Project A4

Michael Diez, Henning Kirchberg, Andreas Galler, Sebastian Schulz, Mykola Biednov, Christina Bömer, Tae-Kyu Choi, Angel Rodriguez-Fernandez, Wojciech Gawelda, Dmitry Khakhulin, Katharina Kubicek, Frederico Lima, Florian Otte, Peter Zalden, Ryan Coffee, Michael Thorwart, and Christian Bressler

X-ray free-electron laser sources enable time-resolved X-ray studies with unmatched temporal resolution. To fully exploit ultrashort X-ray pulses, timing tools are essential. However, new high repetition rate X-ray facilities present challenges for currently used timing tool schemes. Here we address this issue by demonstrating a sensitive timing tool scheme to enhance experimental time resolution in pump-probe experiments at very high pulse repetition rates. Our method employs a self-referenced detection scheme using a time-sheared chirped optical pulse traversing an X-ray stimulated diamond plate. By formulating an effective medium theory, we confirm subtle refractive index changes, induced by sub-milli-Joule intense X-ray pulses, that are measured in our experiment. The system utilizes a Common-Path-Interferometer to detect X-ray-induced phase shifts of the optical probe pulse transmitted through the diamond sample. Owing to the thermal stability of diamond, our approach is well-suited for MHz pulse repetition rates in superconducting linear accelerator-based free-electron lasers.

Nat Commun 14, 2495 (2023)

P. Vester, K. Kubicek, T. Assefa, E. Biasin, M. Christensen, A. O. Dohn, T. B. van Driehl, A. Galler, W. Gawelda, T. C. B. Harlang, N. E. Henriksen, K. S. Kjaer, T. S. Kuhlmann, Z. Németh, Z. Nurekeyev, M. Papai, G. Vankó, H. Yavas, D. B. Zederkof, U. Bergmann, M. M. Nielsen, K. B. Moller, K. Haldrup, and C. Bressler

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.

J. Chem. Phys 157, 22401 (2022)

Miguel Ochmann, Vinícius Vaz da Cruz, Sebastian Eckert, Nils Huse and Alexander Föhlisch

The inherent stability of methylated formamides is traced to a stabilization of the deep-lying σ-framework by resonant inelastic X ray scattering at the nitrogen K-edge. Charge transfer from the amide nitrogen to the methyl groups underlie this stabilization mechanism that leaves the aldehyde group essentially unaltered and explains the stability of secondary and tertiary amides.

Chemical Communications, tba (2022)

Matthias Ruppert,  Hanh Bui, Laxmi Kishore Sagar, Pieter Geiregat, Zeger Hens, Gabriel Bester and Nils Huse

Femtosecond pump–probe spectroscopy reveals ultrafast carrier dynamics in mid-infrared (MIR) colloidal HgTe nanoparticles with a bandgap of 2.5 μm. We observe intraband relaxation processes after photoexcitation ranging from resonant excitation up to the multi-exciton generation (MEG) regime by identifying initially excited states from atomic effective pseudopotential calculations. Our study elucidates the earliest dynamics below 10 ps in this technologically relevant material. With increasing photon energy, we find carrier relaxation times as long as 2.1 ps in the MEG regime close to the ionization threshold of the particles. For all photon energies, we extract a constant mean carrier energy dissipation rate of 0.36 eV ps⁻¹ from which we infer negligible impact of the density of states on carrier cooling.

Nanoscale, 14, 4123-4130 (2022)

A. Britz, S. I. Bokarev, T. A. Assefa, E. G. Bajnóczi, Z. Németh, G. Vankó, N. Rockstroh, H. Junge, M. Beller, G. Doumy, A. M. March, S. H. Southworth, S. Lochbrunner, C. Bressler, W. Gawelda

Time-resolved X-ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X-ray transitions with insights from high-level ab initio calculations. The specific target has been a heteroleptic [Ir^III(ppy)₂(bpy)]⁺ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe₃ (CO)₁₂ as a water reduction catalyst. The relevant molecular transitions have been characterized via high-resolution Ir L-edge X-ray absorption spectroscopy on the picosecond time scale and restricted active space self-consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance.

