arXiv subjects

Matthias F. Kling

Publications and source records attributed to Matthias F. Kling.

At least 19 recordsLinked to original sources

arXiv2026

X-ray Driven Trihydrogen Formation on Silica Nanosurfaces

The trihydrogen cation ($\mathrm{H_3^+}$) initiates the ion-molecule reactions that build molecular complexity in interstellar space. Whether its canonical formation reaction, $\mathrm{H_2^+ + H_2 \rightarrow H_3^+ + H}$, proceeds on inorganic surfaces under radiation-driven ionization has remained untested. Here we drive $\mathrm{H_3^+}$ formation on hydrated silica nanoparticles using intense 1.88 keV X-ray pulses, combining ion velocity map imaging, electron time-of-flight spectroscopy, and single-particle coherent diffractive imaging to resolve this chemistry on individual particles. The self-induced surface electric field on the V/nm scale drives interfacial charge transfer and water fragmentation. This field is the dominant parameter governing the relative yields of $\mathrm{H^+}$, $\mathrm{H_2^+}$, and $\mathrm{H_3^+}$ across particle size, composition, and aggregation. Density functional theory and nonadiabatic quantum molecular dynamics simulations trace this field-driven charge transfer, directly analogous to band bending at semiconductor photoelectrodes. These results establish surface-field-driven charge transfer as a unifying mechanism between radiation dominated astrophysical environments and field-driven surface catalysis.

physics.chem-ph↗

arXiv2026

EquiFiLM: Charge-Conditioned Equivariant Force Fields via Feature-wise Linear Modulation

Foundation machine learning force fields (MLFFs) such as MACE-MP-0 and UMA cover broad chemical space at near density functional theory (DFT) accuracy. However, they assume equilibrium ground-state physics and do not natively handle externally induced changes to the electronic state, such as charging, applied fields, or electronic excitation, which limits their use for driven processes such as photoexcitation and charge injection. We propose EquiFiLM, a lightweight extension that adds continuous external conditioning to any equivariant foundation MLFF via a per-layer Feature-wise Linear Modulation (FiLM) block, learning externally driven changes to the potential energy surface from minimal training data. The block modulates only scalar channels and preserves E(3)-equivariance exactly. We demonstrate the recipe on charged liquid water with the foundation model MACE-MatPES as the backbone, yielding E-MACE. On the four training charges, E-MACE delivers a $3.1\times$ reduction in force RMSE ($21.3$ to $6.96$ meV/$\mathring{A}$) and a $61\times$ reduction in per-atom energy RMSE ($6.1$ to $0.1$ meV/atom) over a baseline without EquiFiLM trained on the same data, at indistinguishable inference cost. Across seven held-out interpolation and extrapolation charges, force RMSE stays within $18-61$ meV/$\mathring{A}$ and energy RMSE within $0.7-5.4$ meV/atom. The model runs stable molecular dynamics across the full range tested and predicts the charge-dependent first-shell response of the reduced pair distribution function probed by ultrafast electron diffraction. Adding this conditioning axis to the foundation requires only a few thousand DFT-labeled frames, against the $\approx 10^8$ structures of a charge-aware foundation trained from scratch. The recipe is backbone- and conditioning-agnostic: it applies without architectural change to any equivariant MLFF with scalar interaction-layer channels.

cs.LG↗

arXiv2026

X-ray Coherent Attosecond Pulse Pair Spectroscopy

X-ray free electron laser (XFEL) experiments using self-amplified spontaneous emission (SASE) pulses typically achieve temporal resolutions of order several femtoseconds, as the pulse duration puts a practical limit to pump-probe or probe-probe schemes. Even with the emerging capabilities to generate pulses with attosecond durations with new single-spike SASE schemes, direct access to attosecond electron dynamics remains an experimental challenge. Here we show how X-ray coherent attosecond pulse-pair spectroscopy (X-CAPPS) provides a powerful new approach to access the ultrashort time-delay window. Coherent attosecond pulse pairs with time delays varying from ~500 as to ~5 fs are generated with Cu K$α_1$ stimulated X-ray emission from a gain medium pumped by intense SASE XFEL pulses. These pulse pairs are analyzed with two subsequent Bragg crystal spectrometers, and the resulting interference spectrum is captured on two sequential 2D image detectors encoding their time separation, relative amplitudes, and phases with high precision. X-CAPPS requires no split-and-delay X-ray optics, nor XFEL pulse modifications, making it broadly implementable across existing facilities. This technique enables the investigation of attosecond processes with $\mathring{A}$ngström resolution, providing a new tool for probing ultrafast dynamics across a wide range of atomic, molecular, and solid systems.

