Capturing the structural changes that molecules undergo during chemical reactions in real space and time is a long-standing dream and an essential prerequisite for understanding and ultimately controlling femtochemistry. A key approach to tackle this challenging task is Coulomb explosion imaging, which has benefited decisively from recently emerging high-repetition-rate X-ray free-electron laser sources. With this technique, information on the molecular structure is inferred from the momentum distributions of the ions produced by the rapid Coulomb explosion of molecules. Retrieving molecular structures from these distributions poses a highly nonlinear inverse problem that remains unsolved for molecules consisting of more than a few atoms. Here, we address this challenge using a diffusion-based Transformer neural network. We show that the network reconstructs unknown molecular geometries from ion-momentum distributions with a mean absolute error below one Bohr radius, which is half the length of a typical chemical bond.

We present the first comprehensive, internally consistent analysis of core-level chemical shifts for aqueous-phase solutes using Electron Spectroscopy for Chemical Analysis of Liquids (ESCAL). An absolute binding-energy calibration enables high accuracy and cross-molecule comparability. The C 1s spectra of oxygenated aliphatic compounds display functional-group-specific shifts that increase with carbon oxidation state. Although these trends depart from gas- and solid-phase behavior, highlighting solvent and hydration effects, they correlate closely with calculated core-level orbital energies, providing a useful first-order predictor. We further resolve secondary, through-bond shifts over one and two bonds, the magnitudes of which depend sensitively on specific functional-group interactions (notably carboxylic acid and ketone motifs). Such element- and oxidation-state-specific structural information establishes the principles and reference data needed to build a predictive ESCAL database for liquid-phase structural and chemical analysis. The results will be contrasted with NMR studies.

When an electronically excited atom or molecule is embedded in a chemical environment as, e.g., in a liquid or a loosely bound cluster, it can de-excite through mechanisms where neighboring atoms or molecules are actively participating in the decay: either by donating or accepting energy or electrons. For such nonlocal decay channels, nuclear dynamics play a crucial role as they have a direct impact on the decay efficiency itself. Here, we present a detailed study of the electron-transfer-mediated decay in a loosely bound triatomic prototype system, combining experimental results from a 5-fold coincidence measurement and theoretical modeling of the decay process. Depending on the decay time, we find that certain classes of molecular geometries are favored for this type of decay. Our findings provide an intuitive picture of how electron-transfer-mediated decay proceeds. In particular, our results confirm a roaming-like behavior of the atoms of the trimer prior to its decay. Our combined theoretical and experimental approach enables a comprehensive tracing of the real-space properties of the decaying system in the time domain.

We report a joint experimental and theoretical study of core ionization of fenchone enantiomers at photon energies in the range 294.9-306.9 eV. Our experimental method enables the measurement of photoelectron momentum distributions with full 4 pi solid-angle coverage in the laboratory frame, yielding photon-energy-resolved dichroic and anisotropy parameters for photoelectrons emitted from C 1s (C=O and C-Hn) orbitals. Our experimental results are supported by ab initio electronic-structure calculations and agree fairly well with values available in the literature. Our findings demonstrate that core-level photoelectron circular dichroism is highly sensitive to both the localization of the emitting orbital and the treatment of background contributions, providing a solid basis for future studies of complex chiral molecules.

The advent of novel free-electron laser sources enabling time-resolved x-ray photoelectron spectroscopy (tr-XPS) provides a unique opportunity to monitor local chemical environments in real time by measuring sub-eV shifts in core-electron binding energies. These shifts reflect the interplay between electronic excitation and nuclear motion, an interplay that remains largely unexplored. In our combined theoretical and experimental study of fluoropyridine (C_5H_4FN), we investigate this link by monitoring the evolving chemical environment at the N and F atomic sites as the photoexcited S_1 state relaxes to the ground state via a conical intersection. We find that the F site responds primarily to vibrational relaxation, showing minimal sensitivity to the electronic excited state. In contrast, excitation to S_1 induces a measurable energy shift at the N site and significantly enhances its sensitivity to local vibrations within the ring. This behavior arises from a photoinduced redistribution of charge, which also increases the Coulomb interaction between the 1s electron at the N atom and the atomic partial charge at an adjacent C atom. This insight opens new avenues for exploring ultrafast dynamics and conical intersection pathways in more complex systems, from photostable DNA bases to light-harvesting materials.