ChemPhysChem 22, 693-700 (2021)

Sebastian Eckert, Vinícius Vaz da Cruz, Miguel Ochmann, Inga von Ahnen, Alexander Föhlisch, and Nils Huse

Symmetry and its breaking crucially define the chemical properties of molecules and their functionality. Resonant inelastic X-ray scattering is a local electronic structure probe reporting on molecular symmetry and its dynamical breaking within the femtosecond scattering duration. Here, we study pyrimidine, a system from the C_2v point group, in an aqueous solution environment, using scattering though its 2a₂ resonance. Despite the absence of clean parity selection rules for decay transitions from in-plane orbitals, scattering channels including decay from the 7b₂ and 11a₁ orbitals with nitrogen lone pair character are a direct probe for molecular symmetry. Computed spectra of explicitly solvated molecules sampled from a molecular dynamics simulation are combined with the results of a quantum dynamical description of the X-ray scattering process. We observe dominant signatures of core-excited Jahn–Teller induced symmetry breaking for resonant excitation. Solvent contributions are separable by shortening of the effective scattering duration through excitation energy detuning.

J. Phys. Chem. Lett. 12, 8637–8643 (2021)

Raphael M. Jay, Sebastian Eckert, Benjamin E. Van Kuiken, Miguel Ochmann, Markus Hantschmann, Amy A. Cordones, Hana Cho, Kiryong Hong, Rory Ma, Jae Hyuk Lee, Georgi L. Dakovski, Joshua J. Turner, Michael P. Minitti, Wilson Quevedo, Annette Pietzsch, Martin Beye, Tae Kyu Kim, Robert W. Schoenlein, Philippe Wernet, Alexander Föhlisch, and Nils Huse

We demonstrate for the case of photoexcited [Ru(2,2′-bipyridine)₃]²⁺ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal–ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.

J. Phys. Chem. Lett. , 12, 6676–6683 (2021)

Yohei Uemura, Ahmed S. M. Ismail, Sang Han Park, Soonnam Kwon, Minseok Kim, Yasuhiro Niwa, Hiroki Wadati, Hebatalla Elnaggar, Federica Frati, Ties Haarman, Niko Höppel, Nils Huse, Yasuyuki Hirata, Yujun Zhang, Kohei Yamagami, Susumu Yamamoto, Iwao Matsuda, Tetsuo Katayama, Tadashi Togashi, Shigeki Owada, Makina Yabashi, Uufuk Halisdemir, Gertjan Koster, Toshihiko Yokoyama, Bert M. Weckhuysen, and Frank M. F. de Groot

Copper tungstate (CuWO₄) is an important semiconductor with a sophisticated and debatable electronic structure that has a direct impact on its chemistry. Using the PAL-XFEL source, we study the electronic dynamics of photoexcited CuWO₄. The Cu L₃ X-ray absorption spectrum shifts to lower energy upon photoexcitation, which implies that the photoexcitation process from the oxygen valence band to the tungsten conduction band effectively increases the charge density on the Cu atoms. The decay time of this spectral change is 400 fs indicating that the increased charge density exists only for a very short time and relaxes electronically. The initial increased charge density gives rise to a structural change on a time scale longer than 200 ps.

J. Phys. Chem. C 125, 7329–7336 (2021)

R. N. Coffee, N. Hartmann, R. Heider, M. S. Wagner, W. Helml, T. Katayama, T. Sato, T. Sato, M. Yabashi, C. Bressler

We present a novel, highly versatile, and self-referenced arrival time monitor for measuring the femtosecond time delay between a hard X-ray pulse from a free-electron laser and an optical laser pulse, measured directly on the same sample used for pump-probe experiments. Two chirped and picosecond long optical supercontinuum pulses traverse the sample with a mutually fixed time delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in between both pulses. Behind the sample the supercontinuum pulses are temporally overlapped to yield near-perfect destructive interference in the absence of the X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero contributions at certain optical wavelengths, which serve as a measure of the relative arrival time of the X-ray pulse with an accuracy of better than 25 fs. We find an excellent agreement of our monitor with the existing timing diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We demonstrate a high sensitivity to measure X-ray pulses with pulse energies as low as 30 μJ. Using a free-flowing liquid jet as interaction sample ensures the full replacement of the sample volume for each X-ray/optical event, thus enabling its utility even at MHz repetition rate XFEL sources.

