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Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA–drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C6H6)2—the simplest prototypes of noncovalent π–π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (6H6+ cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process.

A decade ago, an electron-attachment process called interatomic Coulombic electron capture has been predicted to be possible through energy transfer to a nearby neighbor. It has been estimated to be competitive with environment-independent photorecombination, but its general relevance has yet to be established. Here, we evaluate the capability of alkali and alkaline earth metal cations to capture a free electron by assistance from a nearby water molecule. We introduce a characteristic distance rIC for this energy transfer mechanism in equivalence to the Förster radius. Our results show that water-assisted electron capture dominates over photorecombination beyond the second hydration shell of each cation for electron energies above a threshold. The assisted capture reaches distances equivalent to a fifth to seventh solvation shell for the studied cations. The far reach of the assisted electron capture is of significant general interest to the broad spectrum of research fields dealing with low-energy electrons, in particular radiation-induced damage of biomolecules. The here introduced distance measure will enable quantification of the role of the environment for assisted electron attachment.

Charge transfer processes between weakly bound entities play an important role in various chemical and biological environments. In this combined experimental and theoretical work, we investigate the nature of charge-transfer processes in homogeneous atomic and heterogeneous atomic-molecular clusters. Our results reveal fundamentally different processes to be at play in pure argon clusters compared to mixed argon-nitrogen systems: We demonstrate that the former species decay via photon-mediated charge transfer while a nonradiative direct process is found dominant in the atomic-molecular cases. Our results are of general interest for studies on charge redistribution in more complex and biologically relevant samples where molecules are involved.

We present a simple and efficient method for computing single and double ionization cross sections in ion-atom and ion-molecule collisions using ${L}^{2}$ Gaussian basis sets. Gaussian functions are widely employed to compute bound states of ions, atoms, and molecules. However, the description of continuum states, and therefore ionization phenomena, remains a theoretical challenge. Our approach is tested on the benchmark system antiproton-helium collisions in the so-called intermediate energy range. A good agreement with numerically exact calculations is observed. The proposed method is general and can thus be employed in any collisional systems in the challenging nonperturbative regime. Our work opens the way to investigate multiple ionization processes by ion impact in multicenter polyelectronic systems.

Interatomic processes play a crucial role in weakly bound complexes exposed to ionizing radiation; therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high-resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 fs, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process, where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles, which drastically accelerates ICD., Physical Review X, 11 (2), ISSN:2160-3308

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to a quantum system by transferring the excess energy of the electron capture to a neighbor ionizing it. Using the ab initio R-matrix method, we show that Fano profiles, resulting from interferences between the ICEC final states and resonant states, appear in the ICEC cross sections even at extremely large system-neighbor separations. We identify several types of resonant states depending on their decay pathways which may involve long-range electron and energy transfer. ICEC is a fundamental process and the interferences lead to substantial enhancement or decrease of the cross sections. The present investigation is therefore of general relevance in many contexts wherever electron capture in an environment takes place.

Single- and double-electron-capture processes occurring in the system of hydrogen ions colliding with alkaline-earth atoms, ${\mathrm{H}}^{+}+\mathrm{Mg}$, are investigated in a broad energy domain ranging from 0.25 to 180 keV. Total and state-selective cross sections are calculated using a two-active-electron semiclassical asymptotic-state close-coupling approach. Our results show the best overall agreement with experimental data, and possible reasons for observed discrepancies are discussed. Comparison of our cross sections with previous theoretical results further demonstrates the importance of electronic correlations between the magnesium valence electrons and the strong couplings between various important channels. Furthermore, our investigations suggest that the oscillatory structures observed in the double-electron-capture cross sections stem from complex coherence effects between double-electron capture, electron transfer to excited states, and transfer-excitation processes.

