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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.

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.

Single-site Double-Core Hole (ss-DCH or K−2) and two-site Double-Core Hole (ts-DCH or K−1K−1) photoelectron spectra including satellite lines were experimentally recorded for the aromatic C6H6 molecule using the synchrotron radiation and multielectron coincidence technique. Density functional theory and post-Hartree-Fock simulations providing binding energies and relative intensities allow us to clearly assign the main K−2 line and its satellites. K−1K−1 states’ positions and assignments are further identified using a core-equivalent model. We predict that, contrary to what has been observed in the C2H2n series of molecules, the K−1K−1 energy-level ordering in C6H6 does not reflect the core-hole distances between the two holes.Single-site Double-Core Hole (ss-DCH or K−2) and two-site Double-Core Hole (ts-DCH or K−1K−1) photoelectron spectra including satellite lines were experimentally recorded for the aromatic C6H6 molecule using the synchrotron radiation and multielectron coincidence technique. Density functional theory and post-Hartree-Fock simulations providing binding energies and relative intensities allow us to clearly assign the main K−2 line and its satellites. K−1K−1 states’ positions and assignments are further identified using a core-equivalent model. We predict that, contrary to what has been observed in the C2H2n series of molecules, the K−1K−1 energy-level ordering in C6H6 does not reflect the core-hole distances between the two holes.

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.

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.

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.

We have studied the ${K}^{\ensuremath{-}2}V$ process corresponding to simultaneous $K$-shell ionization and $K$-shell excitation in the $\mathrm{C}{\mathrm{O}}_{2}$ molecule. We define these ${K}^{\ensuremath{-}2}V$ states as super shake-up, at variance with the ``conventional'' ${K}^{\ensuremath{-}1}{v}^{\ensuremath{-}1}V$ shake-up states. While the nature and evolution with photon energy of the conventional shake-up satellites has been the object of many studies, no such data on a large photon-energy range were previously reported on super shake-up. The $\mathrm{C}{\mathrm{O}}_{2}$ molecule is a textbook example because it exhibits two well-isolated ${K}^{\ensuremath{-}2}V$ resonances (with V being $2{\ensuremath{\pi}}_{u}^{*}$ and $5{\ensuremath{\sigma}}_{g}^{*}$) with different symmetries resulting from shake-up processes of different origin populated in comparable proportions. The variation of the excitation cross section of these two resonances with photon energy is reported, using two different experimental approaches, which sheds light on the excitation mechanisms. Furthermore, double-core-hole spectroscopy is shown to be able to integrate and even expand information provided by conventional single-core-hole X-ray Photoelectron Spectroscopy (XPS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS) techniques, revealing, for instance, $g\ensuremath{-}g$ dipole forbidden transitions which are only excited in NEXAFS spectra through vibronic coupling.

International audience; The interest of molecular double core holes was predicted in 1986 by Cederbaum et al who showed that their spectroscopy can be more informative than that of the single core holes, especially when the holes are located at different sites in the molecule (Cederbaum et al J. Chem. Phys. 85 (1986) 6513). Their experimental study in single photon formation had to await 2009-2010 with progress in synchrotron sources and development of efficient multi-electron coincidence experiments based on a magnetic bottle time of flight spectrometer. At the same time the advent of X-rays Free Electron Lasers opened the possibilty to create them in a two-photon process, and motivated new theoretical studies of their properties. We will illustrate here with a few examples, the progress done recently in the field, including formation of double core holes by double core photoionization, their spectroscopy and decay paths, and the related process of simultaneous core ionization and core excitation.

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.

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 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.

International audience; The simultaneous core ionization and core excitation process (or K-2V process) induced by absorption of a single photon provides the basis of a new spectroscopy that offers both advantages of X-ray Photoelectron Spectroscopy (XPS) and near-edge x-ray absorption fine structures (NEXAFS) spectroscopy

We present in detail a theoretical model that provides absolute cross sections for simultaneous core-ionization core-excitation (K(-2)V) and compare its predictions with experimental results obtained on the water molecule after photoionization by synchrotron radiation. Two resonances of different symmetries are assigned in the main K(-2)V peak and comparable contributions from monopolar (direct shake-up) and dipolar (conjugate shake-up) core-valence excitations are identified. The main peak is observed with a much greater width than the total experimental resolution. This broadening is the signature of nuclear dynamics.

