Ultrafast and persistent photoinduced phase transition at room temperature monitored by streaming powder diffraction.

Ultrafast photoinduced phase transitions at room temperature, driven by a single laser shot and persisting long after stimuli, represent emerging routes for ultrafast control over materials' properties. Time-resolved studies provide fundamental mechanistic insight into far-from-equilibrium electronic and structural dynamics. Here we study the photoinduced phase transformation of the Rb0.94Mn0.94Co0.06[Fe(CN)6]0.98 material, designed to exhibit a 75 K wide thermal hysteresis around room temperature between MnIIIFeII tetragonal and MnIIFeIII cubic phases. We developed a specific powder sample streaming technique to monitor by ultrafast X-ray diffraction the structural and symmetry changes. We show that the photoinduced polarons expand the lattice, while the tetragonal-to-cubic photoinduced phase transition occurs within 100 ps above threshold fluence. These results are rationalized within the framework of the Landau theory of phase transition as an elastically-driven and cooperative process. We foresee broad applications of the streaming powder technique to study non-reversible and ultrafast dynamics.

Cerium Photocatalyst in Action: Structural Dynamics in the Presence of Substrate Visualized via Time-Resolved X-ray Liquidography.

[Ce(III)Cl6]3-, with its earth-abundant metal element, is a promising photocatalyst facilitating carbon-halogen bond activation. Still, the structure of the reaction intermediate has yet to be explored. Here, we applied time-resolved X-ray liquidography (TRXL), which allows for direct observation of the structural details of reaction intermediates, to investigate the photocatalytic reaction of [Ce(III)Cl6]3-. Structural analysis of the TRXL data revealed that the excited state of [Ce(III)Cl6]3- has Ce-Cl bonds that are shorter than those of the ground state and that the Ce-Cl bond further contracts upon oxidation. In addition, this study represents the first application of TRXL to both photocatalyst-only and photocatalyst-and-substrate samples, providing insights into the substrate's influence on the photocatalyst's reaction dynamics. This study demonstrates the capability of TRXL in elucidating the reaction dynamics of photocatalysts under various conditions and highlights the importance of experimental determination of the structures of reaction intermediates to advance our understanding of photocatalytic mechanisms.

Anomalous temperature dependence of the experimental x-ray structure factor of supercooled water.

The structural changes of water upon deep supercooling were studied through wide-angle x-ray scattering at SwissFEL. The experimental setup had a momentum transfer range of 4.5 Å-1, which covered the principal doublet of the x-ray structure factor of water. The oxygen-oxygen structure factor was obtained for temperatures down to 228.5 ± 0.6 K. Similar to previous studies, the second diffraction peak increased strongly in amplitude as the structural change accelerated toward a local tetrahedral structure upon deep supercooling. We also observed an anomalous trend for the second peak position of the oxygen-oxygen structure factor (q2). We found that q2 exhibits an unprecedented positive partial derivative with respect to temperature for temperatures below 236 K. Based on Fourier inversion of our experimental data combined with reference data, we propose that the anomalous q2 shift originates from that a repeat spacing in the tetrahedral network, associated with all peaks in the oxygen-oxygen pair-correlation function, gives rise to a less dense local ordering that resembles that of low-density amorphous ice. The findings are consistent with that liquid water consists of a pentamer-based hydrogen-bonded network with low density upon deep supercooling.

Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I.

Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.

New insights into correlated materials in the time domain-combining far-infrared excitation with x-ray probes at cryogenic temperatures.

Modern techniques for the investigation of correlated materials in the time domain combine selective excitation in the THz frequency range with selective probing of coupled structural, electronic and magnetic degrees of freedom using x-ray scattering techniques. Cryogenic sample temperatures are commonly required to prevent thermal occupation of the low energy modes and to access relevant material ground states. Here, we present a chamber optimized for high-field THz excitation and (resonant) x-ray diffraction at sample temperatures between 5 and 500 K. Directly connected to the beamline vacuum and featuring both a Beryllium window and an in-vacuum detector, the chamber covers the full (2-12.7) keV energy range of the femtosecond x-ray pulses available at the Bernina endstation of the SwissFEL free electron laser. Successful commissioning experiments made use of the energy tunability to selectively track the dynamics of the structural, magnetic and orbital order of Ca2RuO4and Tb2Ti2O7at the Ru (2.96 keV) and Tb (7.55 keV)L-edges, respectively. THz field amplitudes up to 1.12 MV cm-1peak field were demonstrated and used to excite the samples at temperatures as low as 5 K.

Enhancement and maximum in the isobaric specific-heat capacity measurements of deeply supercooled water using ultrafast calorimetry.

Knowledge of the temperature dependence of the isobaric specific heat (Cp) upon deep supercooling can give insights regarding the anomalous properties of water. If a maximum in Cp exists at a specific temperature, as in the isothermal compressibility, it would further validate the liquid-liquid critical point model that can explain the anomalous increase in thermodynamic response functions. The challenge is that the relevant temperature range falls in the region where ice crystallization becomes rapid, which has previously excluded experiments. Here, we have utilized a methodology of ultrafast calorimetry by determining the temperature jump from femtosecond X-ray pulses after heating with an infrared laser pulse and with a sufficiently long time delay between the pulses to allow measurements at constant pressure. Evaporative cooling of ∼15-µm diameter droplets in vacuum enabled us to reach a temperature down to ∼228 K with a small fraction of the droplets remaining unfrozen. We observed a sharp increase in Cp, from 88 J/mol/K at 244 K to about 218 J/mol/K at 229 K where a maximum is seen. The Cp maximum is at a similar temperature as the maxima of the isothermal compressibility and correlation length. From the Cp measurement, we estimated the excess entropy and self-diffusion coefficient of water and these properties decrease rapidly below 235 K.