International audience; Potential energy surfaces (PESs) play a central role in our understanding of chemical reactions. Despite the impressive development of efficient electronic structure methods and codes, such computations still remain a difficult task for the majority of relevant systems. In this context, artificial neural networks (NNs) are promising candidates to construct the PES of a wide range of systems. However, the choice of suitable molecular descriptors remains a bottleneck for these algorithms. In this work, we show that a principal components analysis (PCA) is a powerful tool to prepare an optimal set of descriptors and to build an efficient NN: this protocol leads to a substantial improvement of the NNs in learning and predicting a PES. Furthermore, the PCA provides a means to reduce the size of the input space (i.e. number of descriptors) without losing accuracy. As an example, we applied this novel approach to the computation of the high-dimensional PES describing the keto-enol tautomerism reaction occurring in the acetone molecule.

International audience; Interparticle Coulombic Decay (ICD) is an efficient energy transfer process between two weakly interacting systems. ICD was recently proposed as the underlying fundamental mechanism for technological purposes based on quantum dot nanostructures, such as wavelength-sensitive detectors. Via ICD, an excited donor quantum dot releases its excess energy by ionizing a neighbouring acceptor dot. Here, we demonstrate that the presence of a third (ICD inactive) quantum dot can serve as a bridge between the two dots, which is shown to result in an enhancement of the efficiency of the ICD-mediated energy transfer. Furthermore, our results show that this enhancement is found to be robust against the change of the characteristics of the bridge quantum dot and particularly the depth and size. On the other hand, its relative position with respect to the donor and acceptor dots is found to foster the ICD when it is located in between the two dots. Our findings provide new insights for the development of ICD-based nanostructure technologies and particularly for rational design of three coupled quantum dots. 2

Intermolecular Coulombic decay (ICD) is a ubiquitous relaxation channel of electronically excited states in weakly bound systems, ranging from dimers to liquids. As it is driven by electron correlation, it was assumed that it will dominate over more established energy loss mechanisms, for example fluorescence. Here, we use electron–electron coincidence spectroscopy to determine the efficiency of the ICD process after 2a1 ionization in water clusters. We show that this efficiency is surprisingly low for small water clusters and that it gradually increases to 40–50% for clusters with hundreds of water units. Ab initio molecular dynamics simulations reveal that proton transfer between neighboring water molecules proceeds on the same timescale as ICD and leads to a configuration in which the ICD channel is closed. This conclusion is further supported by experimental results from deuterated water. Combining experiment and theory, we infer an intrinsic ICD lifetime of 12–52 fs for small water clusters., Interatomic or intermolecular Coulombic decay is responsible for the generation of slow electrons in clusters and biological samples. Here the authors use electron–electron coincidence detection to find the competitive roles of proton transfer and ICD that occur on similar time scales in water clusters.

International audience; We report the observation of the radiative decay of singly charged noble gas ground-state ions embedded in heterogeneous van der Waals clusters. Electron-photon coincidence spectroscopy and dispersed photon spectroscopy are applied to identify the radiative charge transfer from Kr atoms to a Ne þ 2 dimer, which forms after single valence photoionization of Ne atoms at the surface of a NeKr cluster. This mechanism might be a fundamental decay process of ionized systems in an environment.

Selective bond breaking in a molecule with the use of photons opens the way to control chemical reactions. We demonstrate here that dissociation of a molecule can be efficiently achieved by first photoexciting a neighboring atom or molecule. On the example of the giant $\mathrm{He}\ensuremath{-}{\mathrm{H}}_{2}$ dimer, we show that simultaneous ionization and excitation of the helium atom induces ${\mathrm{H}}_{2}$ dissociation with a high probability. The excited ${\mathrm{He}}^{+}$ ion transfers its excess energy via interatomic Coulombic decay (ICD) or electron transfer mediated decay (ETMD) to ${\mathrm{H}}_{2}$ which is then singly or doubly ionized, respectively. In both cases, the molecular ion dissociates effectively within a few tens of femtoseconds. Molecular-bond breaking induced by ICD and ETMD are expected to be general phenomena, which provide alternatives to standard photochemistry.

