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Hydrogen bonds are ubiquitous in nature and of fundamental importance to the chemical and physical properties of molecular systems in the condensed phase. Nevertheless, our understanding of the structural and dynamical properties of hydrogen-bonded complexes in particular in electronic excited states remains very incomplete. Here, by using formic acid (FA) dimer as a prototype of DNA base pair, we investigate the ultrafast decay process initiated by removal of an electron from the inner-valence shell of the molecule upon electron-beam irradiation. Through fragment-ion and electron coincident momentum measurements and

Photo-ionization induced ultrafast electron dynamics is considered as a precursor to the slower nuclear dynamics associated with molecular dissociation. Here, using ab initio multielectron wave-packet propagation method, we study the overall many-electron dynamics, triggered by the ionization of outer-valence orbitals of different tautomers of a prototype molecule with more than one symmetry element. From the time evolution of the initially created averaged hole density of each system, we identify distinctly different charge dynamics response in the tautomers. We observe that keto form shows charge migration direction away from the nitrogen bonded with tautomeric hydrogen, while in enol - away from oxygen bonded to tautomeric hydrogen. Additionally, the dynamics following ionization of molecular orbitals of different symmetry reveal that a' orbitals show fast and highly delocalized charge in comparison to a" symmetry. These observations indicate why different tautomers respond differently to an XUV ionization, and might explain the subsequent different fragmentation pathways. An experimental schematics allowing detection and reconstruction of such charge dynamics is also proposed. Although the present study uses a simple, prototypical bio-relevant molecule, it reveals the explicit role of molecular symmetry and tautomerism in the ionization-triggered charge migration that controls many ultrafast physical, chemical, and biological processes, making tautomeric forms a promising tool of molecular design for desired charge migration.

After ionization of an inner-valence electron of molecules, the resulting cation-radicals store substantial internal energy which, if sufficient, can trigger ejection of an additional electron in an Auger decay usually followed by molecule fragmentation. In the environment, intermolecular Coulombic decay (ICD) and electron-transfer mediated decay (ETMD) are also operative, resulting in one or two electrons being ejected from a neighbor, thus preventing the fragmentation of the initially ionized molecule. These relaxation processes are investigated theoretically for prototypical heterocycle-water complexes of imidazole, pyrrole, and pyridine. It is found that the hydrogen-bonding site of the water molecule critically influences the nature and energetics of the electronic states involved, opening or closing certain relaxation processes of the inner-valence ionized system. Our results indicate that the relaxation mechanisms of biologically relevant systems with inner-valence vacancies on their carbon atoms can strongly depend on the presence of the electron-density donating or accepting neighbor, either water or another biomolecule.

The creation and dynamical fate of a coherent superposition of electronic states generated in a polyatomic molecule by broadband ionization with extreme ultraviolet pulses is studied using the multiconfiguration time-dependent Hartree method together with an ionization continuum model Hamiltonian. The electronic coherence between the hole states usually lasts until the nuclear dynamics leads to decoherence. A key goal of attosecond science is to control the electronic motion and design laser control schemes to retain this coherence for longer timescales. Here, we investigate this possibility using time-delayed pulses and show how this opens up the prospect of coherent control of charge migration phenomenon.

We present dynamics calculations showing how electron-correlation-driven charge migration occurring in the correlation band of ionized molecules can lead to a redistribution of the charge increasing the stability of the system. These calculations offer an interpretation of recent experimental results obtained for adenine. We discuss the implications of the mechanism for the development of attochemistry and how it can be understood in the context of the ultrafast, non-adiabatic relaxation taking place in highly-excited molecular cations.

Core-excited and core-ionized states created by absorption of X-ray photons carry an enormous energy of hundreds to thousands electronvolts. In the gas phase this energy is dissipated primarily in the Auger decay process or Auger decay cascades, whereby one or several electrons are emitted into the continuum producing multiply charged ions. Two interatomic electronic decay processes, interatomic Coulombic decay (ICD) and electron-transfer mediated decay (ETMD), ensure the efficient dissipation of energy beyond the Auger decay should X-ray absorption take place in a chemical medium characterized by weak van der Waals or hydrogen bonds. Numerous experiments and theoretical results have demonstrated that the Auger–interatomic decay cascades represent a common de-excitation mechanism of core vacancies in medium, whose length and complexity increase with increasing photon energy. Such cascades offer a means for very fast dissipation of the energy which is deposited by the photon into the system. They are also responsible for massive radiation damage to the molecules around the photoabsorption site. Surprisingly, ICD cascades remain a potent mechanism of energy dissipation also when the energy is invested in a cluster by a laser not as a single high energy photon but as multiple low energy ones. The recent research of the electronic decay in chemical media presented in this chapter strives to clarify the physical and chemical consequences of weakly subjecting bound clusters to the highly energetic light provided by modern light sources.

