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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.

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.

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.

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.

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.

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.

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.

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.

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.

The Auger decay processes from the $1s$-core-hole state of Ne have been investigated with a multielectron coincidence method. We have observed the double and triple Auger decays, in which both cascade and direct processes are identified from different ways in energy sharing between the Auger electrons. Branching ratios for the different electronic configurations of the Auger final states are determined from the corresponding electron coincidence yields. Particularly for the double Auger decay, the branching ratios are divided into the individual contributions from direct and cascade paths.

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.

A velocity imaging spectrometer has been developed to observe negative ions from molecular ion-pair dissociation. The imaging spectrometer is equipped with a pair of permanent magnets. The resulting magnetic field prevents efficiently the electrons from reaching the detector, without seriously affecting negative ions’ trajectories. The performance of the imaging spectrometer is demonstrated by the observations of photoelectrons from O 2 and He, and O − from O 2 . Application to ion-pair dissociation of O 2 and N 2 O is presented, and it proves that the present imaging method provides useful information on the assignments of superexcited states and on the dynamics of ion-pair dissociation.

Coincidence spectra of energetic electrons with threshold electrons were measured following photo-excitation of the Xe 4d3/2, 5/2 → np resonances, in order to investigate the origin of threshold electrons, and the mechanism leading to formation of the Xe2+ 5p−2 and 5s−15p−1 final states. A two-step decay process was observed in the production of Xe2+ 5p−2(1D) following decay of the 7p resonance, where the intermediate state is Xe+* 5p−2(1S)8p that autoionizes emitting a pseudo-threshold electron. This process was confirmed in a time-of-flight analysis of the coincidence spectra of the energetic electrons with the threshold photoelectrons. It is suggested that a similar two-step process also contributes to the population of excited Xe2+ states and is the main origin for the production of threshold electrons in decay of the 4d−1 np resonances.

Threshold electron–Auger electron coincidence spectroscopy measurements were carried out near the Ar 2p thresholds. Such a method allows us, for the first time, to observe the threshold electron yields associated with the selected final states of the Ar2+ ion: 3p4(1D2), 3p4(1S0), 3p4(3P) resulting from the 2p hole Auger decay. All the spectra reveal strong PCI distortion. Comparison with calculations carried out in the framework of a quantum-mechanical PCI model allows us to clarify the mechanisms and the dynamics of threshold electron production. In the 3p4(1D2) and 3p4(1S0) channels, contribution to the threshold electron yield comes essentially from the PCI retardation of slow photoelectrons. In the 3p4(3P) final state channel, an additional process of valence multiplet decay of the 3p4(1D2)6d state plays a role at and below the L2, L3 thresholds.

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.

Indirect double photoionization processes of Ne have been studied in the photon energy range from 65.5 to 68.3 eV by using a threshold-photoelectron−photoelectron coincidence method, where intermediately formed Ne+ states have been defined by the detection of threshold photoelectrons and fast electrons from their autoionizing decay have been analysed in coincidence. The present investigation revealed that each Ne+ Rydberg series converging to Ne2+(1Se) shows notable selectivity in the autoionizing decay to the final Ne2+ states.

The formation of fluorescent and metastable fragments from four diatomic molecules, i.e. O2, N2, NO and CO, has been investigated in the vacuum ultraviolet region. The neutral particles are detected by using a microchannel plate stack, where retarding electric potentials prevent charged particles from reaching the detector. Every diatomic molecule investigated here shows the formation of fluorescent and metastable fragments in particular photon energy regions. Three Rydberg states of O2 converging to O2+(a 4Πu) undergo both neutral dissociation processes forming fluorescent [O+(4S)]3s 3S1 and metastable [O+(4S)]3s 5S2; the yield curve for each fragment is determined. Direct and cascade formation of the fluorescent [N+(3P)]3s 4P fragment from N2 are separated, and it is found that the dissociation of the 4s σ Rydberg state converging to N2+(C 2Σu+) preferentially produces [N+(3P)]3p fragments, but not or weakly the [N+(3P)]3s 4P fragment. High -n Rydberg states converging to NO+(c 3Π) and CO+(D 2Π) undergo neutral dissociation described by the core ion model, resulting in large peaks for neutral particle formation.