Interatomic Coulombic decay (ICD) is a mechanism that allows microscopic objects to rapidly exchange energy. When the two objects are distant, the energy transfer between the donor and acceptor species takes place via the exchange of a virtual photon. On the contrary, recent ab initio calculations have revealed that the presence of a third passive species can significantly enhance the ICD rate at short distances due to the effects of electronic wave function overlap and charge transfer states [Phys. Rev. Lett. 119, 083403 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.083403]. Here, we develop a virtual photon description of three-body ICD, allowing us to investigate retardation and geometrical effects which are out of reach for current ab initio techniques. We show that a passive atom can have a significant influence on the rate of the ICD process at fairly large interatomic distances, due to the scattering of virtual photons off the mediator. Moreover, we demonstrate that in the retarded regime ICD can be substantially enhanced or suppressed depending on the position of the ICD-inactive object, even if the latter is far from both donor and acceptor species.

We have investigated the double electron capture process in the ${\mathrm{H}}^{+}+{\mathrm{H}}^{\ensuremath{-}}$ collision system for energies from 60 eV to 20 keV. Despite the apparent simplicity of this highly correlated system, all previous calculations fail to reproduce the experimental total cross sections. Moreover, the latter exhibit oscillations that have been previously attributed to quantum interferences between the gerade and ungerade ionic states of the transient molecule formed during the collision. For this process, we present the absolute cross sections obtained from a fully correlated two-active-electron semiclassical atomic-orbital close-coupling approach. Our results reproduce well the experimental data in both magnitude and shape. Furthermore, we demonstrate that the oscillations stem from coherence effects between double electron capture and other two-electron inelastic channels, namely the transfer-excitation processes. This alternative interpretation is supported by a Rosenthal-like model based on a molecular treatment of the collision. Our results shed new light on this old but challenging problem.

International audience; Using a semiclassical close-coupling approach, we have calculated electron capture, excitation and ionization cross sections for collisions of fully stripped hydrogen, helium and lithium ions with atomic hydrogen in the ground state and in all excited states up to n=3. The cross sections for collision energies between 1 and 100 keV/u are given in table form. Furthermore, we provide estimates of the accuracy of the cross sections. The set of data presented in this work represent the first complete and consistent quantum study of these collision systems and will find use in the modeling and diagnosis of thermonuclear fusion plasma reactors.

International audience; We have theoretically studied the Auger effect after inner-valence ionization of several unsaturated and saturated cyclic and linear hydrocarbon molecules. These prototype molecules were chosen such that the effects of the different characteristics of aromaticity (π electrons, conjugation, cyclic geometry) on the Auger decay can be investigated separately. We show that among these molecules, the ones having π electrons can undergo Auger decay after inner-valence ionization. Furthermore, the results reported here suggest that conjugation allows for several open Auger decay channels while aromaticity limits the range of the latter.

International audience; Interatomic Coulombic Decay (ICD) is an ultrafast energy transfer process. Via ICD, an excited atom can transfer its excess energy to a neighboring atom which is thus ionized. On the example of NeHeNe cluster, we recently reported [Phys. Rev. Lett. 119, 083403 (2017)] that the total ICD widths are substantially enhanced in the presence of an ICD inactive atom. The enhancement occurs due to the coupling of the resonance state to intermediate virtual states of the bridge atom-a mechanism named superexchange ICD. In this follow-up work, we analyze the partial ICD widths in NeHeNe cluster and show that only some channels are affected by the superexchange ICD process. Furthermore, we consider superexchange ICD in NeHeAr. We show that in this system the enhancement is still present but the energy transfer mediated by the superexchange mechanism is less efficient than in NeHeNe owing to the different ionization potentials of Ar and Ne. The behavior of the computed ICD widths is explained with a simple model based on first-order perturbation theory and a Hartree-Fock-like description of the states.

Physical review / A covering atomic, molecular, and optical physics and quantum information 100(2), 022707 (2019). doi:10.1103/PhysRevA.100.022707, We report on the experimental observation of interatomic Coulombic decay (ICD) in pure 4He nanoclustersof mean sizes between N ∼ 5000 and 30 000 and the subsequent scattering of energetic He+ fragments insidethe neutral cluster by using cold target recoil ion momentum spectroscopy. ICD is induced in He clusters byusing vacuum ultraviolet light of hν = 67 eV from the BESSY II synchrotron. The electronic decay createstwo neighboring ions in the cluster at a well-defined distance. The measured fragment energies and angularcorrelations show that a main energy loss mechanism of these ions inside the cluster is a single hard binarycollision with one atom of the cluster., Published by Inst., Woodbury, NY

