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We developed recently [A. Fert\'e, et al., J. Phys. Chem. Lett. 11, 4359 (2020)] a method to compute single site double core hole (ssDCH or K$^{-2}$) spectra. We refer to that method as NOTA+CIPSI. In the present paper this method is applied to the O K$^{-2}$ spectrum of the CO$_2$ molecule, and we use this as an example to discuss in detail its convergence properties. Using this approach, a theoretical spectra in excellent agreement with the experimental one is obtained. Thanks to a thorough interpretation of the shake-up states responsible for the main satellite peaks and with the help of a comparison with the O K$^{-2}$ spectrum of CO, we can highlight the clear signature of the two non equivalent carbon oxygen bonds in the oxygen ssDCH CO$_2$ dication., Comment: 16 pages, 7 figures, 7 tables

Quantum beats in fluorescence decay from Zeeman-split magnetic sublevels have been measured for helium Rydberg states excited by synchrotron radiation. The Zeeman quantum beats observed in this prototypical case were fitted with an equation from a theoretical formulation. It is proposed that Zeeman quantum beat measurement can be a useful way to simply evaluate the polarization characteristics of extreme ultraviolet light.

An undulator is a device widely used in synchrotron light sources to produce a certain quasi-monochromatic light; however, its waveform has never been recorded due to the lack of measurement techniques. Here we propose an approach to characterize the waveforms of electric fields produced by relativistic electrons using an algorithm of spectral phase interferometry for direct electric-field reconstruction, which is one of the most common methods to characterize ultrashort laser pulses. As a result, we have succeeded in retrieving a 10-cycle square enveloped electric field, which corresponds to 10 periods of permanent magnets of the undulator. This method can dramatically improve the development of synchrotron sources including x-ray free electron lasers.

We recently developed [A. Ferté

Ultrafast dynamics of highly excited Rydberg states of ${\mathrm{Xe}}^{+}$ is investigated by time-resolved shot-by-shot photoelectron spectroscopy with EUV--free-electron-laser pump (40.8 eV) and near-infrared probe (1.56 eV) pulses. Sorting the single-shot spectra in the order of relative delay between the two pulses allows for investigating ultrafast decays of intermediate states participating in the double ionization of Xe. Observed photoelectron spectra show a biexponential decay within \ensuremath{\sim}100 fs and \ensuremath{\sim}300 ps in addition to constant signals lasting longer than 1 ns. These ultrafast decays reflect the lifetimes of autoionizing Rydberg states converging to electronically excited states of ${\mathrm{Xe}}^{2+}$. The results demonstrate that time-resolved shot-by-shot EUV--free-electron-laser photoelectron spectroscopywith a synchronized optical laser provides a powerful tool for investigating ultrafast electron emission processes.

Multi-electron−ion coincidence spectroscopy is applied for investigating metastability of the dication states formed by the S 2p Auger decay in OCS. The coincidence spectrometer developed in the present work is based on a magnetic-bottle time-of-flight electron spectrometer equipped with electrostatic retardation and ion detection capabilities. Dication spectroscopy with a fairly-high energy resolution and high statistics enables us to inspect the metastability of the individual electronic states in OCS2+.

The Auger decay process of the $3{d}_{5/2}$ hole is studied experimentally and theoretically along the isoelectronic Br, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Rb}}^{2+}$ series with electron configuration [Ar] $3{d}^{9}4{s}^{2}4{p}^{6}$. The experimental results consist of multielectron coincidence data measured from all three elements and conventional high-resolution Auger spectrum for the Br case. Theoretical interpretation was done by using multiconfiguration Dirac-Fock calculations. It is found that the decay rate of two-Auger-electron emission increases along the series in the direction of decreasing nuclear charge. Also, complexity of the Auger spectrum follows the same trend, requiring a drastic increase to the size of the configuration space for describing the observed spectra.

We report the observation of quantum interference between electron wave packets launched from the inner-shell $4d$ orbital of the Xe atom. Using pairs of femtosecond radiation wave packets from a synchrotron light source, we obtain time-domain interferograms for the inner-shell excitations. This approach enables the experimental verification and control of the quantum interference between the electron wave packets. Furthermore, the femtosecond Auger decay of the inner-shell excited state is tracked. To the best of our knowledge, this is the first observation of wave packet interference in an atomic inner-shell process, and also the first time-resolved experiment on few-femtosecond Auger decay using a synchrotron light source.