Formation of fluorescing and metastable fragments from photon-induced dissociation of O2 has been investigated in the 20.2–21.2 eV and 22.5–25.0 eV photon energy regions under high resolution. The neutral particles are detected using a microchannel plate stack, where electric potentials prevent charged particles from reaching the detector. Yield curves of excited neutral particles exhibit peaks at Rydberg states converging to O2+(B 2Σg−) and O2+(c 4Σu−). Furthermore, these yields are strongly enhanced at high-n, enabling the measurement of their energy location and width. A simple model for the decay of the Rydberg states converging to O2+(c 4Σu−) based on the competition between autoionization and neutral dissociation is used to describe qualitatively the observed spectrum. The results indicate that the lifetime of the O2+(c 4Σu−)v = 0 level is two orders of magnitude longer than that of the O2+(c 4Σu−)v = 1 level.

The photoion yield from the photoionization of O2 has been measured in the 20.4–24.8 eV photon energy range as a function of both the incident photon energy and ion kinetic energy, at 0° and 90° with respect to the electric vector of the light. From the two-dimensional photoion spectra, photoion yield curves via the formation of O2+(B 2 Σ g−) are extracted. Resonances due to the autoionization of the Rydberg states converging to O2+(c 4 Σ u−) appear in both photoion yield curves measured at 0° and 90°, which indicates rotation in the O2+(B 2Σ g−) state prior to dissociation that smears the initial molecular orientation generated by photoabsorption. On the assumption that the lifetime of O2+(B 2Σ g−) is longer than the mean rotational period, photoion yield curves for parallel and perpendicular transitions are obtained. The two Rydberg series appear strongly in the parallel transition curve which implies that they have Σ symmetry. In the two-dimensional photoion spectra, a structure due to O* autoionization following the dissociation of superexcited O2 states is identified. The photoion yield curves extracted for this process indicate the superexcited states responsible for the neutral dissociation and the symmetries of the states are determined from the observed fragment emission anisotropies.

High-resolution threshold electron spectroscopy has been applied to CO in the photon energy region 26–40 eV, where many sharp and broad CO + states are observed. Dissociation of CO + states in the photon energy range 30–40 eV has been investigated by threshold photoelectron-ion coincidence spectroscopy. Fragmentation characteristics of several distinct CO + states, which are interpreted as Rydberg states converging to CO 2+ states, are found to be state-specific, and are not the same as those of the underlying broad bands corresponding to valence excited CO + states.

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.

The multiple Auger electron emission processes after the $2{p}_{3/2}^{\ensuremath{-}1}$ 4$s$ excitation in Ar have been investigated with a multielectron coincidence method. We have observed the double, triple, and quadruple Auger decays of the resonant state, where both cascade and direct processes are identified. The cascade processes in the resonant double and triple Auger decays result mostly from the spectator behavior of the Rydberg electron in the initial core-hole decay: First Auger decay produces ion states with large internal energies, and subsequent electron emission leads mainly formation of ground Ar${}^{2+}$ and Ar${}^{3+}$ states. It is revealed that spectator behavior, which is known to be the dominant path in resonant single Auger decay, is also important in the direct paths of the resonant double and triple Auger decays. In contrast, the participation of the Rydberg electron is predominant in the direct path of the resonant quadruple Auger decay.