physics.optics↗

arXiv2026

Elucidating Norrish Type-I reactive pathways by ultrafast X-ray absorption spectroscopy

Norrish type I reactions selectively cleave carbon-carbon bonds directly adjacent to carbonyl groups. Despite their broad use in combination with aromatic carbonyls for additive manufacturing and dental UV curing applications, the nature of the photochemically active state and its population mechanism remain insufficiently understood. Detailed mechanistic insight requires mapping of the photoexcited population flow involving internal conversion and intersystem crossing. We present a time-domain study of gas phase acetophenone as a prototypical aromatic carbonyl combining soft X-ray time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge with ab initio multiple spawning (AIMS) simulations. Exploiting the specific sensitivity of TR-NEXAFS spectroscopy to states with $nπ^*$ character, we observe population transfer from the initially excited $^1ππ^*$ state to the $^1nπ^*$ state with a time constant of $(0.13 \pm 0.02)$ ps after an initial induction period of $(0.12 \pm 0.02)$ ps without population transfer, in quantitative agreement with the AIMS simulations. The population in the $^1nπ^*$ state subsequently decays via intersystem crossing, likely mediated by a $^3ππ^*$ state, within $(3.17 \pm 0.66)$ ps to a long-lived $^3nπ^*$ state, which is presumed to be active towards Norrish type I chemistry.

physics.chem-ph↗

arXiv2025

Chiral Altermagnon in MnTe

Altermagnetism has surfaced as a novel magnetic phase, bridging the properties of ferro- and anti-ferromagnetism. The momentum-dependent spin-splitting observed in these materials reflects their unique symmetry characteristics, which also establish the conditions for chiral magnons to emerge. Here we provide the first direct experimental evidence for a chiral magnon in the altermagnetic candidate MnTe revealed by circular-dichroism resonant inelastic X-ray scattering (CD-RIXS). This mode which we term chiral altermagnon exhibits a distinct momentum dependence of its spin polarization consistent with the proposed altermagnetic $g-$wave symmetry of MnTe. Our polarization-resolved results corroborate the existence of a new class of magnetic excitations, demonstrating how altermagnetic order shapes spin dynamics and paves the way for advances in spintronic and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Direct Observation of the Lindhard Continuum using Resonant Inelastic X-ray Scattering

Understanding the excitations of quantum materials is essential for unraveling how their microscopic constituents interact. Among these, particle-hole excitations form a particularly important class, as they govern fundamental processes such as screening, dissipation, and transport. In metals, the continuum of electron-hole excitations is described by the Lindhard function. Although central to the theory of Fermi liquids, the corresponding Lindhard continuum has remained experimentally elusive. Here, we report its direct observation in the weakly correlated metal MgB$_{2}$ using ultra-soft resonant inelastic X-ray scattering (RIXS). We resolve a linearly dispersing excitation with velocity comparable to the Fermi velocity and find quantitative agreement with simulations of the non-interacting charge susceptibility. A detailed analysis and decomposition of the simulations reveal the intra-band origin of this low-energy excitation, confirming it as the Lindhard continuum. Our results establish ultra-soft RIXS as a momentum-resolved probe of the fermiology in metals and call for deeper investigations of continuum features in RIXS and related spectroscopy of other materials beyond MgB$_{2}$.

cond-mat.str-el↗

arXiv2025

Imaging valence electron rearrangement in a chemical reaction using hard X-ray scattering

