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We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, …, 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations.

Intramolecular hydrogen transfer, a reaction where donor and acceptor sites of a hydrogen atom are part of the same molecule, is a ubiquitous reaction in biochemistry and organic synthesis. In this work, we report hydronium ion (H₃O⁺) production from aminobenzoic acid (ABA) after core-level ionization with soft X-ray synchrotron radiation. The formation of H₃O⁺ during the fragmentation requires that at least two hydrogen atoms migrate to one of the oxygen atoms within the molecule. The comparison of two structural isomers, ortho- and meta-ABA, revealed that the production of H₃O⁺ depends strongly on the structure of the molecule, the ortho-isomer being much more prone to produce H₃O⁺. The isomer-dependency suggests that the amine group acts as a donor in the hydrogen transfer process. In the case of ortho-ABA, detailed H₃O⁺ production pathways were investigated using photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy. It was found that H₃O⁺ can result from a direct two-body dissociation but also from sequential fragmentation processes.

Despite the boom in catalytic response via constructing interfaces, understanding interfaces’ interaction in heterostructures is still a paradox. In this work, the interaction of Ni with MoS₂ in Ni-Ag-MoS₂ heterostructure are unveiled through synchrotron X-PEEM and what’s more, the missing interaction mechanism at the Ag-MoS₂ interface is probed via Raman mapping. The observed competition between the downshift of the E2g¹ and A1g modes due to charge carrier injection and the upshift of the E2g¹ and A1g modes due to compressive strain during reverse laser power experiment is assigned to the non-uniform growth of Ag nanoparticles, their intimate contact with MoS₂, and Ag intercalated layered MoS₂. The substantial improvement of the H₂ yield of the Ni-Ag-MoS₂ (∼55 μmol h−1 g−1) over the pristine MoS₂ and the binary Ag-MoS₂ evidence successful bonding of Ni, Ag and MoS₂. This study highlights the importance of considering both electronic coupling and strain to optically tune electromechanical properties of MoS₂.

Molecular cationic states with two valence holes and one n-Rydberg electron can be created after spectator Auger decay. Unraveling these states’ dissociation is often very challenging due to the frequent occurrence of conical intersections between cationic potential energy curves. Here, based on an advanced analysis of the experimental multicoincidence data obtained after O1s core excitation in O₂, we achieved an energy-resolution better than we recently exploited in Phys. Rev. A 99, 022511 (2019). We therefore revealed a group of weak channels in the two-dimensional energy-correlation map between the coincident resonant Auger electron and ion in addition to the previously reported strong ones. The fragments in the identified weak channels contain only outmost electrons in valence orbitals; in contrast, the fragments in the strong channels contain an outmost electron in a n’-Rydberg orbital. Compared with the strong channels, the weak channels preferentially occur at smaller principal quantum number n. It indicates that the electron orbital size tends to be conserved during the dissociation process. These weak features are suggested to be created by the Rydberg-valence mixing between the molecular spectator Auger final state and the very dissociative molecular cationic states without the Rydberg electron. A tendency to orbital selection is also suggested in the Rydberg-valence mixing.

Ca- and Cl-containing nanoparticles are common in atmosphere, originating for example from desert dust and sea water. The properties and effects on atmospheric processes of these aerosol particles depend on the relative humidity (RH) as they are often both hygroscopic and deliquescent. We present here a study of surface structure of free-flying CaCl₂ nanoparticles (CaCl₂-NPs) in the 100 nm size regime prepared at different humidity levels (RH: 11–85%). We also created mixed nanoparticles by aerosolizing a solution of CaCl₂ and phenylalanine (Phe), which is a hydrophobic amino acid present in atmosphere. Information of hydration state of CaCl₂-NPs and production of mixed CaCl₂ + Phe nanoparticles was obtained using soft X-ray absorption spectroscopy (XAS) at Ca 2p, Cl 2p, C 1s, and O 1s edges. We also report Ca 2p and Cl 2p X-ray absorption spectra of an aqueous CaCl₂ solution. The O 1s X-ray absorption spectra measured from hydrated CaCl2-NPs resemble liquid-like water spectrum, which is heavily influenced by the presence of ions. Core level spectra of Ca²⁺ and Cl⁻ ions do not show a clear dependence of % RH, indicating that the first coordination shell remains similar in all measured hydrated CaCl₂-NPs, but they differ from aqueous solution and solid CaCl₂.

The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid–vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations.

Linearly polarized synchrotron radiation has been used to record polarization dependent valence shell photoelectron spectra of imidazole in the photon energy range 21–100 eV. These have allowed the photoelectron angular distributions, as characterized by the anisotropy parameter β, and the electronic state intensity branching ratios to be determined. Complementing these experimental data, theoretical photoionization partial cross sections and β-parameters have been calculated for the outer valence shell orbitals. The assignment of the structure appearing in the experimental photoelectron spectra has been guided by vertical ionization energies and spectral intensities calculated by various theoretical methods that incorporate electron correlation and orbital relaxation. Strong orbital relaxation effects have been found for the 15a’, nitrogen lone-pair orbital. The calculations also predict that configuration mixing leads to the formation of several low-lying satellite states. The vibrational structure associated with ionization out of a particular orbital has been simulated within the Franck–Condon model using harmonic vibrational modes. The adiabatic approximation appears to be valid for the X 2A″ state, with the β-parameter for this state being independent of the level of vibrational excitation. However, for all the other outer valence ionic states, a disparity occurs between the observed and the simulated vibrational structure, and the measured β-parameters are at variance with the behavior expected at the level of the Franck–Condon approximation. These inconsistencies suggest that the excited electronic states may be interacting vibronically such that the nuclear dynamics occur over coupled potential energy surfaces.

