Interatomic and Intermolecular Coulombic Decay

Abstract

Interatomic or intermolecular Coulombic decay (ICD) is a nonlocal electronic decay mechanism occurring in weakly bound matter. In an ICD process, energy released by electronic relaxation of an excited atom or molecule leads to ionization of a neighboring one via Coulombic electron interactions. ICD has been predicted theoretically in the mid nineties of the last century, and its existence has been confirmed experimentally approximately ten years later. Since then, a number of fundamental and applied aspects have been studied in this quickly growing field of research. This review provides an introduction to ICD and draws the connection to related energy transfer and ionization processes. The theoretical approaches for the description of ICD as well as the experimental techniques developed and employed for its investigation are described. The existing body of literature on experimental and theoretical studies of ICD processes in different atomic and molecular systems is reviewed.

Figure 1

Schematic of a prototypical ICD process in a dimer. (a) An inner-valence electron of one of the atoms is removed by means of photoionization. (b) As ICD takes places, an outer-valence electron fills the inner-valence vacancy. The energy gained in this transition is transferred to the neighboring atom, which is ionized as a consequence. (c) The doubly charged dimer fragments in a Coulomb explosion. Reprinted with permission from ref (2). Copyright 2004 APS.

Figure 2

Physical pictures behind the two different contributions to the ICD matrix element (ICD occurring after inner-valence ionization of a neon dimer as an example). (a) Direct term, (b) exchange term. Figure adapted with permission from ref (8). Copyright 2007 APS.

Figure 3

Schematic overview of nonradiative electronic decay processes and their relation. An inner box depicts a subcategory of its surrounding outer box. Abbreviations: Interatomic/Intermolecular Coulombic Decay; Exchange (see Section 1); Three electron-ICD (3e-ICD) (see Section 5.1.8); double-ICD (dICD) (see Section 5.1.8); Electron Transfer Mediated Decay (see Section 2.1); Interatomic/Intermolecular Coulombic Electron Capture (see Section 2.2); Collective AutoIonization (see Section 5.7); Förster Resonant Energy Transfer (see Section 2.5); Dexter (electron exchange) energy transfer (see Section 2.5); (Radiative) Charge Transfer (see Section 2.4).

Figure 4

Scheme of ETMD(2): A vacancy in the left atom is filled by an electron of the right atom of a dimer. The energy that is released by this transition is used to emit an electron from the right atom. Bottom: Energy-level representation of the process, depicting the involved inner-valence (iv) and outer-valence (ov) states. Atom A is neutralized after ETMD(2), and atom B is doubly charged (right). The figure has been taken from ref (12).

Figure 5

Scheme of interatomic Coulombic electron capture (ICEC). An electron is captured from the continuum into the left atom of a dimer. The energy released in the capture process is employed to free an electron from the right atom.

Figure 6

ICD and ICD-like processes after simultaneous excitation of several atoms in a cluster by absorption of multiple photons. See text for details. Reprinted with permission from ref (53). Copyright 2014 APS.

Figure 7

Systems, excitation mechanisms, and detection methods (probes) that were used in the study of ICD. Shaded boxes, connected by thick lines, highlight the elements that were used most frequently.

Figure 8

Experimental signatures of ICD. Left panel: ICD leads to a lifetime broadening of the excited state. This can be observed in the energy domain when the excited state is produced, for example, by photoionization or -excitation. Middle panel: Most obviously, the ICD electron can be observed directly. Right panel: The pair of ions created by ICD undergoes a Coulomb explosion. Detection of the final state ions also provides a strong indication for ICD. Experiments using these three characteristics have all been successfully performed in the past.

Figure 9

Cross-section through an apparatus including a molecular jet source for production of rare gas clusters (“CS”), and an expansion chamber (“EC”), separated from the main vacuum chamber “VC” by a skimmer “SK”. Synchrotron radiation coming out of the drawing plane produces electrons that are collected in an electron spectrometer “ES” consisting of a straight transfer lens, a hemispherical dispersive section and a detector. An electron trajectory is drawn in blue color. Reproduced from ref (251), with the permission of AIP Publishing.