International audience; Double core hole spectroscopy is an ideal framework for investigating photoionization shake-up satellites. Their important intensity in a single site double core hole (ssDCH) spectrum allows the exploration of the subtle mix of relaxation and correlation effects associated with the inherent multielectronic character of the shake-up process. We present a high accuracy computation method for single photon double core shell photoelectron spectra that combines a selected configuration interaction procedure with the use of non orthogonal molecular orbitals to obtain unbiased bindingenergy and intensity. This strategy leads to oxygen ssDCH spectrum of the CO molecule that is in excellent agreement with the experimental result. Through a combined wave function and density analysis, we highlight that the most intense shake-up satellites are characterized by an electronicreorganization that opposes the core hole induced relaxation.

Multi-electron coincidence measurements have been performed at the photon energies for the core-to-valence (1s → π

The pulse duration of soft X-ray free-electron laser (FEL) pulses of SACLA BL1 (0.2–0.3 nC per bunch, 0.5–0.8 MeV) were characterized by photoelectron sideband measurements. The intensity of the He 1 s−1 photoelectron sidebands generated by a near-infrared femtosecond laser was measured as a function of the time delay between the two pulses using an arrival time monitor. From the width of the cross-correlation trace thus derived, the FEL pulse duration was evaluated to be 28 ± 5 fs full width at half-maximum in the photon energy range between 40 eV and 120 eV.

International audience; The detailed study of single photon multiple ionization processes requires the detection in coincidence of all the electrons emitted from the same ionization event. The advent of magnetic bottle experiments made possible the development of this multi-electron coincidence spectroscopy. First we will briefly review the achievements of this technique. Then we will illustrate more specifically its high sensitivity taking as an example the decay by emission of three Auger electrons of 2p and 2s holes in argon. Our results show that the processes are completely different depending on the initial core hole: the three Auger electrons are emitted dominantly in a simultaneous path for the 2p case, but in cascade for the 2s one.

Quantum manipulation of populations and pathways in matter by light pulses, so-called coherent control, is currently one of the hottest research areas in optical physics and photochemistry. The forefront of coherent control research is moving rapidly into the regime of extreme ultraviolet wavelength and attosecond temporal resolution. This advance has been enabled by the development of high harmonic generation light sources driven by intense femtosecond laser pulses and by the advent of seeded free electron laser sources. Synchrotron radiation, which is usually illustrated as being of poor temporal coherence, hitherto has not been considered as a tool for coherent control. Here we show an approach based on synchrotron radiation to study coherent control in the extreme ultraviolet and attosecond regime. We demonstrate this capability by achieving wave-packet interferometry on Rydberg wave packets generated in helium atoms., Synchrotron light sources have wide range of tunable parameters like frequency, intensity. Here the authors demonstrate quantum control of Rydberg states of helium using delay controlled XUV wavepackets generated from synchrotron radiation.

We report on the use of pairs of 10-cycle extreme ultraviolet wave packets with attosecond-controlled spacing emitted by individual relativistic electrons within an electron bunch passing through a tandem undulator. Based on the temporal coherent control technique with circular polarization, we succeeded in controlling the excited state alignment in the photoexcitation of helium atoms, which we verified through the observation of oscillation in fluorescence yield depending on the attosecond-controlled delay time. Our work demonstrates the potential of undulator radiation for the generation of longitudinally coherent wave packets suitable for attosecond coherent control, an application which has hitherto been hidden in the incoherent nature of the radiation pulse emitted by a bunch of electrons.

A pulsed extraction of electrons associated with a single light pulse is introduced into electron time-of-flight measurement by using a magnetic bottle electron spectrometer. The pulsed extraction enables us to observe long times-of-flight of electrons with synchrotron radiation pulses of short periods. The feasibility and the performance of this method are demonstrated by multielectron coincidence measurements for Xe 4d excitation/ionization.

Multi-electron coincidence spectroscopy using a magnetic-bottle electron spectrometer has been applied to the study of the Auger decay following O1s photoionization of condensed H 2 O molecules. Coincidence Auger spectra are obtained for three different photoelectron energy ranges. In addition, the energy distribution of the slow electrons ejected in the Auger decay of the O1s core hole is deduced from three-fold coincidences.