Atoms and molecules attached to rare-gas clusters are ionized by an interatomic autoionization process traditionally termed "Penning ionization" when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge exchange at a short interatomic distance and one from virtual photon transfer at a large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence. Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at a large distance from the nanodroplet surface, autoionization is largely dominated by charge-exchange ICD. Furthermore, the measured electron spectra manifest the ultrafast internal relaxation of the droplet mainly into the 1s2s1S state and partially into the metastable 1s2s3S state.

We report on complementary theoretical and laboratory investigations of the 2p ion yield cross sections for the molecular-ion series SiHn + (n = 1, 2, 3), in the 95-108 eV photon energy range, below the L-shell threshold. The experiments used an electron cyclotron resonance (ECR) plasma molecular-ion source coupled with monochromatised synchrotron radiation in a merged-beam configuration. The experimental spectra are compared with total photoabsorption cross-sectional profiles calculated using an ab initio configuration interaction method inclusive of spin-orbit coupling and the vibrational dynamics. The experimental results show vibrationally resolved resonances for SiH2 + in the 98-102 eV range. The calculations indicate twenty four core-excited states below the energy of 102 eV, of which only four contribute significantly to the observed spectrum. These states correspond to the excitation of an atomic-like 2p electron to the SiH2 + (5a1) valence orbital.

We theoretically investigate the keto–enol tautomerism of micro-hydrated acetylacetone molecule and compare it to the isolated acetylacetone. The minimum energy path was computed through the Climbing-Image Nudged Elastic Band method (CINEB). The density functional theory was applied in the Projector Augmented Wavefunction (PAW) formalism using the generalized gradient approximation within Perdew–Wang (PW91) parametrization. Moreover, a topological and energy partition analysis was done in the framework of the Quantum Theory of Atoms In Molecules (QTAIM). We show that the activation barrier energy decreases from the gas phase to clusters with up to three water molecules surrounding the acetylacetone and increases when four water molecules are considered. Our analysis indicates that the hydrogen bond lattice of the water molecules strongly stabilizes the transition state, leading to the decrease of the activation energy. Furthermore, the size of the proton relay formed by the solvent molecules influences the stretch of the $$\hbox {C}_{\alpha }$$ –H bond which explains the increase of the activation energy for the largest cluster.

Electron transfer processes in ${\mathrm{He}}^{+}+\mathrm{He}$ collisions are studied theoretically using a three-electron semiclassical atomic-orbital close-coupling method in a wide energy domain, from 1 to 225 keV/u. Total, state-selective, and angular-differential cross sections are presented and compared with available experimental and theoretical results. A prominent oscillatory energy dependence structure in the transfer-excitation cross sections is observed and explained by a strong competition between these channels and the projectile-excitation processes. Moreover, the angular-differential cross sections considered in this work exhibit an oscillatory structure which is interpreted within a Fraunhofer-type diffraction model. For the two highest considered collision energies, the cross sections show a different pattern for which both Fraunhofer-type diffraction and the Thomas mechanism have to be advocated.