Scientific Reports volume 11, Article number: 3562 (2021)

Robin Y. Engel, Maria Ekimova, Piter S. Miedema, Carlo Kleine, Jan Ludwig, Miguel Ochmann, BenjaminGrimm-Lebsanft, Rory Ma, Melissa Teubner, Siarhei Dziarzhytski, Günter Brenner, Marie Kristin Czwalinna,Benedikt Rösner, Tae Kyu Kim, Christian David, Sonja Herres-Pawlis, Michael Rübhausen, Erik T. J. Nibbering, Nils Huse, and Martin Beye

X-ray absorption near-edge structure (XANES) spectroscopy provides element specificity and is a powerful experimental method to probelocal unoccupied electronic structures. In the soft x-ray regime, it is especially well suited for the study of 3d-metals and light elements suchas nitrogen. Recent developments in vacuum-compatible liquid flat jets have facilitated soft x-ray transmission spectroscopy on molecules insolution, providing information on valence charge distributions of heteroatoms and metal centers. Here, we demonstrate XANES spectros-copy of molecules in solution at the nitrogen K-edge, performed at FLASH, the Free-Electron Laser (FEL) in Hamburg. A split-beamreferencing scheme optimally characterizes the strong shot-to-shot fluctuations intrinsic to the process of self-amplified spontaneousemission on which most FELs are based. Due to this normalization, a sensitivity of 1% relative transmission change is achieved, limited byfundamental photon shot noise. The effective FEL bandwidth is increased by streaking the electron energy over the FEL pulse train tomeasure a wider spectral window without changing FEL parameters. We propose modifications to the experimental setup with the potentialof improving the instrument sensitivity by two orders of magnitude, thereby exploiting the high peak fluence of FELs to enableunprecedented sensitivity for femtosecond XANES spectroscopy on liquids in the soft x-ray spectral region.

Structural Dynamics 8, 014303 (2021)

M. Naumova, A.Kalinko, J. W. L. Wong, S. Alvarez Gutierez, J. Meng, M. Liang, M. Abdellah, H. Geng, W.Lin, K. Kubicek, M. Biednov, F. Lima, A. Galler, P. Zalden, S. Checchia, P. Mante, J. Zimara, D. Schwarzer, S. Demeshko, V. Y. Murzin, D. J Gosztola, M. Jarenmark, J. Zhang, M. Bauer, L. Max Lawson Daku, D. Khakhulin, W. Gawelda, C. Bressler, F. Meyer, K. Zheng, S. E. Canton

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.

J. Chem. Phys. 152, 214301 (2020)

Henning Kirchberg and Michael Thorwart

The time-dependent fluorescence Stokes shift monitors the relaxation of the polarization of a polar solvent in the surroundings of a photoexcited solute molecule but also the structural variation of the solute following photoexcitation and the subsequent molecular charge redistribution. Here, we formulate a simple nonequilibrium quantum theory of solvation for an explicitly time-dependent continuous solvent. The time-dependent solvent induces nonequilibrium fluctuations on the solvent dynamics which are directly reflected in different time components in the time-dependent Stokes shift. We illustrate the structural dynamics in the presence of an explicitly time-dependent solvent by the example of a dynamically shrinking solute which leads to a bimodal Stokes shift. Interestingly, both contributions are mutually coupled. Furthermore, we can explain a prominent long-tail decay of the Stokes shift associated with slow structural dynamical variations.

J. Phys. Chem. B 124, 5717 (2020)

M. Naumova, A.Kalinko, J. W. L. Wong, M. Abdellah, H. Geng, E. Domenichini, J. Meng, S. Alvarez Gutierez, P.-A. Mante, W.Lin, P. Zalden, A. Galler, F. Alves Lima, K. Kubicek, M. Biednov, A. Britz, S. Checchia, V. Kabanova, M. Wulff, J. Zimara, D. Schwarzer, S. Demeshko, V. Y. Murzin, D. J Gosztola, M. Jarenmark, J. Zhang, M. Bauer, L. Max Lawson Daku, W. Gawelda, D. Khakhulin, C. Bressler, F. Meyer, K. Zheng, S. E. Canton

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.

