arXiv subjects

Daniel A. Horke

Publications and source records attributed to Daniel A. Horke.

At least 19 recordsLinked to original sources

arXiv2024

Ultrafast Processes in 1,2-Dichloroethene measured with a Universal XUV probe

The presence of two chlorine atoms in 1,2-dichloroethene allows for isomerisation around the double bond. This isomerisation can lead to rich photochemistry. We present a time-resolved pump-probe photoelectron spectroscopy measurement on both the cis- and trans- isomers of 1,2-dichloroethene. A universal XUV probe of 22.3 eV is used allowing observation of photoelectrons formed anywhere on the potential energy surface, even from the ground-state or dissociation products. Following excitation with a 200 nm probe both ultrafast excited state dynamics and product formation are observed within the time resolution of the experiment. Excited state population begins to return to the ground state on an ultrafast time scale (< 70 fs) and population of products channels is observed on the same timescale. With the aid of ab initio calculations it is found that population transfer from the excited state is facilitated by vibrational modes involving C-C-H bends.

physics.chem-ph↗

arXiv2024

Preparation of tautomer-pure molecular beams by electrostatic deflection

Tautomers are ubiquitous throughout chemistry, and typically considered inseparable in solution. Yet (bio)chemical activity is highly tautomer specific, with common examples being the amino and nucleic acids. While tautomers exist in an equilibrium in solution, in the cold environment of a molecular beam the barrier to tautomerization is typically much too high for interconversion, and tautomers can be considered separate species. Here we demonstrate the separation of tautomers and production of tautomerically-pure gas-phase samples. We show this for the 2-pyridone / 2-hydroxypyridine system, an important structural motif in both uracil and cytosine. Spatial separation of the tautomers is achieved via electrostatic deflection in strong inhomogeneous fields. We furthermore collect tautomer-resolved photoelectron spectra using femtosecond multiphoton ionization. This paves the way for studying the structure-function-dynamic relationship on the level of individual tautomers, using approaches that typically lack the resolution to do so, such as ultrafast dynamics experiments.

physics.chem-ph↗

arXiv2024

Resolving non-equilibrium shape variations amongst millions of gold nanoparticles

Nanoparticles, exhibiting functionally relevant structural heterogeneity, are at the forefront of cutting-edge research. Now, high-throughput single-particle imaging (SPI) with x-ray free-electron lasers (XFELs) creates unprecedented opportunities for recovering the shape distributions of millions of particles that exhibit functionally relevant structural heterogeneity. To realize this potential, three challenges have to be overcome: (1) simultaneous parametrization of structural variability in real and reciprocal spaces; (2) efficiently inferring the latent parameters of each SPI measurement; (3) scaling up comparisons between $10^5$ structural models and $10^6$ XFEL-SPI measurements. Here, we describe how we overcame these three challenges to resolve the non-equilibrium shape distributions within millions of gold nanoparticles imaged at the European XFEL. These shape distributions allowed us to quantify the degree of asymmetry in these particles, discover a relatively stable `shape envelope' amongst nanoparticles, discern finite-size effects related to shape-controlling surfactants, and extrapolate nanoparticles' shapes to their idealized thermodynamic limit. Ultimately, these demonstrations show that XFEL SPI can help transform nanoparticle shape characterization from anecdotally interesting to statistically meaningful.

cond-mat.mes-hall↗

arXiv2023

Photoelectron circular dichroism upon multiphoton ionization of a chiral alcohol

We present the first photoelectron circular dichroism (PECD) measurements of chiral alcohols, and in particular 1-Phenylethanol, using multiphoton ionization at 400 nm. Observed PECD values were rather small at $\sim2$%, but could be reliably extracted using both hemispherical integration and Abel inversion approaches. Experimental uncertainties of $<0.3$% (2$σ$) were achieved with a collection time of around 2 hours. All experiments were conducted in a new compact spectrometer, featuring a continuous flow supersonic expansion and velocity-map imaging detection. The latter is crucial to extract reliable PECD values, as it allows discrimination of different features in the photoelectron spectrum, which exhibit different and opposing PECD signals. The use of a tabletop multiphoton universal ionization scheme is an important step towards a viable analytical chiral spectrometer based on PECD.

physics.chem-ph↗

arXiv2021

Unsupervised learning approaches to characterize heterogeneous samples using X-ray single particle imaging