We report on complementary laboratory and theoretical investigations of the $2p$ photoexcitation cross sections for the molecular-ion series $\mathrm{Si}{{\mathrm{H}}_{n}}^{+}$ ($n=1,2,3$) near the $L$-shell threshold. The experiments used an electron cyclotron resonance (ECR) plasma molecular-ion source coupled with monochromatized synchrotron radiation in a merged-beam configuration. For all three molecular ions, the $\mathrm{S}{\mathrm{i}}^{2+}$ decay channel appeared dominant, suggesting similar electronic and nuclear relaxation patterns involving resonant Auger and dissociation processes, respectively. The total yields of the $\mathrm{S}{\mathrm{i}}^{2+}$ products were recorded and put on absolute cross-section scales by comparison with the spectrum of the $\mathrm{S}{\mathrm{i}}^{+}$ parent atomic ion. Interpretation of the experimental spectra ensued from a comparison with total photoabsorption cross-sectional profiles calculated using ab initio configuration interaction theoretical methods inclusive of vibrational dynamics and contributions from inner-shell excitations in both ground and valence-excited electronic states. The spectra, while broadly similar for all three molecular ions, moved towards lower energies as the number of screening hydrogen atoms increased from one to three. They featured a wide and shallow region below $\ensuremath{\sim}107\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ due to $2p\ensuremath{\rightarrow}{\ensuremath{\sigma}}^{*}$ transitions to dissociative states, and intense and broadened peaks in the $\ensuremath{\sim}107--113\ensuremath{-}\mathrm{eV}$ region merging into sharp Rydberg series due to $2p\ensuremath{\rightarrow}n\ensuremath{\delta},n\ensuremath{\pi}$ transitions converging on the ${L}_{\mathrm{II},\mathrm{III}}$ limits above $\ensuremath{\sim}113\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. This overall spectral shape is broadly replicated by theory in each case, but the level of agreement does not extend to individual resonance structures. In addition to the fundamental interest, the work should also prove useful for the understanding and modeling of astronomical and laboratory plasma sources where silicon hydride molecular species play significant roles.

International audience; Hydride molecular ions are key ingredients of the interstellar chemistry since they are precursors of more complex molecules. In regions located near a soft X-ray source these ions may resonantly absorb an X-ray photon which triggers a complex chain of reactions. In this work, we simulate ab initio the X-ray absorption spectrum, Auger decay processes and the subsequent fragmentation dynamics of two hydride molecular ions, namely CH2+ and CH3+. We show that these ions feature strong X-ray absorption resonances which relax through Auger decay within 7 fs. The doubly-charged ions thus formed mostly dissociate into smaller ionic carbon fragments: in the case of CH2+, the dominant products are either C+/H+/H or CH+/H+. For CH3+, the system breaks primary into CH2+ and H+, which provides a new route to form CH2+ near a X-ray source. Furthermore, our simulations provide the branching ratios of the final products formed after the X-ray absorption as well as their kinetic and internal energy distributions. Such data can be used in the chemistry models of the interstellar medium.

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to an ion, atom or molecule by transferring the excess energy to a neighboring species. Absolute cross sections are necessary to evaluate the relative importance of this process. In this work, we employ the R-matrix method to compute ab initio these cross sections for a singly charged neon ion embedded in small helium clusters. Our results show that the ICEC cross sections are several orders of magnitude higher than anticipated and dominate other competing processes. Electron energy loss spectra on an absolute scale are provided for the Ne+@He20 cluster. Such spectra exhibit an unambiguous signature of the ICEC process. The finding is expected to stimulate experimental observations.

International audience; Almost ten years ago, energetic neutral hydrogen atoms were detected after a strong-field double ionization of H2. This process, called 'frustrated tunneling ionization', occurs when an ionized electron is recaptured after being driven back to its parent ion by the electric field of a femtosecond laser. In the present study we demonstrate that a related process naturally occurs in clusters without the need of an external field: we observe a charge hopping that occurs during a Coulomb explosion of a small helium cluster, which leads to an energetic neutral helium atom. This claim is supported by theoretical evidence. As an analog to 'frustrated tunneling ionization', we term this process 'frustrated Coulomb explosion'.

International audience; X-ray absorption and Auger electron spectroscopies are demonstrated to be powerful tools to unravel the electronic structure of solvated ions. In this work for the first time, we use a combination of these methods in the tender X-ray regime. This allowed us to address electronic transitions from deep core levels, to probe environmental effects, specifically in the bulk of the solution since the created energetic Auger electrons possess large mean free paths, and moreover, to obtain dynamical information about the ultrafast delocalization of the core-excited electron. In the considered exemplary aqueous KCl solution, the solvated isoelectronic K+ and Cl– ions exhibit notably different Auger electron spectra as a function of the photon energy. Differences appear due to dipole-forbidden transitions in aqueous K+ whose occurrence, according to the performed ab initio calculations, becomes possible only in the presence of solvent water molecules.