Figure 10

Sketch of an electron/ion spectrometer using the velocity map imaging (VMI) principle. Charged particles produced by the interaction of a photon beam (here from synchrotron radiation SR) with a cluster jet are projected upward by static electric fields produced between the ring-shaped electrodes. Depending on the polarity of the field, either electrons or ions can be projected.

Figure 11

Magnetic bottle time-of-flight spectrometer. (a) Magnetic field distribution and some electron trajectories, (b) sketch of a cluster electron spectroscopy experiment using a magnetic bottle spectrometer with a magnetic field pointing vertically upward. The central magnetic field line is indicated by red color in both panels.

Figure 12

Sketch of a COLTRIMS reaction microscope. A supersonic gas jet is intersected by, for example, a monochromatized photon beam from a synchrotron radiation source (SR). Charged particles created in the interaction region are guided by electric and magnetic fields toward two position- and time-sensitive detectors. Here, the ions are deflected to the upper detector, while the electrons are imaged on the bottom detector after performing a cyclotron motion (due to the magnetic field) inside the spectrometer volume. By measuring the flight times along the spectrometer (for example, the distance s for electrons) and the positions of impact (x, y) on the detectors, the particles’ momenta can be deduced. The three white rings symbolize electrodes, employed for generating the electric extraction fields.

Figure 13

Illustration of the combination of a photon spectrometer with position-sensitive detection. Light emitted in a source volume enters the spectrometer through an entrance slit and is dispersed by a spherical grating. In reality, the detector is fixed in space and the grating is moved to project different spectral ranges onto the detector. The inset shows an exemplary detector image, from which the emission spectrum is obtained by integrating the signal within a region of interest (dashed rectangle) over the vertical coordinate. Figure and caption reprinted from ref (325), used under CC BY.

Figure 14

Sketch of the basic rotational symmetric mirror system. The rotation axis is indicated by a gray dashed line. In the direction of the detector, a parabolic mirror guides the photons onto it is active area (path 1, blue). On the opposite side of the detector, two spherical mirrors reflect photons toward the detector. The radius of the inner spherical mirror R_i is twice its distance to the interaction volume A and photons are reflected in a collimated beam to the detector. The radius of the outer mirror R_o is equal to the distance to the interaction volume A and therefore reflects the photons back into the interaction volume (path 3, violet). From here on, the path coincides with photons of path 1. Reproduced from ref (326) with the permission of AIP publishing. The caption has been taken from ref (326).

Figure 15

Valence electron spectrum of a NeKr cluster jet, ionized at a photon energy of 23 eV. (a) The total electron spectrum shows three distinct features attributed to electrons from Kr and Ne atoms and clusters. In a high-resolution spectrum of the Kr feature (see inset), two 4p fine structure components from Kr atoms and the corresponding cluster signals can be identified. (b) Electron spectrum filtered for true electron-photon coincidences. Reprinted with permission from ref (62). Copyright (2019) American Physical Society. The caption has been taken from ref (62).

Figure 16

(a) Dependence of the shift in electron energy and the decay time. The plot depicts on the y axis the energy a measured electron will have if the decay happens after a certain time (shown on the x axis). The behavior is plotted for different initial photoelectron energies. From bottom to top: 30, 70, 100, 140 meV. (b) Electron energies and kinetic energy releases measured in coincidence. Reprinted with permission from ref (205). Copyright 2013 APS.

Figure 17

Kinetic energy release distribution of the He⁺/He⁺-breakup caused by shakeup induced ICD in He₂ employing two different photon energies for the excitation. Reprinted with permission from ref (187). Copyright 2010 APS.

Figure 18

Coincidence map of kinetic energy release and electron kinetic energy after ionization of neon dimers using photons of hν = 58.8 eV energy. The shakeup states observed in the electron energy spectrum form two sets that differ by their mean internuclear distance at which they decay. States of even parity decay at large, odd parity-states at shorter internuclear distances. Reprinted with permission from ref (8). Copyright 2007 APS.