Single photon simultaneous core ionization/core excitation (K−2V) of the Benzene molecule has been observed experimentally, using synchrotron radiation, by electron coincidence spectroscopy with a magnetic bottle time-of-flight electron spectrometer and reveals a rich spectrum. DFT and Post–Hartree–Fock calculations provide detailed assignments of K−2V states. The specific Auger decay of these states has also been determined experimentally with a new technique to improve the energy resolution.

In this study, it was demonstrated that the polarization control of light from a crossed undulator can be achieved using material response, without any prior monochromatization. Nonmonochromatized horizontally and vertically polarized radiation emitted from two undulators is irradiated to He atoms, and Rydberg states in the extreme ultraviolet region are excited resonantly. The polarization of the photoexcitation is adjustable in a controlled manner by tuning the electron beam orbit between the two undulators, which is verified by observing Zeeman quantum beats in the fluorescence decays of the He Rydberg states.

L23VV Auger decays caused by Si:2p core-level photoionization in Cl3SiSi(CH3)3 vapor have been studied by means of photoelectron Auger-electron coincidence spectroscopy. Upon the Si:2p photoionization, Cl3Si(CH2)nSi(CH3)3 (n = 0–2) shows two core-photoelectron emissions that are distinguishable by the silicon site. Cl3SiCH2CH2Si(CH3)3 (n = 2) and so forth, which have a long inter-site bridge, remember the initially core-ionized silicon-site and show site-specific fragmentation. In contrast, Cl3SiSi(CH3)3, which has direct Si Si bonding (n = 0), forgets the initially core-ionized silicon-site and shows negligible site-specificity in the final fragmentation stage. A reduction of the site specificity in Cl3SiSi(CH3)3 starts during a Si:L23VV Auger decay and its reduction process can be explained in terms of an interatomic-Coulombic-decay-like mechanism. Another site-specificity reduction, caused by inter-site energy randomization, occurs after every Si:L23VV Auger decay.

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.; International audience; Multi-electron coincidence measurements have been performed at the photon energies for the core-to-valence (1s → π *) and core-to-Rydberg (1s → 3sσ and 3pπ) resonant excitations in N 2 , in order to investigate the dynamics of multiple Auger-electron emissions from these core-excited states in detail. Peaks due to slow electrons from superexcited atomic fragments are observed in the decay processes by emission of 2 or 3 Auger electrons, indicating stepwise (cascade) multiple Auger decays that involve faster dissociations than electronic relaxations. Energy partitions between the emitted electrons enable us to reveal the detailed decay mechanisms for these processes. Branching ratios among the decays by emission of 1, 2 or 3 Auger electrons and those between the simultaneous (direct) and stepwise (cascade) processes have been determined for each of the core excited states. Branching ratios of decay channels resulting in molecular or fragment ions have also been substantiated.

Synopsis We report here a detailed experimental study on multiple Auger decay after core excitation into the π* valence and Rydberg orbitals in N2 by an electron-electron coincidence technique.

The single, double, and triple Auger decay processes from the $1{s}^{\ensuremath{-}1}2{p}^{\ensuremath{-}1}np$ shake-up states of Ne have been studied with a multielectron coincidence method. It is revealed that the Rydberg electron promoted in the initial photoionization step generally acts as a spectator both in the single and double Auger decays. The single Auger decay predominantly produces three-hole one-particle final states while leaving the Rydberg electron virtually unchanged. In the cascade double Auger paths, the Rydberg electron is a spectator in the first step of the cascade process but is subsequently released in the second step, leading to the formation of three-hole final states. The direct paths of double Auger decays, which are much weaker than the cascade paths, produce preferably four-hole one-particle states due to the spectator behavior of the Rydberg electron. The distributions of the $\mathrm{N}{\mathrm{e}}^{4+}$ states populated by the triple Auger decays are also presented.