International audience; We present a theoretical model to investigate double-core-hole-state formation in molecules through sequential absorption of two x-ray photons from a femtosecond laser pulse. A complete time-dependent quantum description taking into account x-ray absorption and nuclear dynamics explicitly and Auger decay phenomenologically is established within the local approximation. Using this model, we assess the impact of the nuclear dynamics on double-core photoionization processes in the case of the carbon monoxide molecule. We show that sequential absorption of two x-ray photons modifies significantly the vibrational distribution of the photoelectron spectra of the double-core-hole states compared to direct single x-ray photon absorption. Depending on the shape of the potential energy curves involved in the sequential absorption processes, lower or higher vibrational levels may be favored. Furthermore, in the case where the final state is dissociative, the electron spectrum is further broaden and blue-shifted in the two-photon process.

WoS title: Probing keto-enol tautomerism using photoelectron spectroscopy (vol 17, pg 19991, 2015)Correction for ‘Probing keto–enol tautomerism using photoelectron spectroscopy’ by Nathalie Capron et al., Phsical Chemistry Chemical Physics., 2015, vol. 17, issue 30, pages 19991–19996. DOI: 10.1039/c5cp02023a

The electron-capture processes in ${\mathrm{C}}^{4+}$ + He collisions have been studied theoretically using a two-active-electron semiclassical atomic-orbital close-coupling method in a wide energy domain. The results of the present calculations are compared with available theoretical predictions and experimental measurements: very good agreements are found for both total and state-selective single-electron-capture (SEC) and double-electron-capture (DEC) cross sections. We extend the understanding on that system to high energies for which only a single series of data exists. Furthermore, the mechanisms responsible for SEC and DEC processes have been investigated by additional restricted two-active-electron and single-active-electron calculations. The role of electronic correlations in the collisions is also discussed.

International audience; A combination of resonant inelastic x-ray scattering and resonant Auger spectroscopy provides complementary information on the dynamic response of resonantly excited molecules. This is exemplified for CH3I, for which we reconstruct the potential energy surface of the dissociative I 3d−2 double-core-hole state and determine its lifetime. The proposed method holds a strong potential for monitoring the hard x-ray induced electron and nuclear dynamic response of core-excited molecules containing heavy elements, where ab initio calculations of potential energy surfaces and lifetimes remain challenging.

Interatomic Coulombic Decay (ICD) is a general mechanism in which an excited atom can transfer its excess energy to a neighbor which is thus ionized. ICD belongs to the family of Feshbach resonance processes, and, as such, states undergoing ICD are characterized by their energy width. In this work, we investigate the computations of ICD widths using the R-matrix method as implemented in the UKRmol package. Helium dimer is used here as a benchmark system. The results are compared with those obtained with the well established Fano-Algebraic Diagrammatic Construction method. It is shown that the R-matrix method in its present implementation provides accurate total and partial widths if the kinetic energy of the ICD electron is lower than 10 eV. Advantages and limitations of the R-matrix method on the computations of ICD widths are discussed.

Soft X-ray absorption spectra of molecular ions are important data for the modeling and understanding of laboratory and astrophysical plasma. In this work, we present an ab-initio method, based on the Configuration Interaction (CI) approach, for the calculations of energy positions and oscillator strengths of X-ray absorption spectra. Furthermore, we investigate the effects of the choice of the nature and number of spin-orbitals used in the CI expansion on the spectra. The method is applied on three hydride molecular ions, namely CH+, OH+ and SiH+. However, the approach proposed here is general and may thus be applied to any kind of molecular ions.

Inner-valence ionized states of atoms and molecules live shorter if these species are embedded in an environment due to the possibility for ultrafast deexcitation known as interatomic Coulombic decay (ICD). In this Letter we show that the lifetime of these ICD active states decreases further when a bridge atom is in proximity to the two interacting monomers. This novel mechanism, termed superexchange ICD, is an electronic correlation effect driven by the efficient energy transfer via virtual states of the bridge atom. The superexchange ICD is discussed in detail on the example of the NeHeNe trimer. We demonstrate that the decay width of the ${\mathrm{Ne}}^{+}(2{s}^{\ensuremath{-}1})\text{ }\text{ }{^{2}\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ resonance increases 6 times in the presence of the He atom at a distance of 4 \AA{} between the two Ne atoms. Using a simple model, we provide a qualitative explanation of the superexchange ICD and we derive analytical expressions for the dependence of the decay width on the distance between the neon atoms.