Modern ultrashort X-ray/XUV (extreme ultraviolet) sources provide unique opportunities to investigate the primary reactions of matter upon energetic excitation. Understanding these processes in molecules on ultrafast timescales is required to improve bespoke high-energy radiation detectors, nanomedicine schemes or to study the molecular composition of interstellar media. However, current experiments struggle to provide a general framework because of the uniqueness and complexity of each system. Here we show the universal role of correlation bands—features created by electron correlation. This is done by studying ultrafast energy relaxation of size-scalable two-dimensional molecules following ionization by an ultrashort XUV pulse. We observed long lifetimes that nonlinearly increase with the number of valence electrons. A general law based on solid-like electron–phonon scattering is proposed, which explains both our results and previously reported measurements. This offers new opportunities in attosecond science and high-energy photophysics. The size-dependent lifetimes observed in the ultrafast molecular relaxation dynamics of an entire class of polycyclic aromatic hydrocarbons can be explained by correlation bands and electron–phonon scattering, reminiscent of solid-state systems.

Tracing ultrafast processes induced by interaction of light with matter is often very challenging. In molecular systems, the initially created electronic coherence becomes damped by the slow nuclear rearrangement on a femtosecond timescale which makes real-time observations of electron dynamics in molecules particularly difficult. In this work, we report an extension of the theory underlying the attosecond transient absorption spectroscopy (ATAS) for the case of molecules, including a full account for the coupled electron-nuclear dynamics in the initially created wave packet, and apply it to probe the oscillations of the positive charge created after outer-valence ionization of the propiolic acid molecule. By taking advantage of element-specific core-to-valence transitions induced by x-ray radiation, we show that the resolution of ATAS makes it possible to trace the dynamics of electron density with atomic spatial resolution.

The removal of electrons located in the core shells of molecules creates transient states that live between a few femtoseconds to attoseconds. Owing to these short lifetimes, time-resolved studies of these states are challenging and complex molecular dynamics driven solely by electronic correlation are difficult to observe. Here, we obtain few-femtosecond core-excited state lifetimes of iodine monochloride by using attosecond transient absorption on iodine 4d−16p transitions around 55 eV. Core-level ligand field splitting allows direct access of excited states aligned along and perpendicular to the ICl molecular axis. Lifetimes of 3.5 ± 0.4 fs and 4.3 ± 0.4 fs are obtained for core-hole states parallel to the bond and 6.5 ± 0.6 fs and 6.9 ± 0.6 fs for perpendicular states, while nuclear motion is essentially frozen on this timescale. Theory shows that the dramatic decrease of lifetime for core-vacancies parallel to the covalent bond is a manifestation of non-local interactions with the neighboring Cl atom of ICl., Here the authors report a study measuring lifetimes of core-hole states of ICl molecule using attosecond transient absorption spectroscopy. They find that lifetimes depend on the alignment of the orbital relative to the molecular axis.

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.

This chapter gives a short introduction to the theoretical foundations of molecular quantum dynamics. The basic concepts for describing quantum mechanically the correlated motion of electrons and nuclei are discussed. The emphasis is put on presenting an ab initio methodology allowing a description of the ultrafast, pure electron dynamics triggered by ionization of a molecule—a process known as charge migration. Analysis of the basic mechanisms of the correlation-driven charge migration is provided together with some explicit examples. Finally, the concept of manipulating the chemical reactivity of a molecule by controlling the initially created, pure electronic coherences by ultrashort laser pulses, or the paradigm of the ”attochemistry”, is presented and discussed.

The increasing availability of X-ray free-electron lasers (XFELs) has catalyzed the development of single-object structural determination and of structural dynamics tracking in real-time. Disentangling the molecular-level reactions triggered by the interaction with an XFEL pulse is a fundamental step towards developing such applications. Here we report real-time observations of XFEL-induced electronic decay via short-lived transient electronic states in the diiodomethane molecule, using a femtosecond near-infrared probe laser. We determine the lifetimes of the transient states populated during the XFEL-induced Auger cascades and find that multiply charged iodine ions are issued from short-lived (∼20 fs) transient states, whereas the singly charged ones originate from significantly longer-lived states (∼100 fs). We identify the mechanisms behind these different time scales: contrary to the short-lived transient states which relax by molecular Auger decay, the long-lived ones decay by an interatomic Coulombic decay between two iodine atoms, during the molecular fragmentation., X線照射で始まる超高速反応の観測に成功 --レントゲンによるX線の発見から120年で初--. 京都大学プレスリリース. 2019-05-28.

