A setup for hard x-ray time-resolved resonant inelastic x-ray scattering at SwissFEL

We present a new setup for resonant inelastic hard x-ray scattering at the Bernina beamline of SwissFEL with energy, momentum, and temporal resolution. The compact R = 0.5 m Johann-type spectrometer can be equipped with up to three crystal analyzers and allows efficient collection of RIXS spectra. Optical pumping for time-resolved studies can be realized with a broad span of optical wavelengths. We demonstrate the performance of the setup at an overall ∼180 meV resolution in a study of ground-state and photoexcited (at 400 nm) honeycomb 5d iridate α-Li2IrO3. Steady-state RIXS spectra at the iridium L3-edge (11.214 keV) have been collected and are in very good agreement with data collected at synchrotrons. The time-resolved RIXS transients exhibit changes in the energy loss region <2 eV, whose features mostly result from the hopping nature of 5d electrons in the honeycomb lattice. These changes are ascribed to modulations of the Ir-to-Ir inter-site transition scattering efficiency, which we associate to a transient screening of the on-site Coulomb interaction.

A compact gas attenuator for the SwissFEL ATHOS beamline realized using additive manufacturing

Gas attenuators are important devices providing accurate variation of photon intensity for soft X-ray beamlines. In the SwissFEL ATHOS beamline front-end the space is very limited and an innovative approach has been taken to provide attenuation of three orders of magnitude up to an energy of 1200 eV. Additive manufacturing of a differential pumping system vacuum manifold allowed a triple pumping stage to be realized in a space of less than half a meter. Measurements have shown that the response of the device is as expected from theoretical calculations.

The SwissFEL soft X-ray free-electron laser beamline: Athos

The SwissFEL soft X-ray free-electron laser (FEL) beamline Athos will be ready for user operation in 2021. Its design includes a novel layout of alternating magnetic chicanes and short undulator segments. Together with the APPLE X architecture of undulators, the Athos branch can be operated in different modes producing FEL beams with unique characteristics ranging from attosecond pulse length to high-power modes. Further space has been reserved for upgrades including modulators and an external seeding laser for better timing control. All of these schemes rely on state-of-the-art technologies described in this overview. The optical transport line distributing the FEL beam to the experimental stations was designed with the whole range of beam parameters in mind. Currently two experimental stations, one for condensed matter and quantum materials research and a second one for atomic, molecular and optical physics, chemical sciences and ultrafast single-particle imaging, are being laid out such that they can profit from the unique soft X-ray pulses produced in the Athos branch in an optimal way.

SwissFEL Aramis beamline photon diagnostics. Erratum

The list of authors in the paper by Juranić et al. (2018) [J. Synchrotron Rad. 25, 1238-1248] is corrected.

Towards X-ray transient grating spectroscopy

The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using ∼3  keV X-ray pulses.

Silicon carbide X-ray beam position monitors for synchrotron applications

In this work, the performance of thin silicon carbide membranes as material for radiation hard X-ray beam position monitors (XBPMs) is investigated. Thermal and electrical behavior of XBPMs made from thin silicon carbide membranes and single-crystal diamond is compared using finite-element simulations. Fabricated silicon carbide devices are also compared with a 12 µm commercial polycrystalline diamond XBPM at the Swiss Light Source at the Paul Scherrer Institute. Results show that silicon carbide devices can reach equivalent transparencies while showing improved linearity, dynamics and signal-to-noise ratio compared with commercial polycrystalline diamond XBPMs. Given the obtained results and availability of electronic-grade epitaxies on up to 6 inch wafers, it is expected that silicon carbide can substitute for diamond in most beam monitoring applications, whereas diamond, owing to its lower absorption, could remain the material of choice in cases of extreme X-ray power densities, such as pink and white beams.

