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2 pp. (2020).

A new particle-injection approach is demonstrated that achieves very low background in the measurement of diffraction from macromolecular nanocrystals by using an aerosol-focusing injector with an X-ray free-electron laser., High-resolution Bragg diffraction from aerosolized single granulovirus nanocrystals using an X-ray free-electron laser is demonstrated. The outer dimensions of the in-vacuum aerosol injector components are identical to conventional liquid-microjet nozzles used in serial diffraction experiments, which allows the injector to be utilized with standard mountings. As compared with liquid-jet injection, the X-ray scattering background is reduced by several orders of magnitude by the use of helium carrier gas rather than liquid. Such reduction is required for diffraction measurements of small macromolecular nanocrystals and single particles. High particle speeds are achieved, making the approach suitable for use at upcoming high-repetition-rate facilities.

Optics express 27(25), 36580 - (2019). doi:10.1364/OE.27.036580, Imaging biological molecules in the gas-phase requires novel sample delivery methods, which generally have to be characterized and optimized to produce high-density particle beams. A non-destructive characterization method of the transverse particle beam profile is presented. It enables the characterization of the particle beam in parallel to the collection of, for instance, x-ray-diffraction patterns. As a rather simple experimental method, it requires the generation of a small laser-light sheet using a cylindrical telescope and a microscope. The working principle of this technique was demonstrated for the characterization of the fluid-dynamic-focusing behavior of 220 nm polystyrene beads as prototypical nanoparticles. The particle flux was determined and the velocity distribution was calibrated using Mie-scattering calculations., Published by Soc., Washington, DC

X-ray free-electron lasers have enabled the possibility of “single particle imaging”, in which the morphology of aerosolized particles may be uncovered through lensless diffractive imaging using intense femtosecond x-ray pulses that “freeze” atomic motion. Diffraction patterns are produced from a stream of particles such as atmospheric aerosol particulates or viruses that are intercepted by x-ray pulses focused to a sub-micrometer spot. In order to enable single-particle imaging, it is necessary to generate high-density particle beams to achieve sufficient diffraction data rates. Here we experimentally demonstrate the feasibility of increasing the density of an aerosolized beam of 300 nm virus particles via the light-induced forces. To achieve this, an optical “funnel” was constructed by re-imaging a first-order quasi Bessel beam, which we used to transversely compress the stream of viruses. We have observed up to a 4-fold increase in the density of particles in the beam.

Imaging biological molecules in the gas-phase requires novel sample delivery methods, which generally have to be characterized and optimized to produce high-density particle beams. A non-destructive characterization method of the transverse particle beam profile is presented. It enables the characterization of the particle beam in parallel to the collection of, for instance, x-ray-diffraction patterns. As a rather simple experimental method, it requires the generation of a small laser-light sheet using a cylindrical telescope and a microscope. The working principle of this technique was demonstrated for the characterization of the fluid-dynamic-focusing behavior of 220 nm polystyrene beads as prototypical nanoparticles. The particle flux was determined and the velocity distribution was calibrated using Mie-scattering calculations.

Angewandte Chemie / International edition International edition 130(42), 13971 - 13975 (2018). doi:10.1002/ange.201807646, Die Abbildung isolierter Moleküle mit atomarer Auflösung, z. B. in Experimenten zur kohärenten Röntgenbeugung an Freie‐Elektronen‐Lasern, benötigt Molekülstrahlen identischer Moleküle mit hoher Dichte. Allerdings besetzen selbst Biomoleküle in kalten Molekülstrahlen typischerweise mehrere Konformerzustände. Hier wird die Produktion von sehr kalten $(T_{rot}≈2.3 K)$ Molekülstrahlen aus intakten Dipeptiden demonstriert, in denen anschließend die einzelnen besetzten Konformerzustände räumlich getrennt werden. Dies wird durch die Kombination von Überschallexpansion mit Laserdesorptionsverdampfung und elektrostatischer Ablenkung in starken inhomogenen Feldern erreicht. Dies ist die erste Demonstration von konformerselektierten und rotationskalten Molekülstrahlen von Peptiden und ermöglicht in Zukunft die Untersuchung von konformerspezifischer Chemie durch nicht‐Konformer‐spezifische Techniken. Des Weiteren ist dies ein Meilenstein, um direkte Strukturabbildungen von isolierten biologischen Molekülen mit atomarer Auflösung mittels ultraschneller Beugungsexperimente zu ermöglichen., Published by Wiley-VCH, Weinheim