International audience; Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

Using direct search algorithms for solving the quantum optimization problem, we demonstrate on model systems that with specifically tailored Gaussian-form laser pulses one can achieve a very good control over the dynamics in complicated quantum systems. We show that by manipulating a very limited number of laser-pulse parameters, one is able to control the charge migration process in molecules. In particular, by combining two identical Gaussian laser pulses with an appropriate delay between them, one can stop the pure electronic, few-femtosecond oscillation of the charge, redistributing it along the molecule of propiolic acid.

Using the multielectron wave-packet propagation method the electronic decay of O2s vacancy in fluorinated cumulenones, OC n F 2 , containing a chain of up to five carbons is traced in time and space. It is shown that in all studied cases this state decays non-locally by emitting an electron from the remote fluorines. Even in the pentatetraenone case, where the oxygen and the flourines are more than 7 A apart, this non-local decay is extremely efficient, with a time constant of about 5 fs. The process can be viewed as an ultrafast energy transfer through the carbon chain and thus our systematic study allows to shed some light on the dependence of the time scale of the electron-correlation driven energy transfer through a medium.

The recent availability of intense and ultrashort extreme ultraviolet sources opens the possibility to investigate ultrafast electronic relaxation processes in matter in an unprecedented regime. In this work we report on the observation of two-photon excitation of interatomic Coulombic decay (ICD) in neon dimers using the tunable intense pulses delivered by the free electron laser FERMI@Elettra. The unique characteristics of FERMI (narrow bandwidth, spectral stability, and tunability) allow one to resonantly excite specific ionization pathways and to observe a clear signature of the ICD mechanism in the ratio of the ion yield created by Coulomb explosion. The present experimental results are explained by \emph{ab initio} electronic structure and nuclear dynamics calculations.

The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born−Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts., The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Here, the authors employ ultrafast spectroscopic methods to explore the dynamics of highly excited organic molecules, revealing many-body effects and hints of coherent vibronic dynamics which persist despite their molecular complexity.

Ionization of molecules very often populates several cationic states launching pure electron dynamics that appear as ultrafast migration of the hole charge throughout the system. A crucial question in the emerging field of attochemistry is whether these pure electronic coherences last long enough to allow for their efficient observation and eventual manipulation with ultrashort laser pulses. We report a full-dimensional quantum calculation of concerted electron-nuclear dynamics initiated by outer-valence ionization of propiolic acid molecule, showing that the charge will oscillate between the carbon triple bond and the carbonyl oxygen for more than 10 fs before getting trapped by the nuclear motion. This time is enough for the charge migration to be observed and controlled. We argue that the molecule is very suitable for experimental studies.

X-ray free-electron lasers (XFELs) made available a new regime of x-ray intensities, revolutionizing the ultrafast structure determination and laying the foundations of the novel field of nonlinear x-ray optics. Although earlier studies revealed nanoplasma formation when an XFEL pulse interacts with any nanometer-scale matter, the formation process itself has never been decrypted and its timescale was unknown. Here we show that time-resolved ion yield measurements combined with a near-infrared laser probe reveal a surprisingly ultrafast population (∼ 12 fs), followed by a slower depopulation (∼ 250 fs) of highly excited states of atomic fragments generated in the process of XFEL-induced nanoplasma formation. Inelastic scattering of Auger electrons and interatomic Coulombic decay are suggested as the mechanisms populating and depopulating, respectively, these excited states. The observed response occurs within the typical x-ray pulse durations and affects x-ray scattering, thus providing key information on the foundations of x-ray imaging with XFELs., プラズマ誕生の瞬間を観測 --X線自由電子レーザー照射によるプラズマ生成機構を解明--. 京都大学プレスリリース. 2018-09-20.