Figure 19

Hole occupation density of NeAr after Ne(2s) ionization in dependence of time. The initial vacancy (Ne(2s)) is filled while the two other holes (Ne(2p) and Ar(3p)) open during ICD. The blue line corresponds to the emitted ICD electron density (depicted in terms of an electron hole). Reprinted with permission from ref (227). Copyright 2007 APS.

Figure 20

Norm of the intermediate excited state prior to ICD for two different time ranges. Reprinted with permission from ref (205). Copyright 2013 APS.

Figure 21

Ne 2s photoelectron spectrum from <N> = 480 Ne clusters recorded with a magnetic bottle time-of-flight spectrometer at a photon energy of 52 eV. The bulk and surface component cannot be resolved at the chosen spectrometer settings. The black trace shows the energy spectrum of all electrons that were recorded, the blue dashed trace is the same spectrum after a background due to inelastic scattering (red solid trace) has been subtracted. The green dashed trace shows the contributions of those 2s photoelectrons e_ph that were recorded in coincidence with an ICD electron e_ICD. An area comparison of the two dashed traces in the region between vertical bars allows to quantitatively determine the efficiency of ICD of the 2s^–1 state, yielding a value of 0.99(11). See text for details. Reprinted from ref (279), with permission from Elsevier.

Figure 22

Experimental results for the ICD branching ratio α_ICD of the inner-valence (2a₁) ionized state in water clusters of different mean size, recorded as a function of mean cluster size <N> (top axis) or inverse cluster radius, ∼ <N>^–1/3 (bottom axis). Data were recorded at a photon energy of 62 eV. A significant isotope effect in the efficiency was also found (see text). Several models for quantifying the background shown in Figure 21 were used (two sets of symbols). The parameter region compatible with the measurements is highlighted by gray shading. Figure adapted from ref (284), used under CC BY. See original publication for details.

Figure 23

Calculated potential energy curves for singly and doubly ionized states of the water dimer. Energies are shown along the proton transfer coordinate, that is, the O–H distance in the proton donor. Gray shading highlights the parameter range in which ICD is energetically allowed; arrows indicate relaxation via ICD or internal conversion (IC), respectively. Absolute energies were calculated relative to the minimum of the electronic ground state of a water dimer. Figure adapted from ref (284), used under CC BY.

Figure 24

Reprinted in part with permission from ref (192). Copyright 2018 APS. Theoretical and experimental MFADs of sRICD electrons from neon dimers. (a) Predicted distributions for the 5pσ and 5pπ spectator electrons. (b, c) Measured distributions in case the dimer is oriented in parallel or perpendicularly to the polarization direction of the linearly polarized light.

Figure 25

Total decay width of the Ne⁺(2s^–1)HeNe ²Σ⁺_g state as a function of the distance R between the He atom and the center of mass of neon dimer. The distance between the two neon atoms R is fixed to 4 Å. The total decay width of the corresponding state of Ne₂ is shown as a black dotted line. Reprinted with permission from ref (364). Copyright 2017 APS.

Figure 26

Low kinetic-energy electron spectra of Ne clusters excited with photon energies around the Ne 2s threshold. Resonantly enhanced ICD-like features can be identified at two photon energies below the 2s threshold. Moreover, two lines related to inelastic energy loss of 2p photoelectrons by intracluster creation of excitonic states can be seen at all photon energies. The latter occur at fixed binding energy, while the ICD-like features appear at fixed kinetic energy. Spectra in this graph are smoothed binomially over 10 points corresponding to an energy region of 120 meV for better clarity. Reprinted with permission from ref (370). Copyright 2005 AIP.

Figure 27

(a) Photon-induced VUV fluorescence (λ_fl < 130 nm) from an atomic Ar jet versus excitation energy. The absolute cross-section scale was calibrated to the data of ref (381) at 29.4 eV. (b) Analogous data for a partially condensed jet with a mean cluster size of <N> = 36. The right-hand side ordinate is valid for the shaded region and corresponds to the 3s-photoionization cross-section of isolated atoms ((a)). The left-hand side ordinate (white region) indicates the VUV fluorescence emission cross-sections for clusters with the given mean size. Figure and caption reprinted from ref (329), with permission from IOP publishing.

