Disulfide bonds are ubiquitous molecular motifs that influence the tertiary structure and biological functions of many proteins. Yet, it is well known that the disulfide bond is photolabile when exposed to ultraviolet C (UVC) radiation. The deep-UV–induced S─S bond fragmentation kinetics on very fast timescales are especially pivotal to fully understand the photostability and photodamage repair mechanisms in proteins. In 1,2-dithiane, the smallest saturated cyclic molecule that mimics biologically active species with S─S bonds, we investigate the photochemistry upon 200-nm excitation by femtosecond time-resolved x-ray scattering in the gas phase using an x-ray free electron laser. In the femtosecond time domain, we find a very fast reaction that generates molecular fragments with one and two sulfur atoms. On picosecond and nanosecond timescales, a complex network of reactions unfolds that, ultimately, completes the sulfur dissociation from the parent molecule.

Mechanistic insights into photodissociation dynamics of transition metal carbonyls, like Fe(CO)5, are fundamental for understanding active catalytic intermediates. Although extensively studied, the structural dynamics of these systems remain elusive. Using ultrafast X-ray scattering, we uncover the photochemistry of Fe(CO)5 in real space and time, observing synchronous oscillations in atomic pair distances, followed by a prompt rotating CO release preferentially in the axial direction. This behavior aligns with simulations, reflecting the interplay between the axial Fe-C distances' potential energy landscape and non-adiabatic transitions between metal-to-ligand charge-transfer states. Additionally, we characterize a secondary delayed CO release associated with a reduction of Fe-C steady state distances and structural dynamics of the formed Fe(CO)4. Our results quantify energy redistribution across vibration, rotation, and translation degrees of freedom, offering a microscopic view of complex structural dynamics, enhancing our grasp on Fe(CO)5 photodissociation, and advancing our understanding of transition metal catalytic systems.

hearings in Los Alamos and Santa Fe, New Mexico, on May 31 and June 1, 1995. In addition, DOE made the draft classified supplement to the DARHT EIS available for review by appropriately cleared individuals with a need to know the classified information. Reviewers of the classified material included the State of New Mexico, the U.S. Environmental Protection Agency, the Department of Defense, and certain Indian Tribes. Volume 2 of the final DARHT EIS contains three chapters. Chapter 1 includes a collective summary of the comments received and DOE`s response. Chapter 2 contains the full text of the public comments on the draft DARHT EIS received by DOE. Chapter 3 contains DOE`s responses to the public comments and an indication as to how the comments were considered in the final EIS.

Mechanistic insights into photodissociation dynamics of transition metal carbonyls, like Fe(CO)$_5$, are fundamental for understanding active catalytic intermediates. Although extensively studied, the ultrafast structural dynamics of these systems remain elusive. Using ultrafast X-ray scattering, we uncover the ultrafast photochemistry of Fe(CO)$_5$ in real space and time, observing synchronous oscillations in atomic pair distances, followed by a prompt rotating CO release preferentially in the axial direction. This behavior aligns with simulations, reflecting the interplay between the axial Fe-C distances' potential energy landscape and non-adiabatic transitions between metal-to-ligand charge-transfer states. Additionally, we characterize a secondary delayed CO release associated with a reduction of Fe-C steady state distances and structural dynamics of the formed Fe(CO)$_4$. Our results quantify energy redistribution across vibration, rotation, and translation degrees of freedom, offering an ultrafast microscopic view of complex structural dynamics, enhancing our grasp on Fe(CO)$_5$ photodissociation and advancing our understanding of transition metal catalytic systems.

Supercooled water droplets are widely used to study supercooled water 1 , 2 , ice nucleation 3 – 5 and droplet freezing 6 – 11 . Their freezing in the atmosphere affects the dynamics and climate feedback of clouds 12 , 13 and can accelerate cloud freezing through secondary ice production 14 – 17 . Droplet freezing occurs at several timescales and length scales 14 , 18 and is sufficiently stochastic to make it unlikely that two frozen drops are identical. Here we use optical microscopy and X-ray laser diffraction to investigate the freezing of tens of thousands of water microdrops in vacuum after homogeneous ice nucleation around 234–235 K. On the basis of drop images, we developed a seven-stage model of freezing and used it to time the diffraction data. Diffraction from ice crystals showed that long-range crystalline order formed in less than 1 ms after freezing, whereas diffraction from the remaining liquid became similar to that from quasi-liquid layers on premelted ice 19 , 20 . The ice had a strained hexagonal crystal structure just after freezing, which is an early metastable state that probably precedes the formation of ice with stacking defects 8 , 9 , 18 . The techniques reported here could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.