The formalism developed in the companion Paper I is used here for the interpretation of spectra obtained recently on the nitrogen molecule. Double core-hole ionization K(-2) and core ionization-core excitation K(-2)V processes have been observed by coincidence electron spectroscopy after ionization by synchrotron radiation at different photon energies. Theoretical and experimental cross sections reported on an absolute scale are in satisfactory agreement. The evolution with photon energy of the relative contribution of shake-up and conjugate shake-up processes is discussed. The first main resonance in the K(-2)V spectrum is assigned to a K(-2)π(∗) state mainly populated by the 1s→ lowest unoccupied molecular orbital dipolar excitation, as it is in the K(-1)V NEXAFS (Near-Edge X-ray Absorption Fine Structure) signals. Closer to the K(-2) threshold Rydberg resonances have been also identified, and among them a K(-2)σ(∗) resonance characterized by a large amount of 2s/2p hybridization, and double K(-2)(2σ(∗)/1π/3σ)(-1)1π(∗2) shake-up states. These resonances correspond in NEXAFS spectra to, respectively, the well-known σ(∗) shape resonance and double excitation K(-1)(2σ(∗)/1π/3σ)(-1)1π(∗2) resonances, all being positioned above the threshold.

International audience; In the last five years, a strong experimental and theoretical activity has been dedicated to the study of inner-shell double photoionization. Almost simultaneously, between 2009 and 2011, different experiments were performed using two different light sources: well established third generation synchrotron light sources that exist for more than two decades and recently developed x-ray free electron lasers (X-FEL) that provide brightness about 10 orders of magnitude higher. The high photon flux of X-FEL has made possible the successive absorption of many x-ray photons by inner-shell electrons in femtosecond timescale. The possibility to create multiple inner-shell vacancies with this new X-FEL light source has resuscitated “prophetic” 25 years old calculations by Cederbaum et al. [1][2] concerning double K-shell ionization that could be of great interest when conventional Electron Spectroscopy for Chemical Analysis (ESCA) finds some limitations. As a feedback, this has stimulated new theoretical developments. On the other hand, it was considered practically out of reach to observe such states with conventional third generation synchrotron light sources due to the very weak probability to eject two inner-shell electrons by a single photon when the dominant process, that could mask everything else, is single inner-shell ionization. However, due to the development of very efficient detection techniques, it was also possible to perform spectroscopy of double core-hole states with excellent accuracy using third generation synchrotron sources.In this paper we give an outlook, on the basis of recently published results, of the complementary of these light sources for double core-hole spectroscopy. Some new developments and results are also presented with a prospective of what could be done in the future with the technical breakthrough that will be necessary to improve further our understanding of chemical analysis.

Threshold photoelectron–Auger electron coincidence spectroscopy has been applied to the Coster–Kronig transitions from the Cl 2s core-hole state of HCl. The use of coincidence spectroscopy reveals Auger lines associated with the Cl 2s core-hole state in the sub-natural linewidth regime. The coincidence Auger spectrum reveals two band structures corresponding to the formation of HCl2+[(Cl 2p)−1 (V)−1] doubly ionized states via Coster–Kronig transitions, which are only weakly observed in the normal Auger spectrum. These structures are tentatively assigned to HCl2+[(Cl 2p)−1 (V)−1] doubly ionized states with the 2 π or 5 σ valence (V) holes.

Metastable formation has been investigated in the inner-shell region of N2. Enhanced N* formation has been observed around the 1s threshold, and is shown to result from dissociation of high N2+* Rydberg states. These N2+* states are populated by spectator Auger decay from core-excited states, as well as by the recapture of slow photoelectrons into the Rydberg orbitals. The present measurement demonstrates that the metastable observation is a new and sensitive tool to study the decay dynamics of core-excited states and the photoelectron recapture due to post-collision interaction.

Threshold photoelectron-Auger electron coincidence spectroscopy has been applied to the sulphur 2s region of H 2 S, where the Auger electron spectrum is measured in coincidence with threshold electrons. Formation of H 2 S ++ [(S2p) −1 ( V ) −1 ] states via the L 1 L 2,3 V Coster–Kronig transitions is observed above the S2s photoionization threshold. Even below threshold, formation of these H 2 S ++ states is seen and is due to direct double ionization. A sub-natural linewidth regime is achieved with this coincidence method and three structures resulting from overlaps of the H 2 S ++ [(S2p) −1 ( V ) −1 ] states appear in the spectra. The three structures probably correspond to H 2 S ++ states with the 2b 1 , 5a 1 and 2b 2 holes.