We report a joint experimental and theoretical study of K -shell photoionization of S and F atoms in sulfur hexafluoride, SF 6 . In our experiment, we employed tunable synchrotron radiation to generate photoelectrons emitted from the 1 s shells of the S and F atoms with different kinetic energies. Coincident electron- and fragment-ion detection, carried out using cold target recoil-ion momentum spectroscopy, enabled access to polarization-averaged molecular-frame photoelectron angular distributions. In addition, we performed electronic structure calculations within the relaxed-core Hartree-Fock approximation by using a stationary single-center method. The calculations are in good agreement with the experiment. Our results support previous findings on the feasibility of imaging three-dimensional atomic arrangements in molecules using polarization-averaged molecular-frame angular distributions. In particular, we demonstrate that such imaging is possible even at intermediate photoelectron kinetic energies and even for six experimentally undistinguished F 1 s emitters.

Transition-metal complexes are central for catalysis, material sciences and biochemistry, owing to their diverse non-radiative decay pathways driven by femtosecond-scale metal-ligand charge transfer. Resonant double-core-hole (DCH) ion-yield spectroscopy enables direct observation of ultrafast electronic and nuclear dynamics of such complexes with a direct coupling between strongly localized, transient charges and the chemically active valence electrons. Yet, previous DCH investigations were confined to atoms and small molecules at the K -edge. Here, we extend resonant DCH ion-yield spectroscopy to the L -edge of the transition-metal carbonyl Fe(CO) 5 in the gas phase to investigate fragmentation dynamics and sequential DCH ionization of the Fe center. Using intense femtosecond X-ray pulses, we observe signatures of multiply charged Fe cations, up to Fe 15+ . Comparison with atomic-level ionization-pathway modeling indicates a photon-energy range between 740eV and 780eV in which cations originating from core-excited DCH states dominate. Competing DCH channels, (i) resonant excitation following ionization or (ii) resonant-resonant excitation, govern the ionization and subsequent relaxation cascade. These channels enable charge build-up far beyond the single-core-hole (SCH) limit and induce fluence-dependent depletion of lower-charge fragments. Our results establish L -edge DCH resonance spectroscopy as a powerful approach for probing ultrafast charge redistribution and fragmentation in transition-metal complexes.

The advent of novel free-electron laser sources enabling time-resolved X-ray photoelectron spectroscopy (tr-XPS) provides a unique opportunity to monitor local chemical environments in real time by measuring sub-eV shifts in core-electron binding energies. These shifts reflect the interplay between electronic excitation and nuclear motion, an interplay that remains largely unexplored. In our combined theoretical and experimental study of fluoropyridine (C5H4FN), we investigate this link by monitoring the evolving chemical environment at the N and F atomic sites as the photoexcited S1 state relaxes to the ground state via a conical intersection. We find that the F site responds primarily to vibrational relaxation, showing minimal sensitivity to the electronic excited state. In contrast, excitation to S1 induces a measurable energy shift at the N site and significantly enhances its sensitivity to local vibrations within the ring. This behavior arises from a photoinduced redistribution of charge, which also increases the Coulomb interaction between the 1s electron at the N atom and the atomic partial charge at an adjacent C atom. This insight opens new avenues for exploring ultrafast dynamics and conical intersection pathways in more complex systems, from photostable DNA bases to light-harvesting materials.

Using the photon-ion merged-beams technique at the PETRA III synchrotron light source, we have measured cross sections for double and up to tenfold photoionization of La ^+ ions by a single photon in the energy range 820–1400 eV, where resonances and thresholds occur that are associated with the excitation or ionization of one M -shell electron. These cross sections represent experimental benchmark data for the further development of quantum theoretical methods, which will have to provide the bulk of the atomic data required for the modeling of nonequilibrium plasmas such as kilonovae. In the present work, we have upgraded the Jena Atomic Calculator and pushed the state-of-the-art of quantum calculations for heavy many-electron systems to new limits. In particular, we have performed large-scale calculations of the La ^+ photoabsorption cross section and of the deexcitation cascades, which set in after the initial creation of a 3 d hole. Our theoretical results largely agree with our experimental findings. However, our theoretical product-ion charge-state distributions are somewhat narrower than the experimental ones, which is most probably due to the simplifications necessary to keep the cascade calculations tractable.