Figure 28

Electron spectra of neon clusters with an average size of <N> = 70 atoms and neon monomers (i.e., uncondensed beam). As a reference, the electron spectrum recorded at hν = 40 eV is repeated as a shaded area in every panel. Spectra recorded above the cluster 2s ionization threshold (solid, red traces) show a surplus of low energy electrons occurring in the cluster spectra (see horizontal bars). This depicted the first evidence of the existence of ICD. Spectra of an atomic beam are shown for comparison in two panels (dotted lines). Reprinted with permission from ref (3). Copyright 2003 APS.

Figure 29

Scheme of a supersonic jet setup for cluster generation. The setup consists of three vacuum chambers. Bottom to top: Expansion chamber: gas is expanded from a small nozzle into a vacuum. A skimmer is used to extract a thin gas beam from the zone-of-silence. Target chamber: the supersonic jet is crossed with a projectile beam, for example, light from a synchrotron, triggering the cluster reaction under investigation. Jet dump: the part of the supersonic jet which did not react is discarded after leaving the target chamber through a small aperture.

Figure 30

Electron spectrum of large Ne clusters excited at a photon energy above the 2s ionization threshold. The spectrum was corrected for the detection efficiency of the hemispherical electron analyzer, and a contribution of uncondensed atomic Ne was subtracted. Primary 2s photoelectrons, split into a bulk and a surface component, the latter at higher kinetic energy, and electrons due to ICD of the 2s^–1 vacancy are clearly seen. Reprinted from ref (255), with permission from Elsevier.

Figure 31

Electron spectra of large, photoionized NeAr clusters produced at different mixing Ne/Ar ratios. (a) Electrons that were detected in coincidence with a primary Ne 2s photoelectron, at a kinetic energy of approximately 4 eV (shaded region). Spectra were recorded at a photon energy of 52.1 eV. The fraction of Ar in the respective cluster set is indicated in the figure legend. ICD of Ne 2s^–1 to both Ne⁺Ne⁺ and Ne⁺Ar⁺ final states is observed. (b) Calculated Ne 2s^–1 ICD spectrum. All curves were normalized to equal height of the Ne⁺Ar⁺-ICD feature. Gray bars highlight ICD to final states that are separated by more than the Ne–Ne distance between neighboring shells. See text for details. Figure adapted from ref (281), used under CC BY.

Figure 32

Electron−electron coincidence spectrum of mixed Ar–Kr clusters (3% Kr in the initial gas mixture), recorded with hν = 32 eV photons. The first of the two electrons that were recorded, e₁, pertains to the Ar inner-valence region, the second one, e₂, is of low energy and identified as the ETMD electron. (a) Energy spectrum of all secondary (ETMD) electrons e₂, before (black symbols) and after (red symbols) background subtraction. (b) Number of coincident electron pairs on a linear color scale. The region between the two black bars was summed up to produce the ETMD signal in panel a. (c) Energy spectrum of primary electrons e₁, irrespective of the energy of the secondary electron (summation of the coincidence map along horizontal lines). Regions marked by red bars in panel b were used for background determination. Estimated background intensity is also shown by a red trace in panel c. Intensity is expressed (b) as coincident events/pixel of 30 meV² or (a, c) as coincident events per interval of 30 meV. A cluster size around 100 was estimated (order of magnitude). In total, approximately 1.2 × 10⁵ events are shown. See text for details. Figure adapted from ref (283), with permission from Elsevier.

Figure 33

Energy of the two-hole final states after photodouble-ionization of Ne clusters, recorded by electron−electron coincidence spectroscopy. Two different cluster sizes were probed. The feature at approximately 46.5 eV binding energy is produced by ICD to (Ne⁺(2p^–1))₂ final states. In larger clusters, at lower binding energy, final states populated by intracluster inelastic scattering are seen. Figure adapted from ref (276), with permission from Elsevier.