A multiphoton ionization process of He in intense extreme ultraviolet free-electron laser fields is investigated in a photon energy range of $h\mathit{v}=20.1\ensuremath{-}20.8\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ using shot-by-shot photoelectron spectroscopy. A photoelectron peak resulting from sequential double ionization of He due to five-photon absorption is observed around $h\mathit{v}=20.4\phantom{\rule{0.16em}{0ex}}$. Shot-by-shot analysis of photoelectron spectra shows that two-photon resonance to the $\mathrm{H}{\mathrm{e}}^{+}(2s{\phantom{\rule{0.16em}{0ex}}}^{2}S)$ state from the ${\mathrm{He}}^{+}$ ground state is responsible for the substantial enhancement of five-photon double ionization. Effective cross sections for two-photon absorption between the $1s$ and $2s$ states of ${\mathrm{He}}^{+}$ and for one-photon absorption from the ${\mathrm{He}}^{+}$ $2s$ state are obtained.

Angular distributions of photoelectrons from both C and O K-shells of the fixed-in-space CO molecule have been measured using the angle-resolved photoelectron-photoion coincidence technique. The measurements have been performed at several photon energies from the ionization thresholds up to about 30 eV above them, where the σ* shape resonances occur. Experimental results are compared with the multiple-scattering calculations of Dill et al (1976 J. Chem. Phys. 65 3158) and with our new calculations in the relaxed-core Hartree-Fock approximation. Our calculations are in a better agreement with the experimental data though numerical discrepancies remain. The experimental angular distributions are fitted by the expansion in Legendre polynomials containing up to ten terms and the extracted parameters are compared with the corresponding theoretical values.

A new technique, based on velocity imaging, has been developed to examine correlations in energy and angle between the several particles formed in dissociative single and double photoionisation at VUV wavelengths. Electrons and positive fragment ions are imaged in coincidence at position-sensitive detectors; the resulting multidimensional data sets contain separable energy distributions, angular distributions and correlations in energy and angle between the particles. In some cases internal or external evidence indicates that pure axial recoil occurs, without molecular rotation. In such cases fixed-molecule photoelectron angular distributions can be extracted and parametrised. Results for H2, N2, NO, CO and O2 are presented. Effects of shape resonances in the ionisation channels are apparent in several cases.

Single-shot photoelectron spectroscopy of nonresonant two-color two-photon ionization of He has been carried out using an intense free-electron laser (FEL) at 59.7 nm and a femtosecond UV laser at 268 nm. By shot-by-shot analysis of photoelectron spectra, the cross section for nonresonant two-color ionization of He is determined to be ${\ensuremath{\sigma}}^{(2)}=4.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}$ cm${}^{4}$ s. The cross-correlation trace derived from the nonresonant signals shows that the temporal resolution of the system is 0.36(3) ps, which is mainly governed by the timing jitter between the FEL and UV laser pulses.

Shot-by-shot photoelectron spectroscopy has been performed to study the three-photon double ionization of Ar in intense extreme-ultraviolet free-electron laser fields over a photon energy range of 19.6--22.0 eV. It is found that the double ionization to Ar${}^{2+}$ 3${p}^{\ensuremath{-}2}$ proceeds sequentially via the formation of singly charged Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states and is enhanced around photon energies of 20.5 and 21.5 eV. Two types of resonance are identified in the two-photon ionization of the Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states: (i) resonances to intermediate states at the one-photon energy level and (ii) those to autoionizing states above the Ar${}^{+}$ ionization threshold.

We have observed single photon double $K$-shell photoionization in the ${\mathrm{C}}_{2}{\mathrm{H}}_{2n}$ ($n=1--3$) hydrocarbon sequence and in ${\mathrm{N}}_{2}$ and CO, using synchrotron radiation and electron coincidence spectroscopy. Our previous observations of the ${K}^{\ensuremath{-}2}$ process in these molecules are extended by the observations of a single photon double photoionization with one core hole created at each of the two neighboring atoms in the molecule (${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ process). In the ${\mathrm{C}}_{2}{\mathrm{H}}_{2n}$ sequence, the spectroscopy of ${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ states is much more sensitive to the bond length than conventional electron spectroscopy for chemical analysis spectroscopy based on single $K$-shell ionization. The cross section variation for single photon ${K}^{\ensuremath{-}1}{K}^{\ensuremath{-}1}$ double core ionization in the ${\mathrm{C}}_{2}{\mathrm{H}}_{2n}$ sequence and in the isoelectronic ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$ and CO molecules validates a knock-out mechanism in which a primary ionized $1s$ photoelectron ejects another $1s$ electron of the neighbor atom. The specific Auger decay from such states is clearly observed in the CO case.