J. Phys. Chem. Lett. 11, 2133-2141 (2020)

D. Khakhulin, F. Otte, M. Biednov, C. Bömer, T.-K. Choi, M. Diez, A. Galler, Y. Jiang, K. Kubicek, F. A. Lima, A. Rodriguez-Fernandez, P. Zalden, W. Gawelda, C. Bressler

Time-resolved X-ray methods are widely used for monitoring transient intermediates over the course of photochemical reactions. Ultrafast X-ray absorption and emission spectroscopies as well as elastic X-ray scattering deliver detailed electronic and structural information on chemical dynamics in the solution phase. In this work, we describe the opportunities at the Femtosecond X-ray Experiments (FXE) instrument of European XFEL. Guided by the idea of combining spectroscopic and scattering techniques in one experiment, the FXE instrument has completed the initial commissioning phase for most of its components and performed first successful experiments within the baseline capabilities. This is demonstrated by its currently 115 fs (FWHM) temporal resolution to acquire ultrafast X-ray emission spectra by simultaneously recording iron Kα and Kβ lines, next to wide angle X-ray scattering patterns on a photoexcited aqueous solution of [Fe(bpy)₃]²⁺, a transition metal model compound.

Appl. Sci. 10, 995 (2020)

Alexander Britz, Wojciech Gawelda, Tadesse A. Assefa, Lindsey L. Jamula, Jonathan T. Yarranton, Andreas Galler, Dmitry Khakhulin, Michael Diez, Manuel Harder, Gilles Doumy, Anne Marie March, Éva Bajnóczi, Zoltán Németh, Mátyás Pápai, Emese Rozsályi, Dorottya Sárosiné Szemes, Hana Cho, Sriparna Mukherjee, Chang Liu, Tae Kyu Kim, Robert W. Schoenlein, Stephen H. Southworth, Linda Young, Elena Jakubikova, Nils Huse, György Vankó, Christian Bressler, and James K. McCusker

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]²⁺ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the ⁵T₂/³T₁ crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the ⁵T₂ state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a ⁵T₂ state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited ⁵T₂ state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin ¹A₁ ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means.

Inorg. Chem. , 58, 14, 9341–9350 (2019)

Henning Kirchberg, Peter Nalbach, Christian Bressler, and Michael Thorwart
When a hydrophilic solute in water is suddenly turned into a hydrophobic species, for instance, by photoionization, a layer of hydrated water molecules forms around the solute on a time scale of a few picoseconds. We study the dynamic buildup of the hydration shell around a hydrophobic solute on the basis of a time-dependent dielectric continuum model. Information about the solvent is spectroscopically extracted from the relaxation dynamics of a test dipole inside a static Onsager sphere in the nonequilibrium solvent. The growth process is described phenomenologically within two approaches. First, we consider a time-dependent thickness of the hydration layer that grows from zero to a finite value over a finite time. Second, we assume a time-dependent complex permittivity within a finite layer region around the Onsager sphere. The layer is modeled as a continuous dielectric with a much slower fluctuation dynamics. We find a time-dependent frequency shift down to the blue of the resonant absorption of the dipole, together with a dynamically decreasing line width, as compared to bulk water. The blue shift reflects the work performed against the hydrogen-bonded network of the bulk solvent and is a directly measurable quantity. Our results are in agreement with an experiment on the hydrophobic solvation of iodine in water.
J. Phys. Chem. B 123, 2106 (2019)

A. Cordones, J. Hyuk Lee, K. Hong, H. Cho, K. Garg, M. Boggio-Pasqua, J. Rack, N. Huse, R. W. Schoenlein, T. Kyu Kim

Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.

Nature Communications (2018)

M. Ochmann, A. Hussain, I. von Ahnen, A. A. Cordones, K. Hong, J. H. Lee, R. Ma, K. Adamczyk, O. Vendrell, T. K. Kim, R. W. Schoenlein, and N. Huse

We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH₂S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

J. Am. Chem. Soc. 140, 6554 (2018)

B. Maerz, D. Göries, M. Naumova, M. Biednov, G. Neuber, A. Wetzel, S. M. Hofmann, P. Roedig, A. Meents, J. Bielecki, J. Andreasson, K. R. Beyerlein, H. N. Chapman, C. Bressler, W. Zinth, M. Rübhausen, and S. Herres-Pawlis

The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex—with a specifically designed constraining ligand geometry—that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine–quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet–visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.