Core-valence double ionization electron spectra near the S2p, F1s and S1s edges of SF6 are presented, analyzed and compared with conventional valence photoelectron spectra and quantum chemical calculations. The core-valence spectra are energetically stretched out, revealing salient structures between 15 and 40 eV that are sufficiently separated for molecular orbital analysis. The spectra offer new insights into the electronic structure, showing that the core hole substantially rearranges the molecular orbital order. This effect can be traced to orbital localization and nodal structure, as well as to valence-to-core penetration. The singlet and triplet splittings of the dicationic states progressively decrease in all three core-valence spectra towards deeper core levels, with larger splittings for S2p than S1s, reflecting greater valence-to-core penetration and exchange interaction for S2p. By large, the MO interpretation holds in frozen or self-consistent-field representations, except for the inner parts of the F1s spectra, which require analysis in terms of MO breakdown effects. An intensity model for CV spectra is derived using an independent-channel approach, where primary core ionization is treated by dipolar coupling to a continuum and the valence electron is promoted to a second continuum through shake-off. Full spin coupling and spin selectivity between the discrete doubly ionized states and the two continuum electrons are maintained. The primary molecular photoelectron intensity is expressed as a weighted sum of atomic subshell cross sections using a one-center atomic orbital projection of the relevant molecular orbital. The calculated spectra agree well with experiment and allow detailed assignment. In particular, symmetry breaking at the F1s core-ionization site is identified and discussed.

Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-heptadiyne. The measured fragment-ion momentum distributions show discernible differences between the three isomers and provide signatures of specific carbon and hydrogen sites in the molecule. In contrast to previous work, we demonstrate that distinct 'marker atoms' are not strictly required for constructing a meaningful molecular frame of reference for the interpretation of the momentum-space data. Our work paves the way for tracking the ultrafast motion of nuclei during isomerization reactions in pure hydrocarbons.

Understanding the light-driven responses of chiral molecular systems at a fundamental level—electronically—via photoelectron circular dichroism (PECD) provides a pathway to unlock the intricate mechanisms governing the functionality of chiral molecules. The specific role of oxygen atoms in chiral compounds remains under-explored, despite their pivotal role in numerous functional systems. We report a joint experimental and theoretical study of the PECD of O 1s electrons from the randomly oriented chiral molecule fenchone (C_{10}H_{16}O) in the gas phase across a kinetic-energy range from 3 to 15 eV. We observe a substantial forward-backward asymmetry in the electron emission patterns across several eV, which is well reproduced by relaxed-core Hartree-Fock calculations. Our results provide a site-specific perspective on the chiral sensitivity of an oxygen atom directly connected to the stereocenter of a chiral molecule.

Ultrafast charge transfer (CT) processes redistribute electronic charge within and between molecular units and play a central role in many physical, chemical, and biological phenomena. However, the microscopic pathways of multiple CT events, including the coupled structural evolution and energy redistribution, are challenging to disentangle experimentally in complex systems. To obtain controlled insight into such dynamics, well-defined properties are required. Here, we investigate the N2Ar dimer, which combines a covalent bond with a weak van der Waals interaction, using site-selective synchrotron photoionization and coincident detection of electrons and ions. Combined with ab initio calculations, this approach enables step-by-step tracking of ultrafast CT and fragmentation dynamics. We find that the dimer's structural evolution triggers a second CT event, opening complex reaction pathways in which electrons are transferred back and forth between Ar and N2, through two nonadiabatic transitions involving conical intersections. These results demonstrate that sequential multiple CT-induced transitions, even in a simple dimer, provide controlled insight into nonadiabatic reaction mechanisms relevant to complex systems.

Experimental insights into low-kinetic-energy electron scattering in aqueous solutions are essential for an improved understanding of electron-driven chemistry and radiobiology, and the development and informed application of aqueous-phase electron-based spectroscopy and dichroism methods. Generally, in aqueous environments and for electron kinetic energies below 12-15 eV, significant and, thus far, incompletely understood low-energy-transfer inelastic electron scattering with solvent molecules preponderates. This leads to cascades of tens-of-meV kinetic-energy losses that distort nascent photoelectron spectra, prevent direct and accurate electron-binding-energy measurements, and limit possibilities to determine electron-scattering cross sections at especially low electron kinetic energies. Here, we quantify aqueous-phase inelastic-scattering-based energy losses using 1-30 eV kinetic energy photoelectrons and liquid-jet photoemission spectroscopy, specifically by photoionizing an exemplary surface-active solute and comparing the results with those from the homogeneously distributed aqueous solvent. Thereby, we identify a general ≳17 eV electron-kinetic-energy requirement for the direct and accurate measurement of aqueous-phase electron binding energies, irrespective of interfacial concentration profiles. Further, at electron kinetic energies from 10 eV down to a few-eV above the ionization threshold, we observe and quantify lower degrees of scattering for photoelectrons generated from surface-active solutes, allowing moderately distorted surface-active-solute photoemission peaks to be resolved down to just few-eV electron kinetic energies. These results demonstrate that liquid-jet photoemission spectroscopy can be used to probe interfacial surface-active-solute dynamics and dichroism effects close to ionization thresholds, in stark contrast to similar experiments on homogeneously distributed solution components. Furthermore, they offer novel insights into low-electron-kinetic-energy scattering in aqueous environments, thereby addressing the current lack of reliable experimental data in this critical energy range.