We have observed the signatures of valence electron rearrangement in photoexcited ammonia using ultrafast hard X-ray scattering. Time-resolved X-ray scattering is a powerful tool for imaging structural dynamics in molecules because of the strong scattering from the core electrons localized near each nucleus. Such core-electron contributions generally dominate the differential scattering signal, masking any signatures of rearrangement in the chemically important valence electrons. Ammonia represents an exception to the typically high core-to-valence electron ratio. We measured 9.8 keV X-ray scattering from gas-phase deuterated ammonia following photoexcitation via a 200 nm pump pulse to the 3s Rydberg state. We observed changes in the recorded scattering patterns due to the initial photoexcitation and subsequent deuterium dissociation. Ab initio calculations confirm that the observed signal is sensitive to the rearrangement of the single photoexcited valence electron as well as the interplay between adiabatic and nonadiabatic dissociation channels. The use of ultrafast hard X-ray scattering to image the structural rearrangement of single valence electrons constitutes an important advance in tracking valence electronic structure in photoexcited atoms and molecules.

physics.chem-ph↗

arXiv2025

Experimental demonstration of attosecond hard X-ray pulses

We present the first direct experimental confirmation of attosecond pulse generation in the hard X-ray regime with a free-electron laser. Our experiment is based on measurements of a nonlinear optical phenomenon known as amplified spontaneous emission (ASE) from 3d transition metals. By analyzing the yield of the collective X-ray fluorescence induced by ultrashort pulses at the Linac Coherent Light Source, we identify the generation of attosecond pulses and shot-to-shot fluctuations in their duration, ranging from 100 as to 400 as. The observed product of bandwidth and pulse duration for 100 as pulses is approximately 2 fs$\cdot$eV, indicating the generation of nearly transform-limited pulses. Our results extend the photon energy reach of attosecond techniques by one order of magnitude, providing the ability to simultaneously probe matter on the time-scales of electronic phenomena and with atomic spatial resolution. Furthermore, attosecond hard X-ray pulses can outrun the fastest radiation damage processes, paving the way to single-shot damage-free X-ray measurements.

physics.optics↗

arXiv2025

Attosecond Inner-Shell Lasing at Angstrom Wavelengths

Since the invention of the laser nonlinear effects such as filamentation, Rabi-cycling and collective emission have been explored in the optical regime leading to a wide range of scientific and industrial applications. X-ray free electron lasers (XFELs) have led to the extension of many optical techniques to X-rays for their advantages of angstrom scale spatial resolution and elemental specificity. One such example is XFEL driven population inversion of 1s core hole states resulting in inner-shell K$α$ (2p to 1s) X-ray lasing in elements ranging from neon to copper, which has been utilized for nonlinear spectroscopy and development of next generation X-ray laser sources. Here we show that strong lasing effects, similar to those observed in the optical regime, can occur at 1.5 to 2.1 angstrom wavelengths during high intensity (> ${10^{19}}$ W/cm${^{2}}$) XFEL driven inner-shell lasing and superfluorescence of copper and manganese. Depending on the temporal substructure of the XFEL pump pulses(containing ${~10^{6}}$ - ${10^{8}}$ photons) i, the resulting inner-shell X-ray laser pulses can exhibit strong spatial inhomogeneities as well as spectral splitting, inhomogeneities and broadening. Through 3D Maxwell Bloch theory we show that the observed spatial inhomogeneities result from X-ray filamentation, and that the spectral splitting and broadening is driven by Rabi cycling with sub-femtosecond periods. Our simulations indicate that these X-ray pulses can have pulse lengths of less than 100 attoseconds and coherence properties that open the door for quantum X-ray optics applications.

physics.optics↗

arXiv2024

Attosecond Coherent Electron Motion in a Photoionized Aromatic Molecule

In molecular systems, the ultrafast motion of electrons initiates the process of chemical change. Tracking this electronic motion across molecules requires coupling attosecond time resolution to atomic-scale spatial sensitivity. In this work, we employ a pair of attosecond x-ray pulses from an x-ray free-electron laser to follow electron motion resulting from the sudden removal of an electron from a prototypical aromatic system, para-aminophenol. X-ray absorption enables tracking this motion with atomic-site specificity. Our measurements are compared with state-of-the-art computational modeling, reproducing the observed response across multiple timescales. Sub-femtosecond dynamics are assigned to states undergoing non-radiative decay, while few-femtosecond oscillatory motion is associated with electronic wavepacket motion in stable cation states, that will eventually couple to nuclear motion. Our work provides insight on the ultrafast charge motion preceding and initiating chemical transformations in moderately complex systems, and provides a powerful benchmark for computational models of ultrafast charge motion in matter.