With the development of X-ray free-electron lasers (XFELs), producing pulses of femtosecond durations comparable with the coherence times of X-ray fluorescence, it has become possible to observe intensity-intensity correlations due to the interference of emission from independent atoms. This has been used to compare durations of X-ray pulses and to measure the size of a focused X-ray beam, for example. Here it is shown that it is also possible to observe the interference of fluorescence photons through the measurement of the speckle contrast of angle-resolved fluorescence patterns. Speckle contrast is often used as a measure of the degree of coherence of the incident beam or the fluctuations of the illuminated sample as determined from X-ray diffraction patterns formed by elastic scattering, rather than from fluorescence patterns as addressed here. Commonly used approaches to estimate speckle contrast were found to suffer when applied to XFEL-generated fluorescence patterns due to low photon counts and a significant variation of the excitation pulse energy from shot to shot. A new method to reliably estimate speckle contrast under such conditions, using a weighting scheme, is introduced. The method is demonstrated by comparing the speckle contrast of fluorescence observed with pulses of 3 fs to 15 fs duration.

A spatially resolved air-wedge shearing interferometer and shadowgraph diagnostic provides measurements of electron density with a resolution of similar to 40 mu m. A similar to 100-ns-long, monoenergetic electron bunch at 19.8 MeV and a current of 1.4 kA (8.5 x 10(14) e(-)) heats 100-mu m-thick aluminum (Al) foils in a 1-mm-spot to T-e similar to 1 eV. A 5-ns-long, similar to 60 mJ, frequency doubled Nd:YAG laser probes the dense Al plasma. Electron densities up to 10(20) cm(-3) are resolved; the maximum resolvable density is limited by opacity, transmission, and spatial fringe density achievable with the detector. This diagnostic provides measurements of the total phase shift, transmission, and electron density. Several measurements at different time slices provide the ability to determine the velocity of the leading edge of the shadowgraph and compare it to the motion of different density shells. These measurements are also compared to radiation hydrodynamics simulations. A rough quantitative agreement is shown between the hydro simulations and the measurements; there are differences in the exact density distributions. Published under an exclusive license by AIP Publishing.

Temperature and density measurements of range-thin pure aluminum foils heated by an intense, relativistic, monochromatic electron beam have been performed for the first time. Electron density measurements are obtained from a 532-nm air-wedge shearing interferometer and indicate a large, long-lived volume of dense plasma. The plasma grows to extend more than 5 mm off the target face, and lasts more than 500 ns from the initial deposition of energy. Spatially-and temporally-resolved visible spectroscopy normal to the target foil provides a complimentary diagnostic of the plasma plume temperature and density. The spectrometer measures the Al-I 3p–4s resonance lines, which were consistently observed to be optically thick. Stark broadening of the Al-I doublet, after correcting for self-absorption effects, yields electron densities >4x10 17 cm -3 and electron temperatures around 1 eV. Initial atomic kinetics calculations are presented to threshold plasma temperatures based on the Al-I line profiles. A spectroscopic-quality radiation transport model is developed to post-process results of a radiation-hydrodynamics simulation of the energy deposition and subsequent hydrodynamic expansion of the plasma, including estimating the optical depth of the Al-I lines.

Reversibly photoswitchable fluorescent proteins are essential markers for advanced biological imaging, and optimization of their photophysical properties underlies improved performance and novel applications. Here we establish a link between photoswitching contrast, a key parameter that largely dictates the achievable resolution in nanoscopy applications, and chromophore conformation in the non-fluorescent state of rsEGFP2, a widely employed label in REversible Saturable OpticaL Fluorescence Transitions (RESOLFT) microscopy. Upon illumination, the cis chromophore of rsEGFP2 isomerizes to two distinct off -state conformations, trans1 and trans2 , located on either side of the V151 side chain. Reducing or enlarging the side chain at this position (V151A and V151L variants) leads to single off -state conformations that exhibit higher and lower switching contrast, respectively, compared to the rsEGFP2 parent. The combination of structural information obtained by serial femtosecond crystallography with high-level quantum chemical calculations and with spectroscopic and photophysical data determined in vitro suggests that the changes in switching contrast arise from blue- and red-shifts of the absorption bands associated to trans1 and trans2 , respectively. Thus, due to elimination of trans2 , the V151A variants of rsEGFP2 and its superfolding variant rsFolder2 display a more than two-fold higher switching contrast than their respective parent proteins, both in vitro and in E. coli cells. The application of the rsFolder2-V151A variant is demonstrated in RESOLFT nanoscopy. Our study rationalizes the connection between structural and photophysical chromophore properties and suggests a means to rationally improve fluorescent proteins for nanoscopy applications.