Nature Chemistry 10, 355 (2018)

Henning Kirchberg, Peter Nalbach, and Michael Thorwart
We formulate a theory of nonequilibrium quantum solvation in which parameters of the solvent are explicitly depending on time. We assume in a simplest approach a spherical molecular Onsager cavity with a time-dependent radius. We analyze the relaxation properties of a test molecular point dipole in a dielectric solvent and consider two cases: (i) a shrinking Onsager sphere and (ii) a breathing Onsager sphere. Due to the time-dependent solvent, the frequency-dependent response function of the dipole becomes time-dependent. For a shrinking Onsager sphere, the dipole relaxation is in general enhanced. This is reflected in a temporally increasing linewidth of the absorptive part of the response. Furthermore, the effective frequency-dependent response function shows two peaks in the absorptive part which are symmetrically shifted around the eigenfrequency. By contrast, a breathing sphere reduces damping as compared to the static sphere. Interestingly, we find a non-monotonous dependence of the relaxation rate on the breathing rate and a resonant suppression of damping when both rates are comparable. Moreover, the linewidth of the absorptive part of the response function is strongly reduced for times when the breathing sphere reaches its maximal extension.
J. Chem. Phys. 148, 164301 (2018)

A. March, T. Assefa, C. Bömer, C. Bressler, A. Britz, M. Diez, G. Doumy, A. Galler, M. Harder, D. Khakhulin, Z. Németh, M. Pápai, S. Schulz, S. H. Southworth, H. Yavas, L. Young, W. Gawelda, and G. Vankó

We probe the dynamics of valence electrons in photoexcited [Fe(terpy)₂]²⁺ in solution to gain deeper insight into the Fe–ligand bond changes. We use hard X-ray emission spectroscopy (XES), which combines element specificity and high penetration with sensitivity to orbital structure, making it a powerful technique for molecular studies in a wide variety of environments. A picosecond-time-resolved measurement of the complete 1s X-ray emission spectrum captures the transient photoinduced changes and includes the weak valence-to-core (vtc) emission lines that correspond to transitions from occupied valence orbitals to the nascent core-hole. Vtc-XES offers particular insight into the molecular orbitals directly involved in the light-driven dynamics; a change in the metal–ligand orbital overlap results in an intensity reduction and a blue energy shift in agreement with our theoretical calculations and more subtle features at the highest energies reflect changes in the frontier orbital populations.

J. Phys. Chem. C 121, 2620 (2017)

A. Schori, C. Bömer, D. Borodin, S. P. Collins, B. Detlefs, M. Moretti Sala, S. Yudovich, and S. Shwartz

We report the observation of parametrically down-converted x-ray signal photons at photon energies that correspond to idler photons at optical wavelengths. The count-rate dependence on the angles of the input beam and of the detector and on the slit sizes agrees with theory within the experimental uncertainties. The nonlinear susceptibility, which we calculated from the measured efficiencies, is comparable to the nonlinear susceptibility evaluated from the measurements of x-ray and optical wave mixing. The results of the present Letter advance the development of a spectroscopy method for probing valence-electron charges and the microscopic optical response of crystals with atomic-scale resolution.

Physical review letters 119, 25, 253902 (2017)

M. Ochmann, I. von Ahnen, A. A. Cordones, A. Hussain, J. H. Lee, K. Hong, K. Adamczyk, O. Vendrell , T. K. Kim , R. W. Schoenlein, N. Huse
We applied time-resolved sulfur-1s absorption spectroscopy to a model aromatic thiol system as a promising method for tracking chemical reactions in solution. Sulfur-1s absorption spectroscopy allows tracking multiple sulfur species with a time resolution of ∼70 ps at synchrotron radiation facilities. Experimental transient spectra combined with high-level electronic structure theory allow identification of a radical and two thione isomers, which are generated upon illumination with 267 nm radiation. Moreover, the regioselectivity of the thione isomerization is explained by the resulting radical frontier orbitals. This work demonstrates the usefulness and potential of time-resolved sulfur-1s absorption spectroscopy for tracking multiple chemical reaction pathways and transient products of sulfur-containing molecules in solution.

J. Am. Chem. Soc. 139, 4797 (2017)

D. Göries, B. Dicke, P. Roedig, N. Stübe, J. Meyer, A. Galler, W. Gawelda, A. Britz, P. Geßler, H. Sotoudi Namin, A. Beckmann, M. Schlie, M. Warmer, M. Naumova, C. Bressler, M. Rübhausen, E. Weckert, and A. Meents

We report about the development and implementation of a new setup for time-resolved X-ray absorption fine structure spectroscopy at beamline P11 utilizing the outstanding source properties of the low-emittance PETRA III synchrotron storage ring in Hamburg. Using a high intensity micrometer-sized X-ray beam in combination with two positional feedback systems, measurements were performed on the transition metal complex fac-Tris[2-phenylpyridinato-C2,N]iridium(III) also referred to as fac-Ir(ppy)3. This compound is a representative of the phosphorescent iridium(III) complexes, which play an important role in organic light emitting diode (OLED) technology. The experiment could directly prove the anticipated photoinduced charge transfer reaction. Our results further reveal that the temporal resolution of the experiment is limited by the PETRA III X-ray bunch length of ∼103 ps full width at half maximum (FWHM).