physics.chem-ph↗

arXiv2024

Effect of Burn Parameters on PAH Emissions at Conditions Relevant for Prescribed Fires

Wildfire smoke is a health hazard as it contains a mixture of carcinogenic volatile compounds and fine particulate matter. In particular, exposure to polycyclic aromatic hydrocarbons (PAHs) is a major concern, since these compounds have been recognized as important contributors to the overall carcinogenic risk of smoke exposure. In this work, gas and particle-phase PAH emissions from the combustion of Eastern White Pine (pinus strobus) were quantified using time-of-flight mass spectrometry over a range of burn conditions representative of wildfires and prescribed fires. These experiments allow for controlling conditions of fuel moisture, heat flux, and oxygen concentration to understand their impact on PAH emissions. We find that optimal conditions for fuel moisture content of 20 - 30%, heat load onto the sample of 60 - 70 kW/m$^2$, and oxygen concentrations of the burn environment of 5 - 15% can reduce the emissions of the heavy molar weight PAHs by up to 77%. Our analysis shows that the relative carcinogenic risk can be reduced by more than 50% under optimal conditions, offering a way for reducing emission exposure from forest treatment activities.

physics.ao-ph↗

arXiv2024

Catalysis in Extreme Field Environments: The Case of Strongly Ionized $SiO_{2}$ Nanoparticle Surfaces

High electric fields can significantly alter catalytic environments and the resultant chemical processes. Such fields arise naturally in biological systems but can also be artificially induced through localized excitations at nanoscale. Recently, strong field excitation of dielectric nanoparticles has emerged as an avenue for studying catalysis in highly ionized environments producing extreme electric fields. While the dynamics of surface ion emission driven by ultrafast laser ionization has been heavily explored, understanding the molecular dynamics leading to fragmentation has remained elusive. To address this, we employed a multiscale approach utilizing non-adiabatic quantum molecular dynamics (NAQMD) simulations on hydrogenated silica surfaces in both bare and wetted environments under field conditions mimicking those of an ionized nanoparticle. Our findings indicate that hole localization drives fragmentation dynamics, leading to surface silanol dissociation within 50 fs and charge transfer-induced water splitting in wetted environments within 150 fs. Further insight into such ultrafast mechanisms is critical for advancement of catalysis on the surface of charged nanosystems.

physics.chem-ph↗

arXiv2024

Attosecond Delays in X-ray Molecular Ionization

The photoelectric effect is not truly instantaneous, but exhibits attosecond delays that can reveal complex molecular dynamics. Sub-femtosecond duration light pulses provide the requisite tools to resolve the dynamics of photoionization. Accordingly, the past decade has produced a large volume of work on photoionization delays following single photon absorption of an extreme ultraviolet (XUV) photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required x-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. We have now measured the x-ray photoemission delay of core-level electrons, and here report unexpectedly large delays, ranging up to 700 attoseconds in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft x-ray pulses from a free-electron laser (XFEL) to scan across the entire region near the K-shell threshold. Furthermore, we find the delay spectrum is richly modulated, suggesting several contributions including transient trapping of the photoelectron due to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule, and multi-electron scattering effects. The results demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

physics.atom-ph↗

arXiv2024

Experimental Demonstration of Attosecond Pump-Probe Spectroscopy with an X-ray Free-Electron Laser

Pump-probe experiments with sub-femtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of sub-femtosecond pulse pairs from a two-colour X-ray free-electron laser (XFEL). By measuring the delay between the two pulses with an angular streaking diagnostic, we characterise the group velocity of the XFEL and demonstrate control of the pulse delay down to 270 as. We demonstrate the application of this technique to a pump-probe measurement in core-excited para-aminophenol. These results demonstrate the ability to perform pump-probe experiments with sub-femtosecond resolution and atomic site specificity.