Figure 34

Schematic of fluorescence following ICD after resonant excitation in Ne clusters. (a) In the first step, an inner-valence electron of atom A is resonantly excited. For an isolated atom or molecule, this excitation would result in fast relaxation of site A via its autoionization, which totally suppresses possible radiation emission. In the cluster, due to environment the energy can be alternatively transferred to a neighbor B via ICD, (b) which results in the emission of a low-energy electron. The efficiency of ICD grows with the size of the cluster^, and may even dominate over autoionization on site A. (c) Finally, the still excited atom A releases its energy by emission of a photon, which now becomes possible owing to the preceding ICD. Please note that, despite the atomic picture, the (outer) energy levels in the cluster might have changed dramatically, therefore, no atomic state notation is used in the figure. The caption has been taken from ref (280). Reprinted from ref (280), with permission from IOP publishing.

Figure 35

Sketch of the theoretical model used to compute the RCT spectra. In a free dimer, vibrational levels of the Ne²⁺(2p^–2)-Ne state are populated according to the corresponding excitation Franck–Condon factors. The respective densities are visualized by the gray shaded functions. Thereafter, each density is multiplied by the R-dependent decay widths Γ_RCT(R) and mapped onto the RCT final state Ne⁺–Ne⁺. The observed photon spectrum (gray spectrum in right panel) is obtained by incoherent summation of contributions from all vibrational states. If the dimer is embedded in a cluster, it first relaxes vibrationally during the RCT lifetime. Therefore, only the vibrational ground state (visualized by the magenta shadowed functions) does contribute to the photon emission spectrum (magenta spectrum in right panel). The caption has been taken from ref (61). Reprinted from ref (61), with permission from IOP publishing.

Figure 36

Sketch of the process under investigation. In a pure Ne cluster, resonant outer-valence excitation is followed by fluorescence emission (upper left magnification). Contrary, in heterogeneous Ne–Ar clusters, the energy is transferred to ionize a neighboring Ar atom (lower right magnification), leading to a resonantly enhanced ionization cross-section of Ar. Caption from ref (285). Reprinted with permission from ref (285). Copyright 2019 American Chemical Society.

Figure 37

Sketch of a core level photoionization process (1) in a dimer (using an H₂O-dimer as an example), followed by Auger decay into an excited two-hole state (2) and ICD (3). Alternatively, the dicationic state after step (2) may decay by ETMD. Figure reprinted from ref (11), with permission from Taylor and Francis Ltd.

Figure 38

Experimental energy distribution of ICD electrons from decay of the 2a₁ state in water clusters of different mean size, measured in an electron−electron coincidence experiment. All curves are shown normalized to the same height and without background subtraction. Coincident counts were summed up between 26 and 35 eV photoelectron kinetic energy. Figure adapted from ref (284), used under CC BY.

Figure 39

Set of prototypical bimolecular species that contain the types of hydrogen bonds common in biochemically relevant molecules. Inner-valence ionization in most of these systems initiates ICD. Figure adapted from ref (221).

Figure 40

Calculated energy levels and calculated ICD spectrum of the water, formaldehyde (H₂O···O=CH₂) dimer in its ground state equilibrium geometry. (a, b) Energy levels of the singly and doubly ionized states. Colors in panel a indicate the atom, at which the vacancy is mainly localized. Colors in panel b indicate the type of localization, with red: both vacancies on the OCH₂, green: both vacancies on the H₂O and blue: delocalized. Intermolecular Coulombic decay may proceed from any singly ionized state (top panel) to a doubly ionized state (middle panel) that is lower in energy (further to the left on the x-axis). The calculated ICD spectrum, by convoluting all viable transitions with a fixed width Gaussian, is in the bottom panel. See text for details. Figure adapted from ref (221).

Figure 41

Schematic representation of the electronic decay cascade in a microsolvated Mg²⁺ dication. Auger, ICD, and ETMD steps are shown by black, blue, and red arrows, respectively. The time scale on the left shows the estimated lifetimes of different processes. Figure reprinted from ref (410).

Figure 42

Sketch of a liquid jet vacuum chamber for PE spectroscopy measurements using soft X-ray photons from a synchrotron radiation facility. The photograph shows a magnified view on the conical entrance aperture of the hemispherical electron analyzer (EA) and the glass capillary, forming the jet. Pumping by a cold trap (“LN₂”) and a turbomolecular pump together with the narrow entrance cone ensures a sufficiently low vacuum pressure for electron spectroscopy. Figure adapted from ref (425).