One of the outstanding analytical problems in X-ray single particle imaging (SPI) is the classification of structural heterogeneity, which is especially difficult given the low signal-to-noise ratios of individual patterns and that even identical objects can yield patterns that vary greatly when orientation is taken into consideration. We propose two methods which explicitly account for this orientation-induced variation and can robustly determine the structural landscape of a sample ensemble. The first, termed common-line principal component analysis (PCA) provides a rough classification which is essentially parameter-free and can be run automatically on any SPI dataset. The second method, utilizing variation auto-encoders (VAEs) can generate 3D structures of the objects at any point in the structural landscape. We implement both these methods in combination with the noise-tolerant expand-maximize-compress (EMC) algorithm and demonstrate its utility by applying it to an experimental dataset from gold nanoparticles with only a few thousand photons per pattern and recover both discrete structural classes as well as continuous deformations. These developments diverge from previous approaches of extracting reproducible subsets of patterns from a dataset and open up the possibility to move beyond studying homogeneous sample sets and study open questions on topics such as nanocrystal growth and dynamics as well as phase transitions which have not been externally triggered.

eess.IV↗

arXiv2021

Charge-state distribution of aerosolized nanoparticles

In single particle imaging experiments, beams of individual nanoparticles are exposed to intense pulses of x-rays from free-electron lasers to record diffraction patterns of single, isolated molecules. The reconstruction for structure determination relies on signal from many identical particles. Therefore, well-defined-sample delivery conditions are desired in order to achieve sample uniformity, including avoidance of charge polydispersity. We have observed charging of 220 nm polystyrene particles in an aerosol beam created by a gas-dynamic virtual nozzle focusing technique, without intentional charging of the nanoparticles. Here, we present a deflection method for detecting and characterizing the charge states of a beam of aerosolized nanoparticles. Our analysis of the observed charge-state distribution using optical light-sheet localization microscopy and quantitative particle trajectory simulations is consistent with previous descriptions of skewed charging probabilities of triboelectrically charged nanoparticles.

physics.chem-ph↗

arXiv2021

Optical funnel to guide and focus virus particles for X-ray laser imaging

The need for precise manipulation of nanoparticles in gaseous or near-vacuum environments is encountered in many studies that include aerosol morphology, nanodroplet physics, nanoscale optomechanics, and biomolecular physics. Photophoretic forces, whereby momentum exchange between a particle and surrounding gas is induced with optical light, were recently shown to be a robust means of trapping and manipulating nanoparticles in air. We previously proposed a photophoretic "optical funnel" concept for the delivery of bioparticles to the focus of an x-ray free-electron laser (XFEL) beam for femtosecond x-ray diffractive imaging. Here, we describe the formation of a high-aspect-ratio optical funnel and provide a first experimental demonstration of this concept by transversely compressing and concentrating a high-speed beam of aerosolized viruses by a factor of three in a low-pressure environment. These results pave the way toward improved sample delivery efficiency for XFEL imaging experiments as well as other forms of imaging and spectroscopy.

physics.optics↗

arXiv2020

New aerodynamic lens injector for single particle diffractive imaging

An aerodynamic lens injector was developed specifically for the needs of single-particle diffractive imaging experiments at free-electron lasers. Its design allows for quick changes of injector geometries and focusing properties in order to optimize injection for specific individual samples. Here, we present results of its first use at the FLASH free-electron-laser facility. Recorded diffraction patterns of polystyrene spheres are modeled using Mie scattering, which allowed for the characterization of the particle beam under diffractive-imaging conditions and yield good agreement with particle-trajectory simulations.

physics.ins-det↗

arXiv2020

3D diffractive imaging of nanoparticle ensembles using an X-ray laser

We report the 3D structure determination of gold nanoparticles (AuNPs) by X-ray single particle imaging (SPI). Around 10 million diffraction patterns from gold nanoparticles were measured in less than 100 hours of beam time, more than 100 times the amount of data in any single prior SPI experiment, using the new capabilities of the European X-ray free electron laser which allow measurements of 1500 frames per second. A classification and structural sorting method was developed to disentangle the heterogeneity of the particles and to obtain a resolution of better than 3 nm. With these new experimental and analytical developments, we have entered a new era for the SPI method and the path towards close-to-atomic resolution imaging of biomolecules is apparent.

physics.app-ph↗

arXiv2020

Controlled beams of shockfrozen, isolated, biological and artificial nanoparticles