Two-photon Rabi oscillations are observed in He on a timescale of 10 fs by utilizing the strong-field phenomenon known as Freeman resonance. The importance of ponderomotive shifts of both the Rydberg states and the ionization limit is highlighted. Coherent light–matter interaction provides powerful methods for manipulating quantum systems1,2,3. Rabi oscillation is one such process. As it enables complete population transfer to a target state, it is thus routinely exploited in a variety of applications in photonics, notably quantum information processing4,5. The extension of coherent control techniques to the multiphoton regime offers wider applicability, and access to highly excited or dipole-forbidden transition states. However, the multiphoton Rabi process is often disrupted by other competing nonlinear effects such as the a.c. Stark shift, especially at the high laser-field intensities necessary to achieve ultrafast Rabi oscillations6. Here we demonstrate a new route to drive two-photon Rabi oscillations on timescales as short as tens of femtoseconds, by utilizing the strong-field phenomenon known as Freeman resonance7. The scenario is not specific to atomic helium as investigated in the present study, but broadly applicable to other systems, thus opening new prospects for the ultrafast manipulation of Rydberg states8.

The single, double, and triple Auger decays from the 1s shake-up states of O2 have been studied using a multi-electron coincidence method. Efficient populations of two-hole final states are observed in single Auger decays of the π-π* shake-up states, which is understood as a characteristic property of the Auger transitions from shake-up states of an open-shell molecule. The O23+ populations formed by double Auger decays show similar profiles for both the O1s-1 and shake-up states, which is due to the contributions from cascade double Auger processes. While the cascade contributions to the double Auger decays increase with the initial shake-up energy, the probability of direct double Auger processes remains unchanged between the O1s-1 and shake-up states, which implies a weak influence of the excited electron on the double Auger emission that originates from the electron correlation effect.

The observation of an optical vortex beam at 60 nm wavelength, produced as the second-harmonic radiation from a helical undulator, is reported. The helical wavefront of the optical vortex beam was verified by measuring the interference pattern between the vortex beam from a helical undulator and a normal beam from another undulator. Although the interference patterns were slightly blurred owing to the relatively large electron beam emittance, it was possible to observe the interference features thanks to the helical wavefront of the vortex beam. The experimental results were well reproduced by simulation.

Photoelectron angular distributions from helium atoms are measured using the circularly polarized extreme ultraviolet (XUV) vortex produced by a helical undulator as the higher harmonics of its radiation. The XUV vortex has a helical wave front and carries orbital angular momentum as well as the spin angular momentum associated with its circular polarization. While the violation of the electric dipole transition rules has been predicted for interactions between vortices and atoms, the photoelectron angular distributions are well reproduced by assuming electric dipole transitions only. This observation can be explained by the localized nature of the helical phase effect of the vortex on the interaction with atoms, and demonstrates that nondipole interactions induced by the XUV vortex are hardly observable in conventional gas-phase experiments.

We have studied single photon double K-shell ionization of small molecules (N2, CO, C2H2n (n = 1–3), …) and the Auger decay of the resulting double core hole (DCH) molecular ions thanks to multi-electron coincidence spectroscopy using a magnetic bottle time-of-flight spectrometer. The relative cross-sections for single-site (K−2) and two-site (K−1K−1) double K-shell ionization with respect to single K-shell (K−1) ionization have been measured that gives important information on the mechanisms of single photon double ionization. The spectroscopy of two-site (K−1K−1) DCH states in the C2H2n (n = 1–3) series shows important chemical shifts due to a strong dependence on the C C bond length. In addition, the complete cascade Auger decay following single site (K−2) ionization has been obtained.

An Auger-electron–ion coincidence method is applied to study the site-specific formation of metastable CO 2 2+ in the C1s and O1s Auger decays of CO 2 . It is found that only dication states with the electronic configuration of 1π g −2 are relevant to the metastable formation. These dication states are favorably produced in the O1s Auger decay, because of the charge distribution of the 1π g orbital localizing at the O site. Fragmentation channels of dissociative CO 2 2+ states are also investigated.

The energy resolution and the detection efficiency of a magnetic bottle electron spectrometer equipped with an electrostatic retarder are examined, with a particular focus on its application to electron coincidence measurements. The electrostatic retardation properly improves the energy resolving power of the spectrometer. No significant decrease in detection efficiency is observed when retardation voltages are applied. Photoelectron–Auger electron coincidence measurement of the L2,3VV Auger decay in CS2 is performed by using the magnetic bottle spectrometer, which reveals Auger transition intensities varying for the different core holes.