physics.acc-ph↗

arXiv2024

Tracking Surface Charge Dynamics on Single Nanoparticles

Surface charges play a fundamental role in physics and chemistry, particularly in shaping the catalytic properties of nanomaterials. Tracking nanoscale surface charge dynamics remains challenging due to the involved length and time scales. Here, we demonstrate real-time access to the nanoscale charge dynamics on dielectric nanoparticles employing reaction nanoscopy. We present a four-dimensional visualization of the non-linear charge dynamics on strong-field irradiated single SiO$_2$ nanoparticles with femtosecond-nanometer resolution and reveal how surface charges affect surface molecular bonding with quantum dynamical simulations. We performed semi-classical simulations to uncover the roles of diffusion and charge loss in the surface charge redistribution process. Understanding nanoscale surface charge dynamics and its influence on chemical bonding on a single nanoparticle level unlocks an increased ability to address global needs in renewable energy and advanced healthcare.

cond-mat.mes-hall↗

arXiv2023

Far-field Petahertz Sampling of Plasmonic Fields

The collective response of metal nanostructures to optical excitation leads to localized plasmon generation with nanoscale field confinement driving applications in e.g. quantum optics, optoelectronics, and nanophotonics, where a bottleneck is the ultrafast loss of coherence by different damping channels. The present understanding is built-up on indirect measurements dictated by the extreme timescales involved. Here, we introduce a straightforward field sampling method that allows to measure the plasmonic field of arbitrary nanostructures in the most relevant petahertz regime. We compare experimental data for colloidal nanoparticles to finite-difference-time-domain calculations, which show that the dephasing of the plasmonic excitation can be resolved with sub-cycle resolution. Furthermore, we observe a substantial reshaping of the spectral phase of the few-cycle pulse induced by this collective excitation and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The results pave the way towards both a fundamental understanding of ultrafast energy transformation in nanosystems and practical applications of nanostructures in extreme scale spatio-temporal control of light.

physics.optics↗

arXiv2022

Reaction Nanoscopy of Ion Emission from Sub-wavelength Propanediol Droplets

Droplets provide unique opportunities for the investigation of laser-induced surface chemistry. Chemical reactions on the surface of charged droplets are ubiquitous in nature and can provide critical insight into more efficient processes for industrial chemical production. Here, we demonstrate the application of the reaction nanoscopy technique to strong-field ionized nanodroplets of propanediol (PDO). The technique's sensitivity to the near-field around the droplet allows for the in-situ characterization of the average droplet size and charge. The use of ultrashort laser pulses enables control of the amount of surface charge by the laser intensity. Moreover, we demonstrate the surface chemical sensitivity of reaction nanoscopy by comparing droplets of the isomers 1,2-PDO and 1,3-PDO in their ion emission and fragmentation channels. Referencing the ion yields to gas-phase data, we find an enhanced production of methyl cations from droplets of the 1,2-PDO isomer. Density functional theory simulations support that this enhancement is due to the alignment of 1,2-PDO molecules on the surface. The results pave the way towards spatio-temporal observations of charge dynamics and surface reactions on droplets in pump-probe studies.

physics.chem-ph↗

arXiv2022

Resonance effects in Brunel harmonic generation in thin film organic semiconductors

Organic semiconductors have attracted extensive attention due to their excellent optical and electronic properties. Here, we present an experimental and theoretical study of Brunel harmonic generation in two types of porphyrin thin films: tetraphenylporphyrin (TPP) and its organometallic complex derivative Zinc tetraphenylporphyrin (ZnTPP). Our results show that the $π$-$π^\ast$ excitation of the porphyrin ringsystem plays a major role in the harmonic generation process. We uncovered the contribution of an interband process to Brunel harmonic generation. In particular, the resonant ($S_0 \rightarrow S_2$ transition) enhanced multiphoton excitation is found to lead to an early onset of non-perturbative behavior for the 5th harmonic. Similar resonance effects are expected in Brunel harmonic generation with other organic materials.

physics.optics↗