arXiv subjects

Jochen Küpper

Publications and source records attributed to Jochen Küpper.

At least 19 recordsLinked to original sources

arXiv2026

Rotational-state-controlled dissociative ionization dynamics in $\mathrm{CF_2I_2}$

We investigated strong-field dissociative ionization of $\mathrm{CF_2I_2}$ ensembles prepared in different initial rotational-state distributions using an electrostatic deflector. Pronounced changes in ion-channel branching ratios revealed a strong dependence of the fragmentation dynamics on the initial rotational excitation. Analysis of fragment yields and their laser-power dependences identifies resonance-enhanced multiphoton ionization through an intermediate excited state, $\mathrm{CF_2I_2^{\ast}}$. The measured branching behavior indicates competition between stabilization into bound ionic states and dissociative channels, driven by near-threshold non-adiabatic Coriolis-type coupling. Tuning the rotational energy by only a few $\mu$eV is sufficient to significantly alter the ionization dynamics and to redistribute the reaction products. These findings demonstrate the key role of rotational excitation in controlling non-adiabatic dynamics following strong-field ionization of $\mathrm{CF_2I_2}$.

physics.chem-ph↗

arXiv2026

Controlled beams of cryo-cooled protein-like nanoparticles

We report a cryogenic buffer-gas-cell-aerodynamic-lens-stack setup that enables the generation of shock-frozen, dense, and controllable beams of various nanoparticles in the gas phase, including small and low-density species such as isolated proteins. We demonstrate characterization of the setup using strong-field ionization combined with velocity-map imaging, allowing the unambiguous detection of nanoparticles in the protein-size range and full reconstruction of the particle beams including determination of particle flux and number density. The generation and characterization workflow presented here provides a valuable approach for protein-like sample preparation and delivery in single-particle diffractive imaging, microscopy, and low-temperature nanoscience.

physics.ins-det↗

arXiv2026

Proton transfer and hydronium formation in ionized water

Aqueous radiation chemistry emerges through ultrafast proton transfer and ion-radical formation with unexplored energy-redistribution dynamics steering the subsequent reactions. We performed a time-resolved disruptive-probing experiment on pure water dimer, (H$_2$O)$_2$, to disentangle the elementary post-ionization reactions. Through kinetic-energy-resolved ion imaging, we unraveled the dynamics in the (H$_2$O)$_2^+$ ground state: at low-energy ($\sim$0.05~eV) ultrafast proton transfer ($\sim$19~fs) is followed by H$_3$O$^+$+OH fragmentation ($\sim$360~fs). At higher energies, proton transfer becomes hindered ($\sim$60~fs) while the subsequent fragmentation becomes faster ($\sim$210~fs), eventually ($>0.15$~eV) merging into coupled dynamics ($\sim$100~fs). Additionally, we observed H$_2$O)$_2^+$ stabilization proceeding through a Zundel-like structure. These timescales and product energies reveal how ion-radical formation in ionized hydrogen-bonded networks shapes reactivity in aqueous dynamics.

physics.chem-ph↗

arXiv2025

Single-Particle X-ray Scattering Reveals a High Local Supersaturation of Precursors as the Origin of CoO Assembly Formation

Single-particle small-angle X-ray scattering (SP-SAXS) enables quantitative morphological analysis by recording diffraction snapshots from isolated particles using X-ray free-electron laser (XFEL) pulses. Unlike conventional X-ray techniques, which average over the entire illuminated sample volume, SP-SAXS resolves low-contrast, less abundant, or transient species within heterogeneous particle populations that would otherwise remain hidden. Here, we apply SP-SAXS to investigate the solvothermal formation of CoO nanocrystal assemblies from a Co(acac)$_3$ precursor in benzyl alcohol. The single-particle data reveal amorphous, uniform-density Co(acac)$_2$ spheres as transient intermediates that directly crystallize into cavernous CoO nanocrystal assemblies, which explains why CoO forms as hierarchical aggregates rather than as isolated nanocrystals. These results demonstrate that SP-SAXS provides a powerful framework for disentangling morphological heterogeneity in nanoparticle formation processes.

cond-mat.mes-hall↗

arXiv2025

Single-particle incoherent diffractive imaging and amplified spontaneous emission in copper nanocubes