Review of Scientific Instruments 87, 053116 (2016)

E. Basin, T. B. van Driel, K. S. Kjaer, A. O. Dohn, M. Christensen, T. Harlang, P. Chabera, Y. Liu, J. Uhlig, M. Pápai, Z. Németh, R. Hartsock, W. Liang, J. Zhang, R. Alonso-Mori, M. Chollet, J. M. Glawnia, S. Nelsen, D. Sokara, T. A. Assefa, A. Britz, A. Galler, W. Gawelda, C. Bressler, K. J. Gaffney, H. T. Lemke, K. B. Moller, M. M. Nielsen, V. Sundström, G. Vankó, K. Wärmark, S. E. Canton, and K. Haldrup

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.

Phys. Rev. Lett. 117, 013002 (2016)

A. Britz, T. Assefa, A. Galler, W. Gawelda, M. Diez, P. Zalden, D. Khakhulin, B. Fernandes, P. Gessler, H. Sotuodi, A. Beckmann, M. Harder, H. Yavas, and C. Bressler

The technical implementation of a multi-MHz data acquisition scheme for laser-X-ray pump-probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high-repetition rates of X-ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60-bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X-ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12-bit resolution. These traces are then processed to deliver an integrated value for each recorded single X-ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single-shot values are averaged over ~107 pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X-ray probe energy. The sensitivity reaches down to the shot-noise limit, and signal-to-noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse-1 and is only technically limited by the utilized APD.

J. Synchr. Rad. 23, 1409 (2016)

A. A. Guda, S. A. Guda, M. A. Soldatov, K. A. Lomachenko, A. L. Bugaev, C. Lamberti, W. Gawelda, C. Bressler, G. Smolentsev, A. V. Soldatov, Y. Joly

Finite difference method (FDM) implemented in the FDMNES software [Phys. Rev. B, 2001, 63, 125120] was revised. Thorough analysis shows, that the calculated diagonal in the FDM matrix consists of about 96% zero elements. Thus a sparse solver would be more suitable for the problem instead of traditional Gaussian elimination for the diagonal neighbourhood. We have tried several iterative sparse solvers and the direct one MUMPS solver with METIS ordering turned out to be the best. Compared to the Gaussian solver present method is up to 40 times faster and allows XANES simulations for complex systems already on personal computers. We show applicability of the software for metal-organic [Fe(bpy)₃]²⁺ complex both for low spin and high spin states populated after laser excitation.

J. Phys. Conf. Series 712, 012004 (2016)

K. Haldrup, W. Gawelda, R. Abela, R. Alonso-Mori, U. Bergmann, A. Bordage, M. Cammarata, S. Canton, A. O. Dohn, T. Brandt van Driel, D. M. Fritz, A. Galler, P. Glatzel, T. Harlang, K. S. Kjaer, H. T. Lemke, K. B. Moller, Z. Németh, M. Papai, N. Sas, J. Uhlig, D. Zhu, G. Vankó, V. Sundström, M. M. Nielsen, C. Bressler

In liquid phase chemistry dynamic solute–solvent interactions often govern the path, ultimate outcome, and efficiency of chemical reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramolecular electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermolecular dynamics of a model chromophore, aqueous [Fe(bpy)₃]²⁺, with complementary X-ray tools in a single experiment exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering analysis. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramolecular structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can extract new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent density increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio molecular dynamics simulations. Structural changes in the aqueous solvent associated with density and temperature changes occur with ∼1 ps time constants, characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute molecules well before the solvent contributions become dominant.

J. Phys. Chem. B120, 1158-1168 (2016)

S. Canton, K. Kjaer, G. Vankó, T. van Driel, S. Adachi, A. Bordage, C. Bressler, P. Chabera, M. Christensen, A. Dohn, A. Galler, W. Gawelda, D. Gosztola, K. Haldrup, T. Harlang, Y. Liu, K. Moller, Z. Németh, S. Nozawa, M. Pápai, T. Sato, Ta. Sato, K. Suarez-Alcantara, T. Togashi, K. Tono, J. Uhlig, D. Vithanage, K. Wärnmark, M. Yabashi, J. Zhang, V. Sundström, and M. Nielsen

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.