X-ray free-electron lasers (XFELs) promise the diffractive imaging of single molecules and nanoparticles with atomic spatial resolution. This relies on the averaging of millions of diffraction patterns of identical particles, which should ideally be isolated in the gas phase and preserved in their native structure. Here, we demonstrated that polystyrene nanospheres and Cydia pomonella granulovirus can be transferred into the gas phase, isolated, and very quickly shockfrozen, i.e. cooled to 4~K within microseconds in a helium-buffer-gas cell, much faster than state-of-the-art approaches. Nanoparticle beams emerging from the cell were characterized using particle-localization microscopy with light-sheet illumination, which allowed for the full reconstruction of the particle beams, focused to $<100\:μ\text{m}$, as well as for the determination of particle flux and number density. The experimental results were quantitatively reproduced and rationalized through particle-trajectory simulations. We propose an optimized setup with cooling rates for few-nanometers particles on nanoseconds timescales. The produced beams of shockfrozen isolated nanoparticles provide a breakthrough in sample delivery, e.g. for diffractive imaging and microscopy or low-temperature nanoscience.

physics.bio-ph↗

arXiv2019

Megahertz single-particle imaging at the European XFEL

The emergence of high repetition-rate X-ray free-electron lasers (XFELs) powered by superconducting accelerator technology enables the measurement of significantly more experimental data per day than was previously possible. The European XFEL will soon provide 27,000 pulses per second, more than two orders of magnitude more than any other XFEL. The increased pulse rate is a key enabling factor for single-particle X-ray diffractive imaging, which relies on averaging the weak diffraction signal from single biological particles. Taking full advantage of this new capability requires that all experimental steps, from sample preparation and delivery to the acquisition of diffraction patterns, are compatible with the increased pulse repetition rate. Here, we show that single-particle imaging can be performed using X-ray pulses at megahertz repetition rates. The obtained results pave the way towards exploiting high repetition-rate X-ray free-electron lasers for single-particle imaging at their full repetition rate.

physics.ins-det↗

arXiv2019

Light-sheet imaging for the recording of transverse absolute density distributions of gas-phase particle-beams from nanoparticle injectors

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.

physics.optics↗

arXiv2019

Pure Molecular Beam of Water Dimer

Spatial separation of water dimer from water monomer and larger water-clusters through the electric deflector is presented. A beam of water dimer with $93~\%$ purity and a rotational temperature of $1.5~$K was obtained. Following strong-field ionization using a $35~$fs laser pulse with a wavelength centered around $800~$nm and a peak intensity of $10^{14}~\text{W}/\text{cm}^2$ we observed proton transfer and $46~\%$ of the ionized water dimer broke apart into a hydronium ion $\text{H}_3\text{O}^+$ and neutral OH.

physics.atm-clus↗

arXiv2018

Laser-induced acoustic desorption of thermally stable and unstable biomolecules

We evaluated the effect of the laser-induced acoustic desorption (LIAD) process on thermally stable and unstable biomolecules. We found that the thermally labile glycine molecule fragmented following desorption via LIAD, due to the production of hot molecules from the LIAD process. We furthermore observed a rise in translational temperature with increasing desorption laser intensity, while the forward velocity was invariant with respect to the desorption laser intensity for both glycine and adenine molecules. The forward kinetic energy was in the range of the surface stress energy, which supports the previously proposed stress-induced desorption model for the laser-induced acoustic desorption process.

physics.chem-ph↗

arXiv2018

Optimizing aerodynamic lenses for single-particle imaging

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.

physics.flu-dyn↗

arXiv2018

Spatially separating the conformers of the dipeptide Ac-Phe-Cys-NH$_\text{2}$

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 ($T_\text{rot}\sim2.3~\text{K}$) molecular beams of intact dipeptide molecules, which we then spatially separate into the individual populated conformational states. This is achieved using the combination of supersonic expansion laser-desorption vaporisation with electrostatic deflection in strong inhomogeneous fields. This represents the first demonstration of a conformer-separated and rotationally-cold molecular beam of a peptide, and will enable future single biomolecule x-ray diffraction measurements.

physics.chem-ph↗

arXiv2018

Mapping the Complete Reaction Path of a Complex Photochemical Reaction

We probe the dynamics of dissociating CS$_2$ molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

physics.chem-ph↗

arXiv2018

Optimized cell geometry for buffer-gas-cooled molecular-beam sources

We have designed, constructed, and commissioned a cryogenic helium buffer-gas source for producing a cryogenially-cooled molecular beam and evaluated the effect of different cell geometries on the intensity of the produced molecular beam, using ammonia as a test molecule. Planar and conical entrance and exit geometries are tested. We observe a three fold enhancement in the NH$_3$ signal for a cell with planar-entrance and conical-exit geometry, compared to that for a typically used `box'-like geometry with planar entrance and exit. These observations are rationalized by flow-field simulations for the different buffer-gas cell geometries. The full thermalization of molecules with the helium buffer-gas is confirmed through rotationally-resolved REMPI spectra yielding a rotational temperature of 5 K.

physics.chem-ph↗