We demonstrate element-specific incoherent diffractive imaging (IDI) of single copper nanocubes using intensity correlations of K$\alpha$ fluorescence at a hard X-ray free-electron laser. Combining single particle diffraction classification with IDI, we retrieve the form factor of 88 nm cubes with 20 nm resolution, extending IDI to the destructive single-particle regime with a large gain in resolution. IDI visibility drops sharply above a fluence of $10^2$ J/cm$^2$, consistent with the assumption of amplified spontaneous emission. Our results reveal fundamental limits for high-fluence nanoimaging towards future single-particle X-ray imaging.

physics.optics↗

arXiv2025

Nonlinear phenomena in X-ray fluorescence from single nanoparticles under extreme conditions

Materials exposed to intense femtosecond X-ray pulses with energies above their K-shell absorption edge can enter an extremely ionized state, which could give rise to nonlinear phenomena, such as saturable absorption and reverse saturable absorption. In this work, we investigate these effects on single copper nanoparticles irradiated by an X-ray free-electron laser pulse. We study the properties of the K$\alpha$ fluorescence for two different short pulse durations and three X-ray incident energies below and above the K-shell absorption edge, and correlate these with incident fluence estimates based on coherent diffraction. We observe that the incident fluence of the pulse and not its duration, is the main factor that modulates the nonlinear response, which leads to an effective shortening of the fluorescence emission. Our findings have implications for fluorescence-based methods for imaging single particles using transiently coherent fluorescence, or diffractive imaging through transient resonances.

physics.optics↗

arXiv2025

Controlling rovibrational state populations of polar molecules in inhomogeneous electric fields of the Stark deceleration: molecular dynamics and quantum chemistry simulations

We propose a modified Stark-chirped rapid adiabatic passage technique for a robust rovibrational population transfer in the gas phase molecules in the presence of certain inhomogeneous electric fields. As an example application, the new state switching scheme is shown to greatly enhance the process of slowing polar ammonia molecules in the Stark decelerator. High-level quantum mechanical simulations show that a virtually complete population inversion between a selected pair of weak-field and strong-field seeking states of NH$_3$ can be achieved. Strong dc electric fields within the Stark decelerator are used as part of the rovibrational population transfer protocol. Classical-dynamics simulations for ammonia demonstrate notable improvements in the longitudinal phase space acceptance of the Stark decelerator upgraded with the state switching and an increased deceleration efficiency with respect to the standard Stark deceleration technique.

physics.atom-ph↗

arXiv2025

Direct observation of the exciton polaron by serial femtosecond crystallography on single CsPbBr$_3$ quantum dots

The outstanding opto-electronic properties of lead halide perovskites have been related to the formation of polarons. Nevertheless, the observation of the atomistic deformation brought about by one electron-hole pair in these materials has remained elusive. Here, we measure the diffraction patterns of single CsPbBr$_3$ quantum dots (QDs) with and without resonant excitation in the single exciton limit using serial femtosecond crystallography (SFX). By reconstructing the 3D differential diffraction pattern, we observe small shifts of the Bragg peaks indicative of a crystal-wide deformation field. Building on DFT calculations, we show that these shifts are consistent with the lattice distortion induced by a delocalized electron and a localized hole, forming a mixed large/small exciton polaron. This result creates a clear picture of the polaronic deformation in CsPbBr$_3$ QDs, highlights the exceptional sensitivity of SFX to lattice distortions in few-nanometer crystallites, and establishes an experimental platform for future studies of electron-lattice interactions.

cond-mat.mtrl-sci↗

arXiv2024

Learning phase-space flows using time-discrete implicit Runge-Kutta PINNs

We present a computational framework for obtaining multidimensional phase-space solutions of systems of non-linear coupled differential equations, using high-order implicit Runge-Kutta Physics- Informed Neural Networks (IRK-PINNs) schemes. Building upon foundational work originally solving differential equations for fields depending on coordinates [J. Comput. Phys. 378, 686 (2019)], we adapt the scheme to a context where the coordinates are treated as functions. This modification enables us to efficiently solve equations of motion for a particle in an external field. Our scheme is particularly useful for explicitly time-independent and periodic fields. We apply this approach to successfully solve the equations of motion for a mass particle placed in a central force field and a charged particle in a periodic electric field.

cs.LG↗

arXiv2024

Molecular influence on nuclear-quadrupole-coupling effects in laser induced alignment

We studied the effect of nuclear-quadrupole interactions on the field-free impulsive alignment of different asymmetric-top molecules. Our analysis is focused on the influence of the hyperfine- and rotational-energy-level structures. These depend on the number of nuclear spins, the rotational constants, and the symmetry of the tensors involved in the nuclear spin and external field interactions. Comparing the prototypical large-nuclear-spin molecules iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, and 2,5-diiodobenzonitrile, we demonstrate that the magnitude of the hyperfine splittings compared to the rotational-energy splittings plays a crucial role in the spin-rotational dynamics after the laser pulse. Moreover, we point out that the impact of the quadrupole coupling on the rotational dynamics decreases when highly excited rotational states dominate the dynamics.