Photoionization and photodissociation processes of SO2 and CS2 in vacuum UV are studied by using two-dimensional photoelectron spectroscopy with a monochromatized synchrotron radiation source. The principal focus is on the mechanisms of autoionization and neutral dissociation of superexcited states. Photoelectron spectra of SO2 exhibit characteristic peaks at the electron kinetic energy below 1.8 eV which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, into the ground S+( 4 S° ) state. These S* atoms are in the singlet Rydberg states converging to S+(2D°). The precursor molecular states, SO2*, are considered to be multiple-electron excited Rydberg states lying at the photon energy above ∼22 eV. The onset of the photoelectron yield due to the atomic autoionization accords with that expected from the thermochemical threshold for the formation of S* through three-body dissociation SO2*→S*+O+O. The two-dimensional photoelectron spectrum of CS2 provides tangible evidence for the formation of a dipole-forbidden Rydberg state (6σg)−1(3dσg)11Σg+ at the photon energy of 14.88 eV which autoionizes into the v3=1 vibrational state of the antisymmetric stretch ν3 mode of CS2+ (X2Πg,Ω, Ω=1/2 and 3/2). This Rydberg state is expected to borrow substantial oscillator strength from the (6σg)−1(5pσu)11Σu+ state through vibronic coupling involving the ν3 vibration.

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.

Using the experimental angular distributions of photoelectrons from the K-shells of an oriented CO molecule reported in a companion paper, we have performed a so-called complete experiment and determined 18 dynamical parameters (ten moduli of transition moments and eight phase differences) for the O K-shell, and 16 dynamical parameters (nine moduli of transition moments and seven phase differences) for the C K-shell, and compared them with the results of our calculations in the relaxed-core Hartree-Fock (RCHF) approximation. The agreement between theory and experiment is only qualitative, therefore the model has to be improved by including electron correlations. From the analysis of experimental data we proved that the σ* shape resonance is due to not only the f-wave, as was widely believed earlier, but is due to approximately equal contributions of three partial waves with 1≤l≤3 for the C K-shell, and four partial waves with 0≤l≤3 for the O K-shell, with a rather substantial contribution of other partial waves with l≤5. From the analysis of the transition moments determined from the experiment it follows that several Cooper minima are likely to exist in partial photoionization cross sections, in particular, in the C 1sσ→esσ and in the O 1sσ→edσ transitions.

The laser-synchrotron radiation combination technique has recently been incorporated into an apparatus for two-dimensional photoelectron spectroscopy of atoms and molecules in order to investigate photoionization dynamics of polarized atoms. Ground state Ar atoms are excited with linearly polarized synchrotron radiation to Rydberg states lying below the first ionization potential. The aligned atoms thus formed are ionized by irradiation of a laser which is also linearly polarized. Photoelectrons emitted in the direction of the electric vector of the synchrotron radiation are sampled and energy analysed. The photoelectron angular distribution is measured with respect to the electric vector of the laser. Expressions which correlate the asymmetric coefficients for the angular distribution with theoretical dynamic parameters involving transition dipole matrix elements are derived. The anisotropy of the present angular distribution can be reasonably explained, assuming that the matrix elements and phase shift differences are essentially independent of the total angular momentum quantum number of the final state and that the spin-orbit interaction in the continuous spectrum is small.

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.

A two-dimensional photoelectron spectrum of NO has been measured in the photon energy range of 10.7–13.4eV. The spectrum exhibits many salient features due to autoionization. The vibrational distribution for NO+ (X1∑+) is simulated by means of Franck-Condon analysis, using a harmonic or anharmonic potential energy curve for the autoionizing state. The vibrational levels of this state are substantially perturbed by the Rydberg states converging to NO+ (X1∑+).

The Auger decay channels of the Ar 2$p$ satellite states have been investigated using a multielectron coincidence technique, using a magnetic bottle time-of-flight electron spectrometer. For the Ar${}^{+}$(2${p}^{\ensuremath{-}1}$3${p}^{\ensuremath{-}1}np$) satellite states the 2$p$ hole is filled first, while for the Ar${}^{+}$(2${p}^{\ensuremath{-}1}$3${s}^{\ensuremath{-}1}$4$s$) satellite states the 3$s$ hole is filled first with leading to Ar${}^{2+}$(2${p}^{\ensuremath{-}1}$3${p}^{\ensuremath{-}1}$) states, which subsequently undergo an Auger decay leading to the filling of the 2$p$ hole.

