Hyperspectral imaging (HSI) has become widely used in cultural heritage (CH). This very efficient method for artwork analysis is connected with the generation of large amounts of spectral data. The effective processing of such heavy spectral datasets remains an active research area. Along with the firmly established statistical and multivariate analysis methods, neural networks (NNs) represent a promising alternative in the field of CH. Over the last five years, the application of NNs for pigment identification and classification based on HSI datasets has drastically expanded due to the flexibility of the types of data they can process, and their superior ability to extract structures contained in the raw spectral data. This review provides an exhaustive analysis of the literature related to NNs applied for HSI data in the CH field. We outline the existing data processing workflows and propose a comprehensive comparison of the applications and limitations of the various input dataset preparation methods and NN architectures. By leveraging NN strategies in CH, the paper contributes to a wider and more systematic application of this novel data analysis method.

Interatomic Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process wherein an excited atom transfers its excess energy to a neighboring species leading to the ionization of the latter. In helium clusters, ICD can take place, for example, after simultaneous ionization and excitation of one helium atom within the cluster. After ICD, two helium ions are created and the system undergoes a Coulomb explosion. In this work, we investigate theoretically ICD in small helium clusters containing between two and seven atoms and compare our findings to two sets of coincidence measurements on clusters of different mean sizes. We provide a prediction on the lifetime of the excited dimer and show that ICD is faster for larger clusters. This is due to (i) the increased number of neighboring atoms (and therefore the number of decay channels) and (ii) the substantial decrease of the interatomic distances. In order to provide more details on the decay dynamics, we report on the kinetic-energy distributions of the helium ions. These distributions clearly show that the ions may undergo charge exchange with the neutral atoms within the cluster, such process is known as frustrated Coulomb explosion. The probability for these charge-exchange processes increases with the size of the clusters and is reflected in our calculated and measured kinetic-energy distributions. These distributions are therefore characteristics of the size distribution of small helium clusters.

In our work we study the interatomic Coulombic decay (ICD) in the NeKr dimer, where after 2s ionization of the Ne, the system relaxes and the excess energy is utilized to ionize the Kr. The temporal evolution of the ICD process in NeKr has been recently measured and theoretically explained by Trinter et al. [Chem. Sci. 13, 1789 (2022)]. Here we focus on two other main goals. The first goal regards the found interference effects in the photoemission (PE) spectrum, which are unusual phenomena in noble gas dimers. They result from the coherently excited vibrational energy levels and substantial dependence of the large ICD decay width on the internuclear distance. The PE spectrum reacts sensitively to changes in the potential energy curve (PEC) of the 2s ionized state, and we modified the available ab initio PEC in such a way that satisfactory agreement between theoretical and experimental data is achieved. The impact of isotope masses on the PE spectrum is briefly discussed and used in the determination of the PEC. Our second main goal concerns the nuclear motion during the ICD process. Here we investigate the impact of different vibrationally excited states of the electronic ground state on the ICD-electron and kinetic energy release (KER) spectra. To transfer our vibrationally selected ICD model to a realizable experiment, we also present the impact of temperature on the ICD-electron spectrum. Finally, our studies are complemented by comparing the directly computed KER spectrum to the mirror image of the ICD-electron spectrum, which coincide under certain conditions.

Radiative charge transfer (RCT) from cationic to neutral atoms is a fundamental and frequently occurring process in ion-neutral collisions and in van der Waals clusters. In contrast to collisions with only two collision partners, RCT in clusters is more involved as it may depend on the cluster size, the cluster stoichiometry, and the local arrangement of atoms in the cluster. Here, we present a systematic investigation of an RCT photon spectrum as a function of cluster size, stoichometry, and local atom arrangement. For this purpose, we utilize RCT in heterogeneous NeKr and NeXe clusters after Ne 2p photoionization. Our results confirm that Ne dimer ions form prior to RCT if enough Ne is available and we observe that different features dominate the photon spectrum depending on the cluster production parameters. Additionally, we find that the lifetime of the radiative decay is sensitive to the cluster stoichiometry, which we explain and interpret by theoretical calculations on the RCT decay width for different local geometric structures of the involved atoms. We conclude that RCT properties such as photon spectrum and lifetime in turn exhibit information on the mean size and stoichiometry of a cluster jet and the arrangement of the involved atoms.

We report on an original full ab initio quantum molecular approach designed to simulate Cu 2p X-ray photoelectron spectra. The description includes electronic relaxation/correlation and spin-orbit coupling effects and is implemented within nonorthogonal sets of molecular orbitals for the initial and final states. The underlying mechanism structuring the Cu 2p photoelectron spectra is clarified thanks to a correlation diagram applied to the CuO4C6H6 paradigm. This diagram illustrates how the energy drop of the Cu 3d levels following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbital (H-SOMO) from dominant metal to dominant ligand character. It also reveals how the repositioning of the Cu 3d levels induces the formation of new bonding and antibonding orbitals from which shakeup mechanisms toward the relaxed H-SOMO operate. The specific nature, ligand → ligand and metal → ligand, of these excitations building the satellite lines is exposed. Our approach finally applied to the real Cu(acac)2 system clearly demonstrates how a definite interpretation of the XPS spectra can be obtained when a correct evaluation of binding energies, intensities, and relative widths of the spectral lines is achieved.