Figure 43

Schematic of the electronic and nuclear relaxation pathways in water with the help of potential energy curves. The respective curve for ground-state water is shown in black, for the dissociative core-ionized state in orange, and for the final doubly charged states in green. Also shown are the wave packets for both cases, H₂O and D₂O, in the ground state, and the ones in the excited states, that is, following photoionization. For the latter, the temporal evolution of the wave packets is depicted as well, reflecting the proton motion along the hydrogen (deuteron)-bond axis on the 4 fs-time scale of the core-hole lifetime. The wave packet for light water is seen to reach out further, which corresponds to a larger distance traveled. Autoionization occurs at any point on the core-excited potential energy curve which at small O–H/D distance leads to formation of local dicationic states and at larger distance to charge-separated dicationic states (illustrated by thin orange arrows turning green, labeled 2h and 1h1h, respectively). Figure reprinted from ref (427).

Figure 44

(A) Auger decay, ICD, and ETMD processes in liquid water after O 1s core-level ionization, illustrated for a water pentamer. The left (green) column shows the processes in the ground-state water structure, and the center (orange) column shows the processes for a proton-transferred Zundel-type cationic structure. This is illustrated for all three processes: PTM-Auger, PTM-ICD, and PTM-ETMD; molecules participating in the decay processes are enclosed in brackets. The decay outcomes are shown below each structure. ET denotes energy transfer. The right-hand column depicts the schematic energy diagrams of the (A) Auger decay, (B) ICD, and (C) ETMD processes generated by core-level ionization. Starting point is a molecule with a core-level hole. Different hole-refill routes are indicated by a black arrow. The respective resulting electron–hole in the valence level of either molecule, and the subsequently emitted autoionization electron are shown by white and yellow circles, respectively. Figure adapted from ref (34).

Figure 45

Main figure: Experimental (tier (b)) and computed (tier (c)) oxygen 1s Auger electron spectra from light (in blue) and heavy (red) liquid water. The experimental spectra were measured at 600 eV photon energy. Simulations were performed for a water pentamer. The theoretical spectra are shifted to larger energies by 3.4 eV to account for long-range polarization effects that are apparently missing in the finite size pentameric system. Tier (a) is the experimental Auger spectrum of water vapor exhibiting no high-kinetic energy shoulder associated with the 1h1h states (yellow-shaded region). The spectrum is shifted by the gas–liquid phase shift, as explained in the text. Inset: Simulated Auger-electron spectra (black curves) for three explicit geometries of the water pentamer (as depicted), representing structure changes upon proton dynamics. Results are shown for the proton-transfer coordinates 0.95 Å (ground-state geometry, tier (d)), 1.40 Å (Zundel-like structure, tier (e)), and 1.85 Å (water-hydronium complex, tier (f)). The areas under the gray (shaded area), red, and blue curves reflect the contributions of the Auger, ICD, and ETMD processes, respectively, to the total spectral intensity. Figure adapted from refs (34) and (428).

Figure 46

Populations of the final states populated by different, competing electronic relaxation processes in a model water pentamer computed as a function of time after core-level ionization. Reprinted from ref (34).

Figure 47

Top traces: Nitrogen 1s Auger/autoionization spectra from 2.6 M NH₃ in light water (in blue) and 2.6 M ND₃ in heavy water (in red). Bottom traces: N 1s-autoionization spectra from 2 M NH₄Cl (blue curve) and 2 M ND₄Cl (red circles) aqueous solution. All spectra were measured at 500 eV photon energy. Note that the N 1s binding energy of ammonia in water is approximately 405 eV. Cartoon (III) depicts the complete transfer of the first proton from (NH₄²⁺)* (aq) to a neighbor water molecule forming (NH₃⁺)*(aq)+H₃O⁺ (aq) within 7 fs, followed by local Auger decay. Cartoons (IV): (NH₃⁺)* also releases a proton which travels only half way to another water molecule, forming a Zundel-analogue complex where the proton is shared between the remaining NH₂^* (aq) and a water molecule. The subsequent autoionization processes of the transient structures by Auger decay (IV.1) and ICD and PTM-Auger (IV.2) are shown. These latter processes are the same as for NH₃ aqueous solution. Gray shading highlights the spectral region sensitive to these processes, labels A and B are explained in the text. Figure adapted from ref (437), used under CC BY.