Recent studies on sea spray aerosol indicate an enrichment of Ca2+ in small particles, which are often thought to originate from the very surface of a water body when bubbles burst. One model to explain this observation is the formation of ion pairs between Ca2+(aq) and surface-active organic species. In this study, we have used X-ray photoelectron spectroscopy to probe aqueous salt solutions and artificial sea spray aerosol to study whether ion pairing in the liquid environment also affects the surface composition of dry aerosol. Carboxylic acids were added to the sample solutions to mimic some of the organic compounds present in natural seawater. Our results show that the formation of a core–shell structure governs the surface composition of the aerosol. The core–shell structure contrasts previous observations of the dry sea spray aerosol on substrates. As such, this may indicate that substrates can impact the morphology of the dried aerosol.

The Avobenzone (AVOB) molecule is very photoactive and undergoes irreversible degradation upon irradiation. We studied its valence and core-level (C1s and O1s) photoionisation and subsequent photofragmentation with photoelectron spectroscopy and photoelectron–photoion–photoion coincidence (PEPIPICO) spectroscopy. AVOB is one of the largest molecules studied with this technique. The results show that the AVOB molecule dissociates into an extensive range of fragments by different pathways with little element or site-selectivity. The coincident maps were used to determine selected fragment separation sequences by analysing the slopes of patterns from ion pairs after the core ionisation. Charge delocalisation over the benzene rings and their relative stability favor fragmentation by cleavage of the bridge between them.

When an electron is resonantly excited from a core orbital to a Rydberg one in a molecule, complex multi-time-scale multistep processes are triggered. By using a high-resolution energy-resolved coincidence between Auger electron and ions, we identified the sequences of processes following the spectator Auger decays induced by K-shell excitation in O₂. The first step of the spectator Auger decay can end up to a few metastable O₂⁺ states which autoionize into stable O₂²⁺. However, the majority of O₂⁺ states are dissociative. They evolve into several dissociation limits with complex crossings between potential-energy curves. After the molecular dissociation is finished, the generated fragments can further decay either by fluorescence emission or by autoionization. Interestingly, some electrons are always emitted after molecular dissociation.

Photon-induced fragmentation of a full set of chlorinated methanes (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) has been investigated both experimentally and computationally. Using synchrotron radiation and electron-ion coincidence measurements, the dissociation processes were studied after chlorine 2p electron excitation. Experimental evidence for CH₃Cl and CH₂Cl₂ contains unique features suggesting that fast dissociation processes take place. By contrast, CHCl₃ and CCl₄ molecules do not contain the same features, hinting that they experience alternative mechanisms for dissociation and charge migration. Computational work indicates differing rates of charge movement after the core-excitation, which can be used to explain the differences observed experimentally.

The valence shell photoelectron spectrum of cis-dichloroethene has been studied both experimentally and theoretically. Photoelectron spectra have been recorded with horizontally and vertically plane polarized synchrotron radiation, thereby allowing the anisotropy parameters, characterizing the angular distributions, to be determined. The third-order algebraic-diagrammatic construction approximation scheme for the one-particle Green’s function has been employed to compute the complete valence shell ionization spectrum. In addition, the vertical ionization energies have been calculated using the outer valence Green’s function (OVGF) method and the equation-of-motion coupled-cluster, with single and double substitutions for calculating ionization potentials (EOM-IP-CCSD) model. The theoretical results have enabled assignments to be proposed for most of the structure observed in the experimental spectra, including the inner-valence regions dominated by satellite states. The linear vibronic coupling model has been employed to study the vibrational structure of the lowest photoelectron bands, using parameters obtained from ab initio calculations. The ground state optimized geometries and vibrational frequencies have been computed at the level of the second-order Møller-Plesset perturbation theory, and the dependence of the ionization energies on the nuclear configuration has been evaluated using the OVGF method. While the adiabatic approximation holds for the X̃ ²B₁ state photoelectron band, the à ²B₂, B̃ ²A₁, and C̃ ²A₂ states interact vibronically and form a complex photoelectron band system with four distinct maxima. The D̃ ²B₁ and Ẽ ²B₂ states also interact vibronically with each other. The potential energy surface of the D̃ ²B₁ state is predicted to have a double-minimum shape with respect to the out-of-plane a₂ deformations of the molecular structure. The single photoelectron band resulting from this interaction is characterized by a highly irregular structure, reflecting the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces forming a conical intersection close to the minimum of the Ẽ ²B₂ state.

In non-resonant Auger electron spectroscopies, multi core-ionized states lead to numerous energetically close-lying electronic transitions in Auger spectra, this hampering the assignment and interpretation of the experimental results. Here we reveal a new method to overcome this intrinsic limitation of non-resonant inner-shell spectroscopies. In a proof-of-principle experiment performed for the O₂ molecule, most of the Auger final states are dissociative, and we measure in coincidence the kinetic energy of the photoelectron and the kinetic energy release of the (O⁺, O⁺) ion pairs produced after the Auger decay of the O 1s−1 core-ionized states. The Auger final states are assigned using energy conservation. We fully separate the contributions from the ⁴Σ− and ²Σ− intermediate ionic states and conclusively demonstrate that the Auger decay probability can dramatically depend on the different O₂ 1s−1 intermediate multiplet states. In addition, a metastable Auger final state also exists, with lifetime longer than 3.8 μs, and clear changes are observed in both branching ratio and spectral profile of the O 1s photoelectron spectrum when they are recorded in coincidence with either O₂⁺⁺ or with other ionic species. These changes are attributed to the population of the metastable B′³Σ−u(ν′′=0) Auger final state via different intermediate states.

The experiments were performed at the PLEIADES beamline at the SOLEIL Synchrotron (Proposals No. 20110423 and 20130466).; International audience