physics.atom-ph↗

arXiv2024

Observation of Aerosolization-induced Morphological Changes in Viral Capsids

Single-stranded RNA viruses co-assemble their capsid with the genome and variations in capsid structures can have significant functional relevance. In particular, viruses need to respond to a dehydrating environment to prevent genomic degradation and remain active upon rehydration. Theoretical work has predicted low-energy buckling transitions in icosahedral capsids which could protect the virus from further dehydration. However, there has been no direct experimental evidence, nor molecular mechanism, for such behaviour. Here we observe this transition using X-ray single particle imaging of MS2 bacteriophages after aerosolization. Using a combination of machine learning tools, we classify hundreds of thousands of single particle diffraction patterns to learn the structural landscape of the capsid morphology as a function of time spent in the aerosol phase. We found a previously unreported compact conformation as well as intermediate structures which suggest an incoherent buckling transition which does not preserve icosahedral symmetry. Finally, we propose a mechanism of this buckling, where a single 19-residue loop is destabilised, leading to the large observed morphology change. Our results provide experimental evidence for a mechanism by which viral capsids protect themselves from dehydration. In the process, these findings also demonstrate the power of single particle X-ray imaging and machine learning methods in studying biomolecular structural dynamics.

q-bio.BM↗

arXiv2024

A versatile and transportable endstation for controlled molecule experiments

We report on a new versatile transportable endstation for controlled molecule (eCOMO) experiments providing a combination of molecular beam purification by electrostatic deflection and simultaneous ion and electron detection using velocity-map imaging (VMI). The $b$-type electrostatic deflector provides spatial dispersion of species based on their effective-dipole-moment-to-mass ratio. This enables selective investigation of molecular rotational quantum states, conformers, and molecular clusters. Furthermore, the double-sided VMI spectrometer equipped with two high-temporal-resolution event-driven Timepix3 cameras provides detection of all generated ions independently of their mass-over-charge ratio and electrons. To demonstrate the potential of this novel apparatus, we present experimental results from our investigation of carbonyl sulfide (OCS) after ionization. Specifically, we provide the characterization of the molecular beam, electrostatic deflector, and electron- and ion-VMI spectrometer. The eCOMO endstation delivers a platform for ultrafast dynamics studies using a wide range of light sources from table-top lasers to free-electron-laser and synchrotron-radiation facilities. This makes it suitable for research activities spanning from atomic, molecular, and cluster physics, over energy science and chemistry, to structural biology.

physics.chem-ph↗

arXiv2024

Specific versus Nonspecific Solvent Interactions of a Biomolecule in Water

Solvent interactions, particularly hydration, are vital in chemical and biochemical systems. Model systems unveil microscopic details of such interactions. We uncover a specific hydrogen-bonding motif of the biomolecular building block indole (C$_8$H$_7$N), tryptophan's chromophore, in water: a strong localized $\text{N-H}\cdots\text{OH}_2$ hydrogen bond, alongside unstructured solvent interactions. This insight is revealed from a combined experimental and theoretical analysis of indole's electronic structure in aqueous solution. We have recorded the complete X-ray photoemission and Auger spectrum of aqueous-phase indole, quantitatively explaining all peaks through \emph{ab initio} modeling. The efficient and accurate technique for modeling valence and core photoemission spectra involves the maximum-overlap method and the non-equilibrium polarizable-continuum model. A two-hole electron-population analysis quantitatively describes the Auger spectra. Core-electron binding energies for nitrogen and carbon highlight the specific interaction with a hydrogen-bonded water molecule at the N-H group and otherwise nonspecific solvent interactions.

physics.chem-ph↗

arXiv2024

Controlled molecule injector for cold, dense, and pure molecular beams at the European x-ray free-electron laser