Nat. Commun. 6, 6539 (2015)

S. Guda, A. Guda, M. Soldatov, K. Lomachenko, A. Bugaev, C. Lamberti, W. Gawelda, C. Bressler, G. Smolentsev, A. Soldatov, Y. Joly

Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal–organic frameworks and (ii) light induced dynamics of spin crossover in metal–organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [ Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni–CO configuration was found to be linear, while Ni–NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)₃]²⁺ with a 100 ps delay we identified the Fe–N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

J. Chem. Theory Comput. 11(9), 4512-4521 (2015)

A. M. March, T. A. Assefa, C. Bressler, G. Doumy, A. Galler, W. Gawelda, E. P. Kanter, Z. Németh, M. Pápai, S. H. Southworth, L. Young, G. Vankó

X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump–probe technique.

J. Phys. Chem. C 119 (26), 14571-14578 (2015)

G. Vankó, A. Bordage, M. Pápai, K. Haldrup, P. Glatzel, A. M. March, G. Doumy, A. Britz, A. Galler, T. A. Assefa, D. Cabaret, A. Juhin, T. B. van Driel, K. S. Kjaer, A. O. Dohn, K. B. Moller, H. T. Lemke, E. Gallo, M. Rovezzi, Z. Németh, E. Rozsàlyi, T. Rozgonyi, J. Uhlig, V. Sundström, M. M. Nielsen, L. Young, S. H. Southworth, C. Bressler, W. Gawelda

Theoretical predictions show that depending on the populations of the Fe 3d_xy, 3d_xz, and 3_dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)₂]²⁺. The differences in the structure and molecular properties of these ⁵B₂ and ⁵E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)₂]²⁺ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)₂]²⁺ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as ⁵E, in agreement with our theoretical expectations.

J. Phys. Chem. C 119 (11), 5888-5902 (2015)

P. Nalbach, A. J. A. Achner, M. Frey, M. Grosser, C. Bressler, M. Thorwart
We study the relaxation dynamics of photoexcited Fe-II complexes dissolved in water and identify the relaxation pathway which the molecular complex follows in presence of a hydration shell of bound water at the interface between the complex and the solvent. Starting from a low-spin state, the photoexcited complex can reach the high-spin state via a cascade of different possible transitions involving electronic as well as vibrational relaxation processes. By numerically exact path integral calculations for the relaxational dynamics of a continuous solvent model, we find that the vibrational life times of the intermittent states are of the order of a few ps. Since the electronic rearrangement in the complex occurs on the time scale of about 100 fs, we find that the complex first rearranges itself in a high-spin and highly excited vibrational state, before it relaxes its energy to the solvent via vibrational relaxation transitions. By this, the relaxation pathway can be clearly identified. We find that the life time of the vibrational states increases with the size of the complex (within a spherical model), but decreases with the thickness of the hydration shell, indicating that the hydration shell acts as an additional source of fluctuations.
J. Chem. Phys. 141, 044304 (2014)

C. Bressler, W. Gawelda, A. Galler, M. M. Nielsen, V. Sundström, G. Doumy, A. M. March, S. H. Southworth, L. Young, G. Vankó
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.
Faraday Disc. 2014, Advance article

W. Zhang, R. Alonso-Mori, U. Bergmann, C. Bressler, M. Chollet, A. Galler, W. Gawelda, R. G. Hadt, R. W. Hartsock1, T. Kroll, K. S. Kjær, K. Kubicˇek, H. T. Lemke, H. W. Liang, D. A. Meyer, M. M. Nielsen, C. Purser, J. S. Robinson, et. al
Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons1, 2, 3, 4. 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 dynamics5 and the flux limitations of ultrafast X-ray sources6, 7. 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 controversy6, 7, 8, 9, 10, 11, 12, 13, 14, 15. 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 509, 345 (2014)

K. Haldrup, G. Vankó, W. Gawelda, A. Galler, G. Doumy, A. M. March, E. P. Kanter, A. Bordage, A. Dohn, T. B. van Driel, K. S. Kjaer, H. T. Lemke, S. E. Canton, J. Uhlig, V. Sundström, L. Young, S. Southworth, M. M. Nielsen, C. Bressler
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.
J. Phys. Chem. A116, 9878 (2012)