Clusters and nanoparticles have been widely investigated to determine how plasmonic near fields influence the strong-field induced energetic electron emission from finite systems. We focus on the contrary, i.e., the slow electrons, and discuss a hitherto unidentified low-energy structure (LES) in the photoemission spectra of rare gas clusters in intense near-infrared laser pulses. For Ar and Kr clusters we find, besides field-driven fast electrons, a robust and nearly isotropic emission of electrons with $l4\text{ }\text{ }\mathrm{eV}$ kinetic energies that dominates the total yield. Molecular dynamics simulations reveal a correlated few-body decay process involving quasifree electrons and multiply excited ions in the nonequilibrium nanoplasma that results in a dominant LES feature. Our results indicate that the LES emission occurs after significant nanoplasma expansion, and that it is a generic phenomenon in intense laser nanoparticle interactions, which is likely to influence the formation of highly charged ions.

The ultrafast transfer of excitation energy from one atom to its neighbor is observed in singly charged argon dimers in a time-resolved extreme ultraviolet (XUV)-pump IR-probe experiment. In the pump step, bound 3s-hole states in the dimer are populated by single XUV-photon ionization. The excitation-energy transfer at avoided crossings of the potential-energy curves leads to dissociation of the dimer, which is experimentally observed by further ionization with a time-delayed IR-probe pulse. From the measured pump-probe delay-dependent kinetic-energy release of coincident Ar+ + Ar+ ions, we conclude that the transfer of energy occurs on a time scale of about 800fs . This mechanism represents a fast relaxation process below the energy threshold for interatomic Coulombic decay.

Synopsis We found that the XUV excited caffeine undergoes nondadabatic cascade via more than 100 shake-up states within the time constant of 40 fs by using femtosecond XUV-NIR pump-probe photo-ion spectroscopy and the SAC-CI level of ab-initio many-body theory. This nonadiabatic cascade results in the photofragmentation to specific large fragments. Furether ionization by NIR probe during the cascade increases the small fragments CH3 +, CO+ etc. This results show that both vibronic coupling and electron correlation are the keys for the femtochemistry in the highly-correlated electronic excited states.

Using electron spectroscopy, we investigated the nanoplasma formation process generated in xenon clusters by intense soft x-ray free electron laser (FEL) pulses. We found clear FEL intensity dependence of electron spectra. Multistep ionization and subsequent ionization frustration features are evident for the low FEL-intensity region, and the thermal electron emission emerges at the high FEL intensity. The present FEL intensity dependence of the electron spectra is well addressed by the frustration parameter introduced by Arbeiter and Fennel [New J. Phys. 13, 053022 (2011)].

Due to many-body effects an ultrafast removal of an electron from a molecule can trigger electron dynamics in which the created hole charge migrates throughout the system on a few-femtoseconds time scale. Here we report ab initio calculations of the positive-charge migration following inner-valence ionization of the C-terminally methylamidated dipeptide Gly–Gly–NH–CH3. To investigate the influence of the molecular symmetry on the process, two different conformations of the system are studied. Our results show that in both conformers the charge initially localized on the methylamine end of the system migrates to the remote glycine in only 5–6 fs jumping thereby over an entire amino acid. Our computations also show that the symmetry of the system facilitates the process – a larger fraction of the charge migrates over a larger distance if the molecule is symmetric. Ionization spectra of both studied conformers are also reported.

The hitherto unexplored two-photon doubly excited states [Ne∗(2p-13s)]2 were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne2+ ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

We investigate the interatomic Coulombic decay (ICD) in argon dimers induced by electron-impact ionization (${E}_{0}=90\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$) using a multiparticle coincidence experiment in which the momentum vectors and, consequently, the kinetic energies for electrons and fragment ions are determined. The signature of the ICD process is obtained from a correlation map between ejected electron energy and kinetic energy release (KER) for ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$ fragment ions where low-energy ICD electrons can be identified. Furthermore, two types of ICD processes, termed fast and slow interatomic decay, are separated by the ICD initial-state energies and projectile energy losses. The dependence of the energies of emitted low-energy ICD electrons on the initial-state energy is studied. ICD electron energy spectra and KER spectra are obtained separately for fast and slow decay processes where the KER spectra for the slow decay channel are strongly influenced by nuclear motion. The KER and ICD electron energy spectra are well reproduced by ab initio calculations.

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

After the ionization of a valence electron, the created hole can migrate ultrafast from one end of the molecule to another. Because of the advent of attosecond pulse techniques, the measuring and understanding of charge migration has become a central topic in attosecond science. Here, we pose the hitherto unconsidered question whether ionizing a core electron will also lead to charge migration. It is found that the created hole in the core stays put, but in response to this hole interesting electron dynamics takes place which can lead to intense charge migration in the valence shell. This migration is typically faster than that after the ionization of a valence electron and transpires on a shorter time scale than the natural decay of the core hole by the Auger process, making the subject very challenging to attosecond science.