Site-specific electron relaxations caused by Si:2p core-level photoionizations in F

We investigated neutral species in the matrix-assisted laser desorption and ionization (MALDI) plume using femtosecond laser ionization spectrometry with simultaneous measurement of the standard MALDI spectrum of the identical MALDI event induced by pulsed UV laser irradiation. The ratio of neutral species in the plume [A]p/[M]p (A = phenylalanine (Phe) or alanine (Ala), M = 2,5-dihydroxybenzoic acid (DHB)) was confirmed to be the same as that of the sample mixture in the range of [A]0/[M]0 = 4 × 10–4–1, indicating the validity of the widely adopted approximation [A]p/[M]p = [A]0/[M]0 in the reaction quotient of the proton transfer reaction MH+ + A ⇄ M + AH+. An effective parameter representing the extent of thermal equilibrium in the thermal proton transfer model is introduced for the first time. Numerical simulation based on this semiequilibrium model successfully reproduced variations of MALDI signal intensities AH+ and MH+ with two parameters: the fraction of ionized matrix a ≤ 10–5 and an effective t...

An Auger-electron-photoion coincidence (AEPICO) method has been applied to study the stability and dissociation dynamics of dicationic states after the N K-shell photoionization of nitrogen molecules. From time-of-flight and kinetic energy analyses of the product ions, we have obtained coincident Auger spectra associated with metastable states of N2 (++) ions and dissociative states leading to N2 (++) → N(+) + N(+) and N(++) + N. To investigate the production of dissociative states, we present two-dimensional AEPICO maps which reveal the correlations between the binding energies of the Auger final states and the ion kinetic energy release. These correlations have been used to determine the dissociation limits of individual Auger final states.

Multielectron coincidence spectroscopy has been performed for Ar at a photon energy of only 0.2 eV above the $2{p}_{1/2}$ threshold. It is revealed that a postcollision interaction induced by double Auger decay leads to photoelectron recapture, followed by reemission of the captured electron, where the recapture of the slow photoelectron forms the $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg-excited states which subsequently undergo autoionization. The energy correlation of the emitted electrons discloses that both direct and cascade paths in the double Auger decay contribute to the photoelectron recapture.

Post-collisional interaction following inner-shell ionization of atomic systems has been investigated by multi-coincidence electron spectroscopy, with a special emphasis on the decay by emission of two Auger electrons. The case of the Ar 2p hole is first reviewed. After Ar 2s inner-shell photoionization, the dominant decay channel is cascade Coster–Kronig/Auger decay. The distortion of the photoelectron peak shows a sensitivity to both the energy of the Coster–Kronig electron and to the lifetime of the intermediate Ar 2+ (2p −1 3l −1 ) state that is well reproduced by a theoretical model based on the eikonal approach. The dynamics of cascade double Auger decay can be clearly understood thanks to this study.

Simultaneous two-electron emissions from core-valence double photoionization in Ne and upon core-electron excitation in Ar have been studied using a very efficient multi-electron coincidence method. The former case demonstrates that the population ratio of singlet and triplet doubly-charged ion states reveals the double photoionization dynamics. The latter case shows that a weakly-bound electron can be used as a probe of the double Auger dynamics of a core-hole. In both cases, the inadequacy of the shake-off model is revealed by the experimental observations alone and the necessity to describe better electron correlation for two-electron emission, as done in the knock-out model.

Absorption of one VUV photon by an atom or a molecule can induce the ejection of several electrons through different processes. Such multiple ionization processes, studied by coincidence electron spectroscopy, provide a wealth of information on electron correlations. A magnetic bottle electron time of flight spectrometer implemented on synchrotron radiation centers has allowed the efficient detection in coincidence of two, three and up to five electrons with good energy resolution. The branching ratios of the different processes are easily extracted from the experimental spectra due to the constant transmission of the spectrometer. Multiple Auger decay was observed in rare gases atoms after inner-shell ionization, while core-valence and core-core initial double ionization followed by Auger decay are other pathways to multiple ionization. For molecules, Coulomb explosion with energy released in ionic fragments may occur after multiple ionization, nevertheless, coincidence electron spectroscopy can also provide a clear interpretation for peculiar decay channels in molecules.