Direct measurements of the photoelectrons or Auger electrons associated with inner shell ionization of positively charged ions are extremely difficult and rarely realized. We propose an alternative method to simulate such measurements, based on core valence double photoionization of the neutral species. As an example, we obtain the spectroscopy, lifetimes, and Auger decays of the states arising from $2p$ inner shell ionisation of an ${\mathrm{Ar}}^{+}$ ion. Observations compare well with theoretical predictions obtained within multiconfigurational Dirac-Fock formalism.

Photodissociation of NO followed by autoionization of an N atom has been studied by two‐dimensional photoelectron spectroscopy using synchrotron radiation. In addition to the bands due to molecular ionization, the two‐dimensional spectrum shows several characteristic patterns in the photon energy range of 21.5–27 eV which result from autoionization of the Rydberg states converging to N+(1De) into the ionic ground state N+(3Pe). The electronic states of the counter atomic oxygen can be determined from the lower onset energies of these patterns to be 3Pe and/or 1De. Discussion is made about the transition region for the photodissociation reaction, especially being focused on the nature of the primary molecular states and their dynamical aspects including competitive molecular autoionization.

Double Auger decay of O1s(-1) and its satellite states in H(2)O has been studied with a multi-electron coincidence method, and a process leading to autoionizing O* fragments has been revealed. The breaking of the two O-H bonds producing the autoionizing O* fragments occurs for highly excited H(2)O(2+) populated by the initial Auger decay. The O* fragments are more favorably produced in the decay from the satellite states, resulting from the larger population of highly excited H(2)O(2+) states inheriting the valence excitation in the initial state.

We observe the formation in a single-photon transition of two core holes, each at a different carbon atom of the C2H2 molecule. At a photon energy of 770.5 eV, the probability of this 2-site core double ionization amounts to 1.6 ± 0.4% of the 1-site core double ionization. A simple theoretical model based on the knockout mechanism gives reasonable agreement with experiment. Spectroscopy and Auger decays of the associated double core hole states are also investigated.

The direct observation of triple photoionization involving one inner shell and two valence electrons is reported. The energy distribution of the three photoelectrons emitted from Ne is obtained using a very efficient multielectron coincidence method using the magnetic bottle electron spectroscopic technique. A predominance of the direct path to triple photoionization for the formation of Ne3+ in the 1s 2s2 2p4 configuration is observed. It is demonstrated that the energy distribution evolves with photon energy and indicates a significant difference with triple photoionization involving only valence electrons.

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields ($\ensuremath{\sim}24.1\text{ }\text{ }\mathrm{eV}$, $\ensuremath{\sim}5\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$) is presented. Resonances to the doubly excited states converging to the ${\mathrm{He}}^{+}$ $N=3$ level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr\"odinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.

We present an experimental and theoretical study of triply charged hydrogen bromide ions formed by photoionization of the inner 3d shell of Br. The experimental results, obtained by detecting the 3d photoelectron in coincidence with the two subsequent Auger electrons, are analyzed using calculated potential energy curves of HBr{sup 3+}. The competition between the short-range chemical binding potential and the Coulomb repulsion in the dissociative process is shown. Two different mechanisms are observed for double Auger decay: one, a direct process with simultaneous ejection of two Auger electrons to final HBr{sup 3+} ionic states and the other, a cascade process involving double Auger decay characterized by the autoionization of Br*{sup +} ion subsequent to the HBr{sup 2+} fragmentation.

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21 eV, ∼ 5 TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).

The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.

Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS(2)(3+) ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1+/-0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS(2)(2+) show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

We demonstrate that the observation of slow electrons emitted in the decay of molecular core-excited states can be a sensitive probe of the double Auger processes, and that in combination with electron-electron coincidence spectroscopy, it can provide clear insight into the mechanisms involved. The present study identifies all cascade Auger paths from the C1s-to-Rydberg states in CO to final states of CO{sup 2+}. One pathway includes the first directly identified case of molecular level-to-level autoionization of a cation and shows remarkable selectivity for a specific final state.