Simultaneous two-electron emission upon $2p$ core-electron excitation in Ar has been studied using a state-of-the-art multielectron coincidence method. Simultaneous two-electron emission effectively populates Rydberg-excited ${\text{Ar}}^{2+}$ states, in which the excited electron behaves as a spectator of the direct double Auger decay of the core hole. This observation constitutes experimental evidence that the shake-off mechanism is not sufficient to model direct double Auger decay of a $2p$ hole in Ar and suggests an important contribution from the knock-out mechanism.

We report both experimental and theoretical studies on x-ray absorption measured using partial Auger yields of gas phase nitrogen, carbon monoxide, and oxygen molecules near the N 1s ->pi(*), O 1s ->pi(*), and O 1s ->pi(*) regions, respectively. The main tool of our study is a two-dimensional map in which resonant Auger yields are plotted as a function of photon and kinetic energy. The partial yields of the three molecules are analyzed in detail by extracting profiles along various directions in the map. Narrowing of the absorption resonances is observed along the direction of constant kinetic energy. It is shown that such profiles are similar to the conventional x-ray-absorption spectrum for a broad class of molecules whose potential energy surfaces of the final and core-excited states are almost parallel. However, substantial differences with the conventional x-ray-absorption profiles are observed in the general case of nonparallel surfaces due to the lifetime vibrational interference. Here, we suggest a systematic way to eliminate the lifetime vibrational interference.

Autoionization dynamics of core-valence doubly excited states in ${\mathrm{N}}_{2}$ is studied by measuring angular dependence and photon energy evolution of inner-shell photoelectron spectra. The anisotropic electron emissions in the autoionization are interpreted in terms of symmetries of the doubly excited states. The autoionization features in the photoelectron spectra demonstrate clearly the electronic decay closely related to nuclear dynamics in the femtosecond regime. A nonadiabatic picture is required to describe the autoionization of a Rydberg-type doubly excited state.

We have studied the site-specific formation of metastable OCS2+ in the C1s and the O1s Auger decays of OCS molecules by using an Auger-electron–ion coincidence technique. The coincidence measurement reveals the metastable character of the Auger final states with the (3π)−2 configuration. The formation of metastable OCS2+ is observed negligibly in the C1s Auger decay and only weakly in the O1s Auger decay, in contrast to the favorable metastable formation previously reported for S2p Auger decay. It is found that the origin of the site-specificity in the formation of metastable OCS2+ arises from the localization effect of the valence orbital involved in the Auger transition. The dissociative dication states related to individual fragmentation pathways are also derived from the coincidence measurement.

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.

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.

International audience; Electron correlations in three electron emission associated with innershell photoionization are investigated for the first time through a combined theoretical and experimental approach. Namely, different kinds of Post Collisional Interactions (PCI) occurring in the decay of Argon 2p holes by emission of two Auger electrons are isolated experimentally with a powerful coincidence technique and are modeled with an eikonal approach. It is shown that PCI distortion of the photoelectron line shape is stronger than in the single Auger paths and depends critically on the process: in direct Double Auger decays where two Auger electrons are emitted simultaneously, direct interaction of the 2p photoelectron with the two Auger electrons is demonstrated ; in cascade Double Auger processes, the PCI distortion depends only slightly on the last electron emitted in the cascade. This perturbation depends on and reveals the lifetime of the intermediate Ar 2+ state.

We have studied Auger decays of the 1s shake-up and shake-off states in N2 molecules with a state-of-the-art multi-electron coincidence method. The multi-electron coincidences reveal overall features of the Auger transitions from the individual core-hole states. The decay features of the shake-up states are interpreted by spectator and participant roles of the exited electrons. It is observed that the valence holes formed at the initial shake-off process remain unchanged during the Auger decay from the shake-off states.