Figure 48

Unrelaxed two-dimensional cut through the potential-energy surface of a core-ionized NH₄⁺ (H₂O)₃ cluster showing the electronic energy as a function of the N–H distances along the direction of two hydrogen bonds. The N–H ground state distance is 1.1 Å. The minimum energy corresponding to the fully transferred proton is at ∼1.8 Å, marked by black dashed lines. The third water molecule in the molecular sketch is omitted for clarity. Figure reprinted from ref (437), used under CC BY.

Figure 49

Sketch of the most relevant ETMD processes in LiCl aqueous solution. (a) ETMD(2)_W. (b) ETMD(3)_W,W. (c) ETMD(3)_W,Cl. The subscripts W and Cl refer to the species ionized in the final state (water molecules and a chloride anion), and (2) and (3) refer to the numbers of monomers involved in the ETMD process, including the core-ionized Li⁺. The initial step in each case is the 1s-core-level ionization of Li⁺(aq), forming Li²⁺(aq). E_kin denotes the kinetic energies of electrons emitted in ETMD processes (briefly ETMD electrons), which are measured in the experiment. The respective final ETMD states and the relative kinetic energies of the ETMD electrons are indicated on the left-hand side. Figure reprinted from ref (442).

Figure 50

Experimental and simulated ETMD spectra of LiCl aqueous solutions, shown as a function of kinetic energy (top) and versus double-ionization energy (bottom). (a) Experimental ETMD spectra from 3.0 M LiCl solution, resulting from core ionization of Li⁺ (aq) at 171 eV photon energy (red dots). A reference spectrum from neat liquid water has been subtracted. Black symbols result from five-point-binning of the red dots, and the green line results from additional smoothing. (b) Analogous data as in panel a but for 4.5 M concentration, and a photon energy of 175 eV. Error bars in panels a and b represent the standard deviation from five-point-binning. (c–e) Theoretical ETMD spectra (black solid curves) computed for the solvent-separated (SSP), solvent-shared (SShP), and contact (CP) ion pair cluster models, respectively. Energies and intensities of individual transitions are shown also as sticks. Each stick has been convoluted by a Gaussian with full width at half-maximum of 3.6 eV. The geometries of the cluster models are depicted in the insets (red, oxygen; green, Cl^–; gray, Li⁺; white, hydrogen). The theoretical ETMD spectra are decomposed into various contributions corresponding to different ETMD processes (colored solid curves, see the key). Figure adapted from ref (442).

Figure 51

Electron spectrum from ETMD of Li⁺ core ionized states in 4.5 M solutions of LiCl (‘+’ symbols) and Li acetate (LiOAc, dots). The spectra were normalized to equal total area.

Figure 52

Flow diagram of initial, intermediate, and final state in the Auger process following absorption of ionizing X-rays in aqueous electrolytes. (a) Ionization of a cation or its solvation shell; (b) corresponding situation for an anion. In the intermediate state, either a core–hole on the ion or on one of its solvating water molecules has been formed, leading to varying sorts of interactions between the two species (see main text for details). In the Auger decay, a two-hole final state is produced in which either both valence holes remain localized on the species ionized initially (localized decay), or in which one of the holes has migrated to a neighbor (delocalized decay, ICD). Figure reprinted from ref (445), with permission from Elsevier.

Figure 53

Auger spectra recorded from cations in aqueous solution. (a) L_2,3M_2,3M_2,3 Auger spectra of K⁺ (red) and Ca²⁺ (blue) in a 4.0 M KCl and a 1.0 M CaCl₂ solution, respectively. The top energy scale refers to the Ca²⁺ spectrum, and the lower scale to the K⁺ spectrum. (b) M_4,5N_2,3N_2,3 spectra of Rb⁺ (red) and Sr²⁺ (blue) measured from a 1.0 M RbCl and a 0.5 M SrCl₂ solution. The top energy scale refers to the Sr²⁺ spectrum, and the lower one to the Rb⁺ spectrum. Figure reprinted from ref (445), with permission from Elsevier.