Charge transfer driven by ultrafast spin transition in a CoFe Prussian blue analogue.

Photoinduced charge-transfer is an important process in nature and technology and is responsible for the emergence of exotic functionalities, such as magnetic order for cyanide-bridged bimetallic coordination networks. Despite its broad interest and intensive developments in chemistry and material sciences, the atomic-scale description of the initial photoinduced process, which couples intermetallic charge-transfer and spin transition, has been debated for decades; it has been beyond reach due to its extreme speed. Here we study this process in a prototype cyanide-bridged CoFe system by femtosecond X-ray and optical absorption spectroscopies, enabling the disentanglement of ultrafast electronic and structural dynamics. Our results demonstrate that it is the spin transition that occurs first on the Co site within ~50 fs, and it is this that drives the subsequent Fe-to-Co charge-transfer within ~200 fs. This study represents a step towards understanding and controlling charge-transfer-based functions using light.

Photoswitchable 11 nm CsCoFe Prussian Blue Analogue Nanocrystals with High Relaxation Temperature.

Photoswitchable 11 nm nanocrystals with the coordination network Cs{Co[Fe(CN)6]} were obtained using a template-free method. The nanocrystals were recovered from the colloidal solutions as solid materials surrounded by cetyltrimethylammonium (CTA) cations or embedded in the organic polymer polyvinylpyrrolidone (PVP). Complementary magnetic, spectroscopic, and structural techniques, including EPR spectroscopy, reveal a majority (∼70%) of the low-spin and photoactive diamagnetic CoIIIFeII pairs located in the core of the nanocrystals and a mixture of CoIIFeII and CoIIFeIII species present mainly within the shell of the objects. While bulk compounds with similar vacancy concentration do not exhibit noticeable photoinduced charge transfer, the observed photoactivity of the nanocrystals is ascribed to their nanometric size. The relaxation temperature of the photoinduced state shifts upward by ∼55 K when PVP is replaced by CTA. This is ascribed to the larger rigidity of the dense CsCoFe_CTA material, whose metastable state is lower than that for CsCoFe_PVP, leading to a larger relaxation energy barrier and, therefore, to a higher relaxation temperature.

Optical second harmonic generation in LiB3O5 modulated by intense femtosecond X-ray pulses.

Many of the scientific applications for X-ray free-electron lasers seek to exploit the ultrashort pulse durations of intense X-rays to obtain femtosecond time resolution of various processes in a "pump-probe" scheme. One of the limiting factors for such experiments is the timing jitter between the X-rays and ultrashort pulses from more conventional lasers operating at near-optical wavelengths. In this work, we investigate the potential of using X-ray-induced changes in the optical second harmonic generation efficiency of a nonlinear crystal to retrieve single-shot arrival times of X-ray pulses with respect to optical laser pulses. Our experimental results and simulations show changes to the efficiency of the second harmonic generation of 12%, approximately three times larger than the measured changes in the transmission of the 800 nm center-wavelength fundamental pulse. Further experiments showing even larger changes in the transmission of 400 nm center-wavelength pulses show that the mechanism of the second harmonic generation efficiency modulation is mainly the result of X-ray-induced changes in the linear absorption coefficients near 400 nm. We demonstrate and characterize a cross-correlation tool based on this effect in reference to a previously demonstrated method of X-ray/optical cross-correlation.

Experimental station Bernina at SwissFEL: condensed matter physics on femtosecond time scales investigated by X-ray diffraction and spectroscopic methods.

The Bernina instrument at the SwissFEL Aramis hard X-ray free-electron laser is designed for studying ultrafast phenomena in condensed matter and material science. Ultrashort pulses from an optical laser system covering a large wavelength range can be used to generate specific non-equilibrium states, whose subsequent temporal evolution can be probed by selective X-ray scattering techniques in the range 2-12 keV. For that purpose, the X-ray beamline is equipped with optical elements which tailor the X-ray beam size and energy, as well as with pulse-to-pulse diagnostics that monitor the X-ray pulse intensity, position, as well as its spectral and temporal properties. The experiments can be performed using multiple interchangeable endstations differing in specialization, diffractometer and X-ray analyser configuration and load capacity for specialized sample environment. After testing the instrument in a series of pilot experiments in 2018, regular user operation begins in 2019.

Electronic and Structural Dynamics During the Switching of the Photomagnetic Complex [Fe(L222 N5 )(CN)2 ].

The [Fe(L222 N5 )(CN)2 ] compound, where L222 N5 refers to the macrocyclic Schiff-base ligand, 2,13-dimethyl-3,6,9,-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,- 16-pentaene, is a photomagnetic FeII based coordination compound, which undergoes light-induced excited spin-state trapping (LIESST). The low spin state is hexacoordinated and the high spin state heptacoordinated. This system also serves as complex for the design of trinuclear or one-dimensional compounds made of other types of bricks with diverse coordinated metals. Here its ultrafast spin-state photoswitching dynamics are studied, by combining femtosecond optical spectroscopy and femtosecond X-ray absorption measurements at the XPP station of the X-ray free-electron laser LCLS. DFT and TD-DFT calculations are used to interpret experimental findings. These studies, performed in the solution phase, show that LIESST in [Fe(L222 N5 )(CN)2 ] occurs on the 100 fs timescale under different types of photoexcitation. In addition, coherent oscillations were observed, resulting from the structural dynamics accompanying LIESST, which were recently evidenced in more conventional octahedral FeII N6 systems.

Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer.

The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)3)2](4,4'-bipy)2]n is reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order.