Search

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+.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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∑+).