A permanently available molecular-beam injection setup for controlled molecules (COMO) was installed and commissioned at the small quantum systems (SQS) instrument at the European x-ray free-electron laser (EuXFEL). A $b$-type electrostatic deflector allows for pure state-, size-, and isomer-selected samples of polar molecules and clusters. The source provides a rotationally cold ($T\approx1$~K) and dense ($ρ\approx10^8$~cm$^{-3}$) molecular beam with pulse durations up to 100~\us generated by a new version of the Even-Lavie valve. Here, a performance overview of the COMO setup is presented along with characterization experiments performed both, with an optical laser at the Center for Free-Electron-Laser Science and with x-rays at EuXFEL under burst-mode operation. COMO was designed to be attached to different instruments at the EuXFEL, in particular at the small quantum systems (SQS) and single particles, clusters, and biomolecules (SPB) instruments. This advanced controlled-molecules injection setup enables XFEL studies using highly defined samples with soft and hard x-ray FEL radiation for applications ranging from atomic, molecular, and cluster physics to elementary processes in chemistry and biology.

physics.chem-ph↗

arXiv2024

Water is a radiation protection agent for ionised pyrrole

Radiation-induced damage of biological matter is an ubiquitous problem in nature. The influence of the hydration environment is widely discussed, but its exact role remains elusive. Utilising well defined solvated-molecule aggregates, we experimentally observed a hydrogen-bonded water molecule acting as a radiation protection agent for ionised pyrrole, a prototypical aromatic biomolecule. Pure samples of pyrrole and pyrrole(H$_2$O) were outer-valence ionised and the subsequent damage and relaxation processes were studied. Bare pyrrole ions fragmented through the breaking of C-C or N-C covalent bonds. However, for pyrrole(H$_2$O)$^+$, we observed a strong protection of the pyrrole ring through the dissociative release of neutral water or by transferring an electron or proton across the hydrogen bond. Overall, a single water molecule strongly reduces the fragmentation probability and thus the persistent radiation damage of singly-ionised pyrrole.

physics.chem-ph↗

arXiv2024

Reaction Pathways of Water Dimer Following Single Ionization

Water dimer $(\text{H}_2\text{O})_2$ -- a vital component of the earth's atmosphere -- is an important prototypical hydrogen-bonded system. It provides direct insight into fundamental chemical and biochemical processes, e.g., proton transfer and ionic supramolecular dynamics occurring in astro- and atmospheric chemistry. Exploiting a purified molecular beam of water dimer and multi-mass ion imaging, we report the simultaneous detection of all generated ion products of $(\text{H}_2\text{O})^{+}_2$-fragmentation following single ionization. Detailed information about ion yields and reaction energetics of 13 ion-radical pathways, 6 of which are new, of $(\text{H}_2\text{O})^{+}_2$ are presented, including strong ${}^{18}\text{O}$-isotope effects.

physics.atm-clus↗

arXiv2024

Timepix3: single-pixel multi-hit energy-measurement behavior

The event-driven hybrid-pixel detector readout chip, Timepix3, has the ability to simultaneously measure the time of an event on the nanosecond timescale and the energy deposited in the sensor. However, the behaviour of the system when two events are recorded in quick succession of each other on the same pixel was not studied in detail previously. We present experimental measurements, circuit simulations, and an empirical model for the impact of a preceding event on this energy measurements, which can result in a loss as high as 70~\%. Accounting for this effect enables more precise compensation, particularly for phenomena like timewalk. This results in significant improvements in time resolution -- in the best case, multiple tens of nanoseconds -- when two events happen in rapid succession.

physics.ins-det↗

arXiv2024

Accuracy and Performance Evaluation of Low Density Internal and External Flow Predictions using CFD and DSMC

The Direct Simulation Monte Carlo (DSMC) method was widely used to simulate low density gas flows with large Knudsen numbers. However, DSMC encounters limitations in the regime of lower Knudsen numbers (Kn<0.1). In such cases, approaches from classical computational fluid dynamics (CFD) relying on the continuum assumption are preferred, offering accurate solutions at acceptable computational costs. In experiments aimed at imaging aerosolized nanoparticles in vacuo a wide range of Knudsen numbers occur, which motivated the present study on the analysis of the advantages and drawbacks of DSMC and CFD simulations of rarefied flows in terms of accuracy and computational effort. Furthermore, the potential of hybrid methods is evaluated. For this purpose, DSMC and CFD simulations of the flow inside a convergent-divergent nozzle (internal expanding flow) and the flow around a conical body (external shock generating flow) were carried out. CFD simulations utilize the software OpenFOAM and the DSMC solution is obtained using the software SPARTA. The results of these simulation techniques are evaluated by comparing them with experimental data (1), evaluating the time-to-solution (2) and the energy consumption (3), and assessing the feasibility of hybrid CFD-DSMC approaches (4). Keywords: DSMC; SPARTA; Continuum assumption; Transition regime; Rarefied flow; high-performance computing

physics.flu-dyn↗