The Cover Feature illustrates a study on hydrated phosphotyrosine in the gas phase. In their Research Article (DOI: 10.1002/chem.202403665), J. Leroux, S. Bari and co-workers explain how, by combining synchrotron radiation tuned to the oxygen core—shell energy with tandem mass spectrometry, they identified the most favorable position adopted by a single water molecule on the protonated amino acid. Their integrated experimental and theoretical approach revealed that the water molecule bridges the phosphate and carboxyl groups, effectively stabilizing the overall structure.

The interface of the oceans and aqueous aerosols with air drives many important physical and chemical processes in the environment, including the uptake of CO2 by the oceans. Transport across and reactions at the ocean-air boundary are in large part determined by the chemical composition of the interface, i.e., the first few nanometers into the ocean. The main constituents of the interface, besides water molecules, are dissolved ions and amphiphilic surfactants, which are ubiquitous in nature. We have used a combination of surface tension measurements and liquid-jet X-ray photoelectron spectroscopy to investigate model seawater solutions at realistic ocean-water ion concentrations in the absence and in the presence of model surfactants. Our investigations provide a quantitative picture of the enhancement or reduction of the concentration of ions due to the presence of charged surfactants at the interface. We have also directly determined the concentration of surfactants at the interface, which is related to the ionic strength of the solution (i.e., the "salting out" effect). Our results show that the interaction of ions and surfactants can strongly change the concentration of both classes of species at aqueous solution-air interfaces, with direct consequences for heterogeneous reactions as well as gas uptake and release at ocean-air interfaces.

Structure-sensitive methods based on femtosecond light or electron pulses are now making it possible to measure how molecular structures change during light-induced processes. Despite significant progress, high-fidelity imaging of nuclear positions remains a challenge even for relatively small molecular systems and, notably, regarding the positions of hydrogen atoms. As demonstrated in recent work, X-ray-induced Coulomb explosion imaging (CEI) may overcome this obstacle, as its sensitivity does not depend on the mass of the imaged atoms. The photoinduced ring opening of the heterocyclic molecule 2(5H)-thiophenone has attracted recent interest. Here, we show that CEI offers a powerful route to imaging the peripheral H atoms in this molecule and thus, more generally, to tracking detailed nuclear motions (e.g., isomerizations) in organic molecules on ultrafast time scales. Specifically, we record momentum-space Coulomb explosion images that report on the three-dimensional positioning of all nuclei within the molecule, for instance, distinguishing H atoms in C-H bonds that lie within or are directed out of the plane defined by the heavy atoms. The prospect of imaging peripheral H atoms to probe photochemical dynamics is explored by coupling ab initio molecular dynamics with classical Coulomb explosion simulations, thereby differentiating potential photoproduct isomers, including those whose structures primarily differ in the position of the hydrogens.

Gas-phase near-edge X-ray absorption mass spectrometry (NEXAMS) was employed at the carbon and oxygen K-edges to probe the influence of a single water molecule on the protonated phosphotyrosine molecule. The results of the photodissociation experiments revealed that the water molecule forms two bonds, with the phosphate group and another chemical group. By comparing the NEXAMS spectra at the carbon and oxygen K-edges with density functional theory calculations, we attributed the electronic transitions responsible for the observed resonances, especially the transitions due to the presence of the water molecule. We showed that the water molecule leads to a specific spectral feature in the partial ion yield of hydrated fragments at 536.4 eV. Moreover, comparing the NEXAMS spectra with the calculated structures allowed us to identify three possible structures for singly hydrated phosphotyrosine that agree with the observed fragmentation and resonances.

The photochemistry of heterocyclic molecules plays a decisive role for processes and applications like DNA photo-protection from UV damage and organic photocatalysis. The photochemical reactivity of heterocycles is determined by the redistribution of photoenergy into electronic and nuclear degrees of freedom, initially involving ultrafast internal conversion. Most heterocycles are planar in their ground state and internal conversion requires symmetry breaking. To lower the symmetry, the molecule must undergo an out-of-plane motion, which has not yet been observed directly. Here we show using the example of 2-thiouracil, how Coulomb explosion imaging can be utilized to extract comprehensive information on this molecular deformation, linking the extracted deplanarization of the molecular geometry to the previously studied temporal evolution of its electronic properties. Particularly, the protons of the exploded molecule are well-suited messengers carrying rich information on its geometry at distinct times after electronic excitation. We expect that our new analysis approach centered on these peripheral protons can be adapted as a general concept for future time-resolved studies of complex molecules in the gas phase.