Journal of aerosol science 124, 17-29 (2018). doi:10.1016/j.jaerosci.2018.06.010, A numerical simulation infrastructure capable of calculating the flow of gas and the trajectories of particles through an aerodynamic lens injector is presented. The simulations increase the fundamental understanding and predict optimized injection geometries and parameters. Our simulation results were compared to previous reports and also validated against experimental data for 500 nm polystyrene spheres from an aerosol-beam-characterization setup. The simulations yielded a detailed understanding of the radial phase-space distribution and highlighted weaknesses of current aerosol injectors for single-particle diffractive imaging. With the aid of these simulations we developed new experimental implementations to overcome current limitations., Published by Elsevier, Amsterdam

Analytical chemistry 90(6), 3920 - 3927 (2018). doi:10.1021/acs.analchem.7b04797, A laser-induced acoustic desorption source, developed for use at central facilities, such as free-electron lasers, is presented. It features prolonged measurement times and a fixed interaction point. A novel sample deposition method using aerosol spraying provides a uniform sample coverage and hence stable signal intensity. Utilizing strong-field ionization as a universal detection scheme, the produced molecular plume is characterized in terms of number density, spatial extend, fragmentation, temporal distribution, translational velocity, and translational temperature. The effect of desorption laser intensity on these plume properties is evaluated. While translational velocity is invariant for different desorption laser intensities, pointing to a non-thermal desorption mechanism, the translational temperature increases significantly and higher fragmentation is observed with increased desorption laser fluence., Published by American Chemical Society, Columbus, Ohio

Angewandte Chemie / International edition 57(42), 13775 - 13779 (2018). doi:10.1002/anie.201807646, Atomic‐resolution‐imaging approaches for single molecules, such as coherent X‐ray diffraction at free‐electron lasers, require the delivery of high‐density beams of identical molecules. However, even very cold beams of biomolecules typically have multiple conformational states populated. We demonstrate the production of very cold $(T_{rot}≈2.3 K)$ molecular beams of intact dipeptide molecules, which were then spatially separated into the individual populated conformational states. This is achieved using the combination of supersonic expansion laser‐desorption vaporization with electrostatic deflection in strong inhomogeneous fields. This represents the first demonstration of a conformer‐separated and rotationally cold molecular beam of a peptide, which enables the investigation of conformer‐specific chemistry using inherently non‐conformer‐specific techniques. It furthermore represents a milestone toward the direct structural imaging of individual biological molecules with atomic resolution by ultrafast diffractive‐imaging methods., Published by Wiley-VCH, Weinheim

The structure of chitotriose bound to lysozyme after mixing times of 2 and 50 s was determined using a polyimide tape-drive device for mix-and-diffuse serial crystallography at a synchrotron light source., Unravelling the interaction of biological macromolecules with ligands and substrates at high spatial and temporal resolution remains a major challenge in structural biology. The development of serial crystallography methods at X-ray free-electron lasers and subsequently at synchrotron light sources allows new approaches to tackle this challenge. Here, a new polyimide tape drive designed for mix-and-diffuse serial crystallography experiments is reported. The structure of lysozyme bound by the competitive inhibitor chitotriose was determined using this device in combination with microfluidic mixers. The electron densities obtained from mixing times of 2 and 50 s show clear binding of chitotriose to the enzyme at a high level of detail. The success of this approach shows the potential for high-throughput drug screening and even structural enzymology on short timescales at bright synchrotron light sources.