International audience; Tuning hard x-ray excitation energy along Cl 1s→σ∗ resonance in gaseous HCl allows manipulating molecular fragmentation in the course of the induced multistep ultrafast dissociation. The observations are supported by theoretical modeling, which shows a strong interplay between the topology of the potential energy curves, involved in the Auger cascades, and the so-called core-hole clock, which determines the time spent by the system in the very first step. The asymmetric profile of the fragmentation ratios reflects different dynamics of nuclear wave packets dependent on the photon energy.

Spectral probes for the ${\mathrm{CH}}^{+}, {\mathrm{OH}}^{+}$, and ${\mathrm{SiH}}^{+}$ hydride molecular ions that play key roles in astrophysics and plasma processes are presented. The merged-beam technique at the SOLEIL synchrotron was used to record the photoionization (ion yield) spectra of ${\mathrm{CH}}^{+}, {\mathrm{OH}}^{+}$, and ${\mathrm{SiH}}^{+}$ and that of their parent atomic ions, in the $K$-shell and $L$-shell regions, respectively. Energies and oscillator strengths for the $K\ensuremath{\alpha}$ (${\mathrm{CH}}^{+}$ and ${\mathrm{OH}}^{+}$) and $L\ensuremath{\alpha}$ (${\mathrm{SiH}}^{+}$) transitions were determined from the spectra. Ab initio calculations interpret the experimental data in terms of contributions from ground and excited valence electronic states.

Creation of deep core holes with very short (τ≤1 fs) lifetimes triggers a chain of relaxation events leading to extensive nuclear dynamics on a few-femtosecond time scale. Here we demonstrate a general multistep ultrafast dissociation on an example of HCl following Cl 1s→σ^{*} excitation. Intermediate states with one or multiple holes in the shallower core electron shells are generated in the course of the decay cascades. The repulsive character and large gradients of the potential energy surfaces of these intermediates enable ultrafast fragmentation after the absorption of a hard x-ray photon.

International audience; We have studied the excitation of atomic hydrogen by fully-stripped lithium ion impact in the intermediate energy range using a new and efficient implementation of the two-center atomic orbital approach with Gaussian-type orbitals. Partial and state-selective cross sections have been obtained for excitation up to H(6 h). A careful investigation of the convergence of the results with respect to the basis set has been performed which allows us to estimate the accuracy of the cross sections. Furthermore, our calculations provide an explanation for the discrepancies between previous calculations on this collision system.

This study focuses on the two C1s-to-LUMO (lowest unoccupied molecular orbital) excitations of the iso-dichloroethylene (H2C=CCl2) and the subsequent Auger decay. We investigate the effect of the t ...

Since their theoretical prediction in 1997, interatomic (intermolecular) Coulombic decay (ICD) and related processes have been in the focus of intensive theoretical and experimental research. The spectacular progress in this direction has been stimulated both by the fundamental importance of the discovered electronic decay phenomena and by the exciting possibility of their practical application, for example in spectroscopy of interfaces. Interatomic decay phenomena take place in inner-shell-ionized clusters due to electronic correlation between two or more cluster constituents. These processes lead to the decay of inner-shell vacancies by electron emission and often also to disintegration of the resulting multiply ionized cluster. Here we review the recent progress in the study of interatomic decay phenomena in singly and multiply ionized clusters.

We present the realization of a Young-type double-slit experiment, in which single electrons, scattering on two protons, produce interference pattern. The electrons are produced by a Auger effect, following double capture process in low He 2+ + H 2 collisions. Well-defined oscillations are visible in the angular distribution of the electrons emitted towards the receding protons. The presence of these oscillations is a clear demonstration that an electron interferes with itself. We also discuss the dependence of the interference pattern with interference parameters, such as the electron wavelength as well as the distance between the protons when the electron is ejected.