Figure 54

Oxygen 1s Auger spectra of 5 M ammonium halide salts in water. The inset shows the full spectrum while the main figure shows merely the high kinetic energy tail, as indicated by the gray box in the inset. Figure reprinted from ref (445), with permission from Elsevier.

Figure 55

Oxygen 1s resonant and off-resonant Auger spectra of liquid water (blue trace) as well as 6 m LiBr (green trace) and 3 m MgBr₂ (red trace) aqueous solutions. The inset shows an X-ray absorption spectrum of water where the excitation energies used for producing the Auger spectra in traces (a) and (b) are indicated. Difference spectra of the respective solution minus water for each trace are shown as dotted lines. The gray boxes shown in the difference spectra of trace (b) highlight a characteristic spectral redistribution at the main and post edges discussed in the main text. Figure reprinted from ref (445), with permission from Elsevier.

Figure 56

Sketch of the generation mechanism of the catalytic free electron responsible for DNA photolesion repair acting in DNA-photolyases. The excitation energy of the initially excited HDF (orange) is transferred to FADH^– (green) and an electron is emitted, here in the deoxyribodipyrimidine photolyase of Thermus thermophilus. The figure is taken from ref (120).

Figure 57

Angular distribution of the 1s photoelectron in dependence of the orientation of the dimer axis and the direction of the polarization vector ε (horizontal). The dimer is aligned in parallel to the polarization vector but with the doubly charged fragment is located in (a) on the right and in (b) on the left side. The asymmetry, being a result of core hole localization, is clearly visible in the experimental data (circles). Insets, solid line: frozen core Hartree–Fock calculation assuming a localized photoelectron emission. Figure adapted with permission from ref (58). Copyright 2008 IOP.

Figure 58

Upper panel: temporal evolution of the charges during a Coulomb explosion between the ions (a) and (b), with (b) transferring its charge to a neutral atom (c). Lower panel: kinetic energies of the three involved particles. The horizontal line at a kinetic energy of 4.5 eV corresponds to the asymptotic energy of the ions after a two-body Coulomb explosion. The figure and caption were taken from ref (320). Reprinted with permission from ref (320). Copyright 2018 APS.

Figure 59

Schematic representation of ICD of an inner-valence vacancy in a hydrogen-bonded THF–water dimer. (a) Starting point of this intermolecular process is the creation of an inner-valence (iv) vacancy by direct electron-impact ionization of the water molecule. (b) Energy released by de-excitation of an outer-valence (ov) electron at the same molecule is transferred to the neighboring THF molecule, which consequently emits a low-energy electron. (c) Back-to-back emission of the fragment ions after Coulomb explosion of the dimer. Figure and caption reprinted from ref (319).

Figure 60

Electron spectra of He droplets doped with several Mg atoms, forming a cluster after condensation. Spectra were recorded in coincidence with charged Mg fragments with different e/m ratio. Low kinetic energy electrons received together with a doubly charged Mg fragment (all fragment sizes added to improve statistics) are interpreted as result of ETMD from an ionized He atom to the Mg cluster (black trace). Singly charged fragments result from breakup of a doubly charged Mg cluster after ETMD (red, blue trace). The 1s main line from He photoionization is also seen. Reprinted with permission from ref (273). Copyright 2016 APS.

Figure 61

Electron spectra from Ne clusters excited by FEL radiation of hν = 17.12 eV, corresponding to the Ne 2p → 3s excitation for surface sites (a), and of hν = 17.65 eV, corresponding to bulk sites. All traces are normalized according to the FEL intensity, indicated in the legend. The dashed lines mark the energy corresponding to 2hν – IP, at which a small amount of two photon direct photoionization can be seen. Continuum electrons corresponding to collective ICD between two excited Ne* centers are at somewhat lower energy (marked in the figure). See text for details. Reprinted with permission from ref (274). Copyright 2016 APS.

Figure 62

ICD-like de-excitation of a highly charged ion (red) via the passage through a graphene foil. Reprint with permission from ref (47). Copyright 2017 APS.