SwissFEL Aramis beamline photon diagnostics

The SwissFEL Aramis beamline, covering the photon energies between 1.77 keV and 12.7 keV, features a suite of online photon diagnostics tools to help both users and FEL operators in analysing data and optimizing experimental and beamline performance. Scientists will be able to obtain information about the flux, spectrum, position, pulse length, and arrival time jitter versus the experimental laser for every photon pulse, with further information about beam shape and size available through the use of destructive screens. This manuscript is an overview of the diagnostics tools available at SwissFEL and presents their design, working principles and capabilities. It also features new developments like the first implementation of a THz-streaking based temporal diagnostics for a hard X-ray FEL, capable of measuring pulse lengths to 5 fs r.m.s. or better.

THz streak camera method for synchronous arrival time measurement of two-color hard X-ray FEL pulses

The two-color operation of free electron laser (FEL) facilities allows the delivery of two FEL pulses with different energies, which opens new possibilities for user experiments. Measuring the arrival time of both FEL pulses relative to the external experimental laser and to each other improves the temporal resolution of the experiments using the two-color FEL beam and helps to monitor the performance of the machine itself. This work reports on the first simultaneous measurement of the arrival times of two hard X-ray FEL pulses with the THz streak camera. Measuring the arrival time of the two FEL pulses, the relative delay between them was calculated and compared to the set values. Furthermore, we present the first comparison of the THz streak camera method to the method of FEL induced transient transmission. The results indicate a good agreement between the two methods.

PRIGo: a new multi-axis goniometer for macromolecular crystallography

The Parallel Robotics Inspired Goniometer (PRIGo) is a novel compact and high-precision goniometer providing an alternative to (mini-)kappa, traditional three-circle goniometers and Eulerian cradles used for sample reorientation in macromolecular crystallography. Based on a combination of serial and parallel kinematics, PRIGo emulates an arc. It is mounted on an air-bearing stage for rotation around ω and consists of four linear positioners working synchronously to achieve x, y, z translations and χ rotation (0-90°), followed by a ϕ stage (0-360°) for rotation around the sample holder axis. Owing to the use of piezo linear positioners and active correction, PRIGo features spheres of confusion of <1 µm, <7 µm and <10 µm for ω, χ and ϕ, respectively, and is therefore very well suited for micro-crystallography. PRIGo enables optimal strategies for both native and experimental phasing crystallographic data collection. Herein, PRIGo hardware and software, its calibration, as well as applications in macromolecular crystallography are described.

High-precision x-ray FEL pulse arrival time measurements at SACLA by a THz streak camera with Xe clusters

The accurate measurement of the arrival time of a hard X-ray free electron laser (FEL) pulse with respect to a laser is of utmost importance for pump-probe experiments proposed or carried out at FEL facilities around the world. This manuscript presents the latest device to meet this challenge, a THz streak camera using Xe gas clusters, capable of pulse arrival time measurements with an estimated accuracy of several femtoseconds. An experiment performed at SACLA demonstrates the performance of the device at photon energies between 5 and 10 keV with variable photon beam parameters.

Recent developments at the MX beamline X10SA at the SLS

The three macromolecular crystallography (MX) beamlines at the Swiss Light Source (SLS) rank among the most productive in Europe. The very successful design of the first beamline, X06SA, inaugurated in 2001, was the basis for the second beamline, X10SA, operated by the Paul Scherrer Institut and financed by the partners Max Planck Society, Novartis and Hofmann-La Roche. To keep up with the increasing demand for high throughput crystallographic experiments, especially in an industrial environment, as well as the rising interest in more challenging targets, the beamline is under constant development. Here we will present the recent advances in usability and performance, including software integration and automation with the completely new data acquisition software DA+, in-situ screening for diffraction candidates, (serial) micro-crystallography with micro-beam, and beamline hardware.