We investigate interatomic Coulombic decay in NeKr dimers after neon inner-valence photoionization [Ne+(2s-1)] using a synchrotron light source. We measure with high energy resolution the two singly charged ions of the Coulomb-exploding dimer dication and the photoelectron in coincidence. By carefully tracing the post-collision interaction between the photoelectron and the emitted ICD electron we are able to probe the temporal evolution of the state as it decays. Although the ionizing light pulses are 80 picoseconds long, we determine the lifetime of the intermediate dimer cation state and visualize the contraction of the nuclear structure on the femtosecond time scale.

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.

We investigate the accuracy of the Outer-Valence Green’s Function (OVGF) electron propagator approach for the computation of vertical ionization energies (IEs) of several hydrogen-bonded dimers. The vertical IEs were computed with 10 different basis sets (i.e. Pople, correlation-consistent and augmented correlation-consistent) and compared with references obtained at the extrapolated equation-of-motion coupled-cluster singles and doubles level. Our study shows that the OVGF method predicts accurately the IEs of hydrogen-bonded clusters. Furthermore, the choice of an optimal basis set is analyzed: our results suggest that the aug-cc-pVDZ basis set provides the best compromise between accuracy and computational costs. The benchmark study provided in this work represents an essential step to investigate molecular ionization, and thus molecular reduction–oxidation properties, in the liquid phase.

Hyperspectral reflectance imaging in the short-wave infrared range (SWIR, "extended NIR", ca. 1000 to 2500 nm) has proven to provide enhanced characterization of paint materials. However, the interpretation of the results remains challenging due to the intrinsic complexity of the SWIR spectra, presenting both broad and narrow absorption features with possible overlaps. To cope with the high dimensionality and spectral complexity of such datasets acquired in the SWIR domain, one data treatment approach is tested, inspired by innovative development in the cultural heritage field: the use of a pigment spectral database (extracted from model and historical samples) combined with a deep neural network (DNN). This approach allows for multi-label pigment classification within each pixel of the data cube. Conventional Spectral Angle Mapping and DNN results obtained on both pigment reference samples and a Buddhist painting (thangka) are discussed.

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 for 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 component 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.

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 neighboring 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 change in the characteristics of the bridge quantum dot, particularly the depth and size. On the other hand, its relative position with respect to the donor and acceptor dots is found to foster ICD when it is located in between the two dots. Our findings provide new insights for the development of ICD-based nanostructure technologies, particularly for rational design of three coupled quantum dots.

Interatomic-Coulombic-Decay (ICD) processes play an important role in the interaction of X-rays with biological systems. Here, we present the first successful experimental demonstration that enables and precisely times the outcome of an ICD process in an argon dimer, utilizing ultrafast XUV and IR radiation.

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 He-H-2 dimer, we show that simultaneous ionization and excitation of the helium atom induces H-2 dissociation with a high probability. The excited He+ ion transfers its excess energy via interatomic Coulombic decay (ICD) or electron transfer mediated decay (ETMD) to 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 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 the 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 followup work, we analyze the partial ICD widths in the 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.

We report on the experimental observation of interatomic Coulombic decay (ICD) in pure He-4 nanoclusters of mean sizes between N similar to 5000 and 30 000 and the subsequent scattering of energetic He+ fragments inside the neutral cluster by using cold target recoil ion momentum spectroscopy. ICD is induced in He clusters by using vacuum ultraviolet light of h nu = 67 eV from the BESSY II synchrotron. The electronic decay creates two neighboring ions in the cluster at a well-defined distance. The measured fragment energies and angular correlations show that a main energy loss mechanism of these ions inside the cluster is a single hard binary collision with one atom of the cluster.

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_{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.

Charge transfer (CT) at avoided crossings of excited ionized states of argon dimers is observed using a two-color pump-probe experiment at the free-electron laser in Hamburg (FLASH). The process is initiated by the absorption of three 27-eV-photons from the pump pulse, which leads to the population of Ar2+*-Ar states. Due to nonadiabatic coupling between these one-site doubly ionized states and two-site doubly ionized states of the type Ar+*-Ar+, CT can take place leading to the population of the latter states. The onset of this process is probed by a delayed infrared (800 nm) laser pulse. The latter ionizes the dimers populating repulsive Ar2+ -Ar+ states, which then undergo a Coulomb explosion. From the delay-dependent yields of the obtained Ar2+ and Ar+ ions, the lifetime of the charge-transfer process is extracted. The obtained experimental value of (531 ± 136) fs agrees well with the theoretical value computed from Landau-Zener probabilities.

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 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.

Almost ten years ago, energetic neutral hydrogen atoms were detected after a strong-field double ionization of H_2. 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'.