The Cover Feature displays a comparison of chromophore conformations in the X-ray crystal structures of the reversibly photoswitchable fluorescent protein rsEGFP2 in its on state (green) and its V151L (purple) and V151A (yellow) variants in their off states. The latter displays an increased photoswitching contrast, as indicated by the displayed switching kinetics. Cover design by Virgile ADAM. More information can be found in the Research Article by Dominique Bourgeois, Martin Weik and co-workers.

X-ray free-electron lasers (XFELs) enable obtaining novel insights in structural biology. The recently available MHz repetition rate XFELs allow full data sets to be collected in shorter time and can also decrease sample consumption. However, the microsecond spacing of MHz XFEL pulses raises new challenges, including possible sample damage induced by shock waves that are launched by preceding pulses in the sample-carrying jet. We explored this matter with an X-ray-pump/X-ray-probe experiment employing haemoglobin microcrystals transported via a liquid jet into the XFEL beam. Diffraction data were collected using a shock-wave-free single-pulse scheme as well as the dual-pulse pump-probe scheme. The latter, relative to the former, reveals significant degradation of crystal hit rate, diffraction resolution and data quality. Crystal structures extracted from the two data sets also differ. Since our pump-probe attributes were chosen to emulate EuXFEL operation at its 4.5 MHz maximum pulse rate, this prompts concern about such data collection.

Significance Charge transfer plays a fundamental role in many areas of chemistry, physics, biology, and materials science. The spatial redistribution of electrons is typically associated with changes in the molecular geometry. A comprehensive understanding of this process requires atomic spatial resolution and femtosecond temporal resolution. Here, we exploit an X-ray free-electron laser to determine the excited-state structures during charge transfer with sub-Ångström resolution. Important behaviors such as bond elongation and contraction and reorientation of functional groups during the charge transfer are discovered, demonstrating the substantial information on excited-state molecular structure that becomes accessible through ultrafast X-ray scattering measurements.

The interaction of intense femtosecond x-ray pulses with molecules sensitively depends on the interplay between multiple photoabsorptions, Auger decay, charge rearrangement, and nuclear motion. Here, we report on a combined experimental and theoretical study of the ionization and fragmentation of iodomethane (CH_{3}I) by ultraintense (∼10^{19}  W/cm^{2}) x-ray pulses at 8.3 keV, demonstrating how these dynamics depend on the x-ray pulse energy and duration. We show that the timing of multiple ionization steps leading to a particular reaction product and, thus, the product's final kinetic energy, is determined by the pulse duration rather than the pulse energy or intensity. While the overall degree of ionization is mainly defined by the pulse energy, our measurement reveals that the yield of the fragments with the highest charge states is enhanced for short pulse durations, in contrast to earlier observations for atoms and small molecules in the soft x-ray domain. We attribute this effect to a decreased charge transfer efficiency at larger internuclear separations, which are reached during longer pulses.

Shock waves launched by x-ray pulses in sample-carrying liquid jets may affect protein crystallography data collected at MHz repetition rate x-ray free-electron laser (XFEL) facilities, by damaging the crystals before they are probed. We investigated the shock damage in lysozyme microcrystals using a double-pulse operation mode at a low repetition rate x-ray laser facility. The double-pulse mode generated shock waves with pressures that covered and exceeded the shock pressures expected at MHz pulse rate experiments at the European XFEL (EuXFEL) x-ray laser. The quality of the x-ray diffraction data from the crystals was degraded after the shock passed. A decrease in the number of peaks and in the resolution occurred above an estimated shock pressure threshold on the order of tens of MPa. Based on the scaling of the shock pressure with the sample injection parameters and the pulse rates, this threshold was not reached in initial EuXFEL experiments performed at pulse rates of 1.1 MHz but may be exceeded at the maximum pulse rate of 4.5 MHz. The observation of shock damage in lysozyme crystals indicates how experiments can be designed to rapidly detect, and eventually avoid, shock damage in other crystals. Our analysis of shock pressures in liquid jets can also be used to estimate the effect of the shocks in other types of experiments at MHz repetition rate XFELs.

This corrects the article DOI: 10.1103/PhysRevLett.120.265701.

We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV (λ ≈ 130 pm) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the Cornell-SLAC pixel array detector, and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to be within 5%. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data, which should be readily available at upcoming high-repetition-rate facilities.