k 2-megapixel hard X-ray detector at LCLS”. J. Synchrotron Radiat. 27, 608-615 (2020)
Here the first measurements on the new ePix10k detector are presented and the performance under typical XFEL conditions evaluated during an LCLS X-ray diffuse scattering experiment measuring the 9.5 keV X-ray photons scattered from a thin liquid jet. - Kunnus et al., “Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering“. Nat. Comm. 11, 634 (2020)
Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation originating from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces. - Ledbetter et al., “Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy”. J. Chem. Phys. 152, 074203 (2020)
Here we demonstrate the use of the high-sensitive but low yield valence-to-core x-ray emission spectroscopy (VtC XES) to study geometric and electronic structural changes induced by photoexcitation in the fs time domain. Time-resolved VtC XES on a series of ferrous complexes [Fe(CN)2n(2, 2′-bipyridine)3−n]−2n+2, n = 1, 2, 3, are compared with ground state DFT calculations revealing signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps. - Alonso-Mori et al., “Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy”. Sci. Rep. 10 (1), 1-7 (2020)
This paper reports the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in the micron-sized focused LCLS beam. - Nunes et al., "Liquid-phase Mega-electron-volt ultrafast electron diffraction", Struct. Dyn., 7, 024301 (2020)
This instrument paper describes the liquid phase ultrafast electron diffraction endstation and results from static investigations of the structure of water. - Yang et al., "Simultaneous observation of nuclear and electronic dynamics by ultrafast electron diffraction", Science, 368, 885 (2020).
This paper showcases the sensitivity of electron diffraction to electronic structure changes in addition to its sensitivity to structural dynamics. - Wolf and Gühr, "Photochemical Pathways in Nucleobases measured with an X-ray FEL", Philos. Trans. Royal Soc. A, 377, 20170473 (2019).
This is a review paper for time-resolved gas phase spectroscopy with soft X-rays for photochemical dynamcis.
- Britz et al., “Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS”. Phys. Chem. Chem. Phys. 22, 2660-2666 (2019)
First femtosecond-resolved EXAFS measurement in solution phase chemistry at an XFEL - Inhester et al., "Characterization of chemical bond dissociation using femtosecond core-level electron spectroscopy", J. Phys. Chem. Lett., 10, 6536 (2019)
Theoretical study of signatures from bond dissociation in carbon-edge time-resolved XPS. - Shen et al., "Femtosecond gas-phase mega-electron-volt ultrafast electron diffraction", Struct. Dyn., 6, 054305 (2019)
Tihis is an instrument paper describing the status of the R&D effort at the megaelectronvolt ultrafast electron diffraction facility for time-resolved experiments on gas phase samples. - Sierra et al. “The macromolecular femtosecond crystallography instrument at the linac coherent light source”. J. Synchrotron Rad. 26 (2), 346-357 (2019)
Instrument paper for MFX describing capabilities and scientific highlights up to 2019. - Abraham et al., “A high-throughput energy-dispersive tender X-ray spectrometer for shot-to-shot sulfur measurements”. J. Synchrotron Rad. 26 (3), 629-634 (2019)
This paper describes the prototype of a large throughput and high energy resolution tender X-ray emission spectrometer with an overall signal intensity increased by a factor of ∼15 over existing designs. Implementation of this approach will be realized in the design of the tender X-ray spectroscopy endstation for LCLS-II (TXI). - Wolf et al., "The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction", Nat. Chem. 11, 504 – 509, (2019)
This study is the first demonstration of imaging a photochmical reaction in a molecule exclusively comprised of light elements with sub-Angstrom and sub-picosecond resolution. Apart from the ring-opening, we observed coherent nuclear wavepacket motion in the ground state of the photoproduct. - Wolf and Gühr, Photochemical Pathways in Nucleobases measured with an X-ray FEL, Philos. Trans. Royal Soc. A, 377, 20170473 (2019).
- Alonso-Mori et al., “Xray Free Electron Lasers, Chapter X: X-Ray Spectroscopy with XFELs”. Book Chapter. Springer, Cham. 377-399 (2018)
Book chapter providing an inclusive review of XFEL spectroscopic studies on biological samples and focusing on the description of the experimental aspects of such measurements up to 2018 - Yang et al., Imaging CF3I conical intersection and photodissociation dynamics by ultrafast electron diffraction, Science, 361, 64 (2018)
This study represents the first demonstration of imaging photochemistry with femtosecond temporal and sub-Angstrom spatial resolution in a gas phase sample.