D3, the new diffractometer for the macromolecular crystallography beamlines of the Swiss Light Source

A new diffractometer for microcrystallography has been developed for the three macromolecular crystallography beamlines of the Swiss Light Source. Building upon and critically extending previous developments realised for the high-resolution endstations of the two undulator beamlines X06SA and X10SA, as well as the super-bend dipole beamline X06DA, the new diffractometer was designed to the following core design goals. (i) Redesign of the goniometer to a sub-micrometer peak-to-peak cylinder of confusion for the horizontal single axis. Crystal sizes down to at least 5 µm and advanced sample-rastering and scanning modes are supported. In addition, it can accommodate the new multi-axis goniometer PRIGo (Parallel Robotics Inspired Goniometer). (ii) A rapid-change beam-shaping element system with aperture sizes down to a minimum of 10 µm for microcrystallography measurements. (iii) Integration of the on-axis microspectrophotometer MS3 for microscopic sample imaging with 1 µm image resolution. Its multi-mode optical spectroscopy module is always online and supports in situ UV/Vis absorption, fluorescence and Raman spectroscopy. (iv) High stability of the sample environment by a mineral cast support construction and by close containment of the cryo-stream. Further features are the support for in situ crystallization plate screening and a minimal achievable detector distance of 120 mm for the Pilatus 6M, 2M and the macromolecular crystallography group's planned future area detector Eiger 16M.

Automatic loop centring with a high-precision goniometer head at the SLS macromolecular crystallography beamlines

Automatic loop centring has been developed as part of the automation process in crystallographic data collection at the Swiss Light Source. The procedure described here consists of an optional set-up part, in which the background images are taken, and the actual centring part. The algorithm uses boundary and centre-of-mass detection at two different microscope image magnifications. Micromounts can be handled as well. Centring of the loops can be achieved in 15-26s, depending on their initial position, and as fast as manual centring. The alignment of the sample is carried out by means of a new flexural-hinge-based compact goniometer head. The device features an electromagnet for robotic wet mounting of samples. The circle of confusion was measured to be smaller than 1 µm (r.m.s.); its bidirectional backlash is below 2 µm.

Reduction of X-ray-induced radiation damage of macromolecular crystals by data collection at 15 K: a systematic study

The cryocooling of protein crystals to temperatures of around 100 K drastically reduces X-ray-induced radiation damage. The majority of macromolecular data collection is therefore performed at 100 K, yielding diffraction data of higher resolution and allowing structure determination from much smaller crystals. However, at third-generation synchrotron sources radiation damage at 100 K still limits the useful data obtainable from a crystal. For data collection at 15 K, realised by the use of an open-flow helium cryostat, a further reduction of radiation damage is expected. However, no systematic studies have been undertaken so far. In this present study, a total of 54 data sets have been collected from holoferritin and insulin crystals at 15 and 90 K in order to identify the effect of the lower data-collection temperature on the radiation damage. It is shown that data collection at 15 K has only a small positive effect for insulin crystals, whereas for holoferritin crystals radiation damage is reduced by 23% compared with data collection at 90 K.

Protein conformational relaxation and ligand migration in myoglobin: a nanosecond to millisecond molecular movie from time-resolved Laue X-ray diffraction

A time-resolved Laue X-ray diffraction technique has been used to explore protein relaxation and ligand migration at room temperature following photolysis of a single crystal of carbon monoxymyoglobin. The CO ligand is photodissociated by a 7.5 ns laser pulse, and the subsequent structural changes are probed by 150 ps or 1 micros X-ray pulses at 14 laser/X-ray delay times, ranging from 1 ns to 1.9 ms. Very fast heme and protein relaxation involving the E and F helices is evident from the data at a 1 ns time delay. The photodissociated CO molecules are detected at two locations: at a distal pocket docking site and at the Xe 1 binding site in the proximal pocket. The population by CO of the primary, distal site peaks at a 1 ns time delay and decays to half the peak value in 70 ns. The secondary, proximal docking site reaches its highest occupancy of 20% at approximately 100 ns and has a half-life of approximately 10 micros. At approximately 100 ns, all CO molecules are accounted for within the protein: in one of these two docking sites or bound to the heme. Thereafter, the CO molecules migrate to the solvent from which they rebind to deoxymyoglobin in a bimolecular process with a second-order rate coefficient of 4.5 x 10(5) M(-1) s(-1). Our results also demonstrate that structural changes as small as 0.2 A and populations of CO docking sites of 10% can be detected by time-resolved X-ray diffraction.