Based on elaborated ab initio calculations we first study the ionization spectra of the free-base porphyrin (H2-P) and Mg(II) porphyrin (Mg-P). Then we investigate the ultrafast electron dynamics following outer-valence ionization of these systems which constitutes the highlight of this paper. It is shown that the electron correlation effects are very strong in these systems giving rise to pronounced shake-down satellites in the outer-valence part of the ionization spectra. We show that due to these strong correlation effects the removal of an electron from the 3 b 1 g orbital of H2-P and Mg-P leads to an ultrafast reorganization of the electronic cloud. After being initially localized on the B and D pyrrole rings, the hole charge created upon ionization spreads throughout the molecule in only few femtoseconds. In both systems the electron dynamics triggered by the ionization represent alternating ultrafast delocalizations and localizations of the charge. The results may also give a hint on the nuclear dynamics in the free-base and Mg(II) porphyrin radical cations that will follow the ultrafast charge migration.

Intermolecular Coulombic decay (ICD) is a very fast and efficient relaxation pathway of ionized and excited molecules in environment. The ICD and related phenomena initiated by inner-valence ionization are explored for H(2)O···HCHO, H(2)O···H(2)CNH, H(2)O···NH(3), NH(3)···H(2)O, H(2)O···H(2)S, H(2)S···H(2)O, and H(2)O···H(2)O (p-donor···p-acceptor). This set of small hydrogen-bonded systems contains seven types of hydrogen bonding, which are typical for biochemistry, and thus its investigation provides insight into the processes that can take place in living tissues. In particular, an estimate of the ICD in biosystems interacting with water (their usual medium) is made. This decay mode is expected to be a source of low-energy electrons proven to be of extreme genotoxic nature. For the purpose of our study, we have used high-precision ab initio methods in optimizing the geometries and computing the single- and double-ionization spectra of formaldehyde-, formaldimine-, ammonia-, hydrogen sulfide-, and water-water complexes. The energy range of the emitted ICD electrons, as well as the kinetic energy of the dissociating ions produced by ICD, is also reported.

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.

Electronic many-body effects alone can be responsible for the migration of a positive charge created upon ionization in molecular systems. Here, we report an ultrafast charge migration taking place after valence ionization of the molecule 4-methylphenol. The results obtained by a fully ab initio methodology show that the positive charge localized initially on the methyl group can migrate to the hydroxyl group in less than 2 fs jumping over the whole aromatic ring.

The applicability of the reducing-difference procedure for the interpretation of the conventional IR-spectroscopy as successful scientific technique for the analysis of ancient and modern resins has been demonstrated. The new temperature tool for modeling of the ancient resin samples has also been shown. The experimental infrared data are supported by the hydride approach of HPLC–MS–MS with ES-ionisation.

Electron correlation can be the driving force for ultrafast charge migration. Using ab initio calculations this is demonstrated in the present paper for the first time as a consequence of ionizing the outer-valence shell. The example studied is 2-phenylethyl-N,N-dimethylamine (PENNA) on which exciting pump–probe measurements have been recently carried out.

In high-intensity laser light, matter can be ionized by direct multiphoton absorption even at photon energies below the ionization threshold. However on tuning the laser to the lowest resonant transition, the system becomes multiply excited, and more efficient, indirect ionization pathways become operative. These mechanisms are known as interatomic Coulombic decay (ICD), where one of the species de-excites to its ground state, transferring its energy to ionize another excited species. Here we show that on tuning to a higher resonant transition, a previously unknown type of interatomic Coulombic decay, intra-Rydberg ICD occurs. In it, de-excitation of an atom to a close-lying Rydberg state leads to electron emission from another neighbouring Rydberg atom. Moreover, systems multiply excited to higher Rydberg states will decay by a cascade of such processes, producing even more ions. The intra-Rydberg ICD and cascades are expected to be ubiquitous in weakly-bound systems exposed to high-intensity resonant radiation., 原子の集団が数珠つなぎに電子を放出する! : 極紫外自由電子レーザーで誘起される新現象解明. 京都大学プレスリリース. 2016-12-12.

A surprisingly dominant contribution of slow electrons is observed following NIR strong-field ionization of clusters. This is consistent with highly efficient intra-Rydberg correlated electronic decay processes, from which the emission of low-energy electrons is expected.