The aim of this work is to give a clarifying view of the ‘super-high resolution’ scheme employing X-ray absorption spectroscopy in the resonant scattering mode. If the idea is not applied carefully, unrealistic results, as ‘perfect’ resolution, can be produced. The background of the problem is that the technique of extracting lifetime-broadening-free spectra assumes a known location of the resonance in the two-dimensional map of X-ray scattering spectra. The accuracy of the latter is, however, related to the lifetime broadening of the core-excited state. Thus, a compromise must be found between the uncertainty in the position of the X-ray absorption peak and the width of the desired absorption profile.

Autoionization decay from Ne+ Rydberg states has been investigated by using multi-electron spectroscopy at a photon energy of 200.4 eV, where the autoionizing states have been produced through valence photoemission with simultaneous excitation into Rydberg orbitals. The coincidence measurements enable one to resolve individual autoionization lines corresponding to decays from the Ne+ 2p−2(1D)nl, 2p−2(1S)nl, 2s−12p−1(1,3P)nl and 2s−2(1S)nl Rydberg states. The present investigation revealed the autoionization decay features of the Rydberg states: branching ratios, which show strong selectivity towards the Ne2+ final states and asymmetric line shapes in the electron spectra, which have been analysed in terms of the interaction between the discrete state and each continuum. In addition, it is demonstrated that the autoionization decay feature of the Ne+ 2s−12p−1(1P)3p 2S state is characterized by a strong interaction with 2s−12p−1(3P)nl autoionizing states.

Angle-resolved metastable fragment yields following photo-excitation have been measured in the photon energy region 405.7–430 eV in N2. Structures corresponding to multiple excitations associated with parallel and perpendicular transitions are observed in the metastable yield curves recorded using detectors orientated at 0° and 90° to the electric field vector of the linearly polarized incident radiation. The double-excitation structures around 415 eV are interpreted in terms of the overlap of three broad peaks and vibrational progressions. Multiple excitation structures corresponding to high Rydberg states converging to N+2 shake-up satellites are seen above 418 eV. The present work demonstrates that the angle-resolved detection of metastable fragments is an effective spectroscopic tool for probing core-valence multiply excited states in molecules.

The imaging technique has been applied to the negative fragment ion observation in the vicinity of the C1s and O1s ionization thresholds of CO 2 . The O − yield curves obtained exhibit, as well as the π * and Rydberg states bellow the ionization thresholds, doubly excited states above the thresholds. Anisotropic O − emissions with respect to the electric vector are seen on the images observed at the core-excited states, reflecting the symmetries of the states.

We have developed an Auger electron-ion coincidence spectrometer to measure energy- and angular-correlations among the particles ejected on decay of core-excited/ionized molecules. The apparatus consists of a double toroidal electron analyzer and a three-dimensional ion momentum spectrometer, both of which are terminated by time- and position-sensitive detectors. For extracting the ions into the ion momentum spectrometer, a pulsed electric field is applied to the interaction region, according to each Auger electron observation. To evaluate the performance of the ion momentum spectrometer, momentum vectors of fragment ions produced through the dissociation from the selected molecular ion states following N 2 core photoionization have been observed. The momentum, energy and angular resolutions achieved by the spectrometer have been estimated from the events associated with the formation of N 2 2+ and with the pair formation of N + + N+ .

Photoelectron spectroscopy has been applied to investigate electron emissions from excited atomic fragments in the vicinity of the K-shell ionization threshold of N 2 . It is found that low kinetic energy electrons are emitted in the photon energy region both immediately below and above the K-shell ionization threshold. The electron emissions are attributable to autoionization from the excited fragments in low- n Rydberg states. The N 2 +* states leading to the formation of the excited fragments which autoionize with emitting slow electrons, are populated not only by spectator Auger decay from the core-excited states, but also by the recapture of slow photoelectrons into the Rydberg orbitals.

The pair formation of positive and negative fragment ions has been studied in the vacuum ultraviolet region of NO, with negative ion imaging spectroscopy. The negative ion yield curve obtained in the photon energy region of 19–25 eV exhibits many structures which are absent from the photoabsorption spectrum in the same region. The partial yields and asymmetry parameters associated with the dissociations into individual ion pair limits have been extracted from the negative ion images observed. On the basis of these quantities, the assignments for the structures exhibited on the negative ion yield curve are given and the dynamical properties on the ion pair dissociation are discussed.