A molecular movie at 1.8 A resolution displays the photocycle of photoactive yellow protein, a eubacterial blue-light receptor, from nanoseconds to seconds

The photocycle of the bacterial blue-light photoreceptor, photoactive yellow protein, was stimulated by illumination of single crystals by a 7 ns laser pulse. The molecular events were recorded at high resolution by time-resolved X-ray Laue diffraction as they evolved in real time, from 1 ns to seconds after the laser pulse. The complex structural changes during the photocycle at ambient temperature are displayed in a movie of difference electron density maps relative to the dark state. The step critical to entry into the photocycle is identified as flipping of the carbonyl group of the 4-hydroxycinnamic acid chromophore into an adjacent, hydrophobic environment rather than the concomitant isomerization about the double bond of the chromophore tail. The structural perturbation generated at the chromophore propagates throughout the entire protein as a light-induced "protein quake" with its "epicenter" at the carbonyl moiety of the chromophore.

Energy transduction on the nanosecond time scale: early structural events in a xanthopsin photocycle

Photoactive yellow protein (PYP) is a member of the xanthopsin family of eubacterial blue-light photoreceptors. On absorption of light, PYP enters a photocycle that ultimately transduces the energy contained in a light signal into an altered biological response. Nanosecond time-resolved x-ray crystallography was used to determine the structure of the short-lived, red-shifted, intermediate state denoted [pR], which develops within 1 nanosecond after photoelectronic excitation of the chromophore of PYP by absorption of light. The resulting structural model demonstrates that the [pR] state possesses the cis conformation of the 4-hydroxyl cinnamic thioester chromophore, and that the process of trans to cis isomerization is accompanied by the specific formation of new hydrogen bonds that replace those broken upon excitation of the chromophore. Regions of flexibility that compose the chromophore-binding pocket serve to lower the activation energy barrier between the dark state, denoted pG, and [pR], and help initiate entrance into the photocycle. Direct structural evidence is provided for the initial processes of transduction of light energy, which ultimately translate into a physiological signal.

Structure of a protein photocycle intermediate by millisecond time-resolved crystallography

The blue-light photoreceptor photoactive yellow protein (PYP) undergoes a self-contained light cycle. The atomic structure of the bleached signaling intermediate in the light cycle of PYP was determined by millisecond time-resolved, multiwavelength Laue crystallography and simultaneous optical spectroscopy. Light-induced trans-to-cis isomerization of the 4-hydroxycinnamyl chromophore and coupled protein rearrangements produce a new set of active-site hydrogen bonds. An arginine gateway opens, allowing solvent exposure and protonation of the chromophore's phenolic oxygen. Resulting changes in shape, hydrogen bonding, and electrostatic potential at the protein surface form a likely basis for signal transduction. The structural results suggest a general framework for the interpretation of protein photocycles.

Photolysis of the carbon monoxide complex of myoglobin: nanosecond time-resolved crystallography

The biological activity of macromolecules is accompanied by rapid structural changes. The photosensitivity of the carbon monoxide complex of myoglobin was used at the European Synchrotron Radiation Facility to obtain pulsed, Laue x-ray diffraction data with nanosecond time resolution during the process of heme and protein relaxation after carbon monoxide photodissociation and during rebinding. These time-resolved experiments reveal the structures of myoglobin photoproducts, provide a structural foundation to spectroscopic results and molecular dynamics calculations, and demonstrate that time-resolved macromolecular crystallography can elucidate the structural bases of biochemical mechanisms on the nanosecond time scale.

Feasibility and Realization of Single-Pulse Laue Diffraction on Macromolecular Crystals at ESRF

Laue diffraction patterns with an exposure time of ca 60 ps have been acquired at the European Synchrotron Radiation Facility (ESRF) on protein crystals by using the single-bunch mode of the storage ring. A 10 ns laser pulse initiating photodissociation was synchronized with the X-ray pulse. The potential for a quantitative detection of conformational changes in proteins on the nanosecond timescale with this technique is demonstrated using the example of carbonmonoxymyoglobin, from simulations and real data. The instrumental aspects of the experiment (highly intense X-ray beam, fast shutter system, Laue camera, detector, laser apparatus and synchronization technique) are emphasized.