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.

Properties of the interatomic Coulombic decay (ICD) process in Ne dimers have been obtained by tracking the formation of energetic Ne+ ions. The double photoionization cross section, deduced from the Ne+/Ne+ coincidence signal, is dominated by the ICD process and presents a threshold 280 meV below the atomic Ne+2s(-1) threshold. Rydberg excitation of a 2s electron in the dimer creates molecular Rydberg states whose Sigma and Pi symmetries have been resolved. These excited states decay by a resonant ICD process releasing an energetic Ne+ ion and a neutral excited Ne* fragment. Subsequent autoionization of the Ne* fragment explains a double photoionization threshold below the dimer 2s ionization threshold.

The emission of low-energy electrons from H2O has been investigated at photon excitation energies in the vicinity of the O 1s ionization threshold. Neutral oxygen Rydberg atoms (O*) were found to form, and the correlation between the initial inner-shell excited state of H2O and the Rydberg state of O* was determined. The initially excited electron in a Rydberg orbital is shown to remain associated with O* even after the cleavage of two O-H bonds. We also show that the energy discrepancy between two Rydberg states of H2O and O* can be explained by the influence of the post-collision interaction, which becomes stronger as the excitation energy approaches the 1s ionization threshold.

Direct measurements of the photoelectrons or Auger electrons associated with inner shell ionization of positively charged ions are extremely difficult and rarely realized. We use here an alternative method to simulate such measurements, based on core-valence double photoionization of the neutral species. As an example, we obtain the spectroscopy, lifetimes and Auger decays of the states arising from 2p inner shell ionization of an Ar+ ion. Observations compare well with theoretical predictions obtained with multiconfigurational Dirac-Fock formalism.

We have observed single photon one-site (K?2) and two-sites (K?1K?1) double K-shell ionization in C2H2n (n=1-3) sequence and in isoelectronic C2H2, N2 and CO molecules. The spectroscopy of K?1K?1 states is much more sensitive to the bond length than conventional ESCA spectroscopy based on single K-shell ionization. The mechanism of single photon two-sites is due to a knock-out process in which a primary photoionized 1s electrons eject another 1s electron on the neighbor atom.

The dynamical information (ten dipole matrix elements and eight phase differences) has been deduced from the measured angular distributions of photoelectrons from O $K$ shell of oriented CO molecules near the ionization threshold in the region of a ${\ensuremath{\sigma}}^{*}$ shape resonance. Light polarization parallel and perpendicular to the molecular axis has been used. An important contribution of six $l\ensuremath{\sigma}$ partial waves with $0\ensuremath{\le}l\ensuremath{\le}5$ to the ${\ensuremath{\sigma}}^{*}$ shape resonance is demonstrated. A comparison with our calculations in the relaxed core Hartree-Fock approximation reveals only a qualitative agreement, therefore a more advanced theory is needed.

The concept, design and performance of Japan's compact free-electron laser (FEL) facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications in mainly atomic, molecular and optical science are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multi-photon processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump?probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multi-photon ionization by intense x-ray FEL pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.

Single photon double K-shell ionization of small molecules (N2, O2, CO, CO2) has been observed by coincidence detection of two photoelectrons with two Auger electron and reveals properties of hollow molecules.

Cascade Auger electron emission following Xe 3d photoionization has been investigated using a multi-electron coincidence technique, which utilizes an electron spectrometer of magnetic bottle type. It has been found that the Xe2+ states of the 4p−14d−1 configuration, formed by the Auger decay of the Xe+ 3d3/2, 5/2−1 states, dominantly turn into triply charged states of the 4d−25p−1/4d−25s−1 configurations. The Xe2+ 4s−14d−1 states, formed by the 3d Auger decay, yield the 4p−14d−15p−1 states as well as the 4d−3 states. From the coincidence spectrum among three Auger electrons, it is suggested that the Xe2+ 4p−14d−1 states give rise to the following cascade processes: 4p−14d−1 → 4d−25p−1 → 4d−15p−3.

Core–valence doubly ionized states of OCS, which are produced by the Coster–Kronig transitions following the S 2s ionization, have been investigated by using threshold photoelectron-Auger electron coincidence spectroscopy. The sub-natural linewidth regime achieved with this coincidence method enables us to locate the core–valence OCS2+ states, while the corresponding structures are almost smeared out on the S 2s Auger spectrum measured with conventional Auger electron spectroscopy.