Journal of applied physics 121(12), 123106(2017). doi:10.1063/1.4978914, A novel methodology for measuring gas flow from small orifices or nozzles into vacuum is presented. It utilizes a high-intensity femtosecond laser pulse to create a plasma within the gas plume produced by the nozzle, which is imaged by a microscope. Calibration of the imaging system allows for the extraction of absolute number densities. We show detection down to helium densities of $4\times10^{16}$~cm$^{-3}$ with a spatial resolution of a few micrometer. The technique is used to characterize the gas flow from a convergent-nozzle aerosol injector [Struct.\ Dyn.~2, 041717 (2015)] as used in single-particle diffractive imaging experiments at free-electron laser sources. Based on the measured gas-density profile we estimate the scattering background signal under typical operating conditions of single-particle imaging experiments and estimate that fewer than 50 photons per shot can be expected on the detector., Published by American Inst. of Physics, Melville, NY

Journal of synchrotron radiation 24(6), 1296 - 1298 (2017). doi:10.1107/S1600577517011961, The success of diffraction experiments from weakly scattering samples strongly depends on achieving an optimal signal-to-noise ratio. This is particularly important in single-particle imaging experiments where diffraction signals are typically very weak and the experiments are often accompanied by significant background scattering. A simple way to tremendously reduce backgroundscattering by placing an aperture downstream of the sample has been developed and its application in a single-particle X-ray imaging experiment at FLASH is demonstrated. Using the concept of a post-sample aperture it was possible to reduce the background scattering levels by two orders of magnitude., Published by IUCr, Chester

127 pp. (2017)., Published by Verlag Deutsches Elektronen-Synchrotron, Hamburg

Laser-induced acoustic desorption (LIAD) is a promising technique for gentle and efficient preparation of large intact neutral molecules into the gas-phase [1]. Here we demonstrate our newly set-up LIAD source, designed for prolonged measurement time, as required by FEL experiments, through automatic sample replenishment. Using a gas dynamic virtual nozzle we deposit a near-uniform layer of sample on a ∼1 m long, 10 µm thick tantalum foil band. An acoustic wave was initiated by irradiating the backside of the foil with the third harmonic of a nanosecond Nd:YAG laser. The induced photoacoustic shock wave travels through the foil and desorbs samples into the gas-phase. A stable neutral phenylalanine plume was prepared and characterized using strong-field ionization by an intense femtosecond laser field with a linear time of flight mass spectrometer. Spatial-temporal profiles, velocity distributions of the LIAD plume as a function of desorption laser intensity were investigated. Its potential for applications in x-ray or electron diffraction imaging will be discussed. [1] B. Lindner and U. Seydel, Anal. Chem. 57, 895 (1985).

By dispersing molecular beams with strong electric fields, we can routinely produce pure samples of individual conformers or clusters according to their quantum state. Selected molecules can be further controlled using alignment and orientation by laser and static electric fields [1]. We are currently working towards extending these techniques to much large molecules and systems. Laser-induced acoustic desorption (LIAD) is a promising technique for gentle and efficient preparation of large intact neutral molecules in the gas-phase [2]. Here we demonstrate our newly set-up LIAD source, designed for prolonged measurement times through automatic sample replenishment. Molecules are sprayed onto a long 10 µ m tantalum foil band that is irradiated from the backside by a third harmonic Nd:YAG laser (355 nm). The induced shockwave travels through the foil and desorbs molecules from the frontside of the foil. We present results and characterizations of the source, and discuss its potential for applications in x-ray or electron diffraction imaging. [1] Y. P. Chang, D. A. Horke, S. Trippel and J. Küpper, Int. Rev. Phys Chem. 34, 557 (2015); [2] B. Lindner and U. Seydel, Anal. Chem. 57, 895 (1985).

Nachrichten aus der Chemie 64(3), 319-322(2016). doi:10.1002/nadc.20164047276, Published by Wiley-VCH, Weinheim