First (e,e) coincidence measurements on solvated sodium benzoate in water using a magnetic bottle time-of-flight spectrometer.

Understanding the mechanisms of X-ray radiation damage in biological systems is of prime interest in medicine (radioprotection, X-ray therapy…). Study of low-energy rays, such as soft-X rays and light ions, points to attribute their lethal effect to clusters of energy deposition by low-energy electrons. The first step, at the atomic or molecular level, is often the ionization of inner-shell electrons followed by Auger decay in an aqueous environment. We have developed an experimental set-up to perform electron coincidence spectroscopy on molecules in a water micro-jet. We present here the first results obtained on sodium benzoate solutions, irradiated at the oxygen and carbon K-edges.

Fragmentation dynamics of CH3Clq+ (q = 2,3): theory and experiment.

A combined theoretical and experimental study of the dissociation of the di- and trication of the CH3Cl molecule has been performed. Experimentally, these multi-charged ions were produced after interactions of a CH3Cl effusive jet with a mono-energetic beam of H+ or Ar9+ projectile ions. Theoretically, we mapped the multi-dimensional potential energy surfaces of CH3Cl2+, H2CClH2+ and CH3Cl3+ species in their electronic ground and electronically excited states using post-Hartree-Fock configuration interaction methods. In addition to the obvious bond-breaking ionic fragments (i.e. H+ + CH2Cl+, H+ + CH2Cl2+ and CH3+ + Cl+), the formation of H2+ (+CHCl+ or CHCl2+), H3+ (+CCl+) and HCl+ (+CH2+) was observed upon bond rearrangement after ion impact of CH3Cl. The interaction strength of the incident projectiles is found to affect the relative yields on the observed dissociation channels, however, it has no effect on the kinetic energy releases of the fragmentation pathways. For the observed dissociation channels, plausible formation mechanisms were proposed. These reaction pathways take place on the ground and/or electronic excited potential energy surfaces of the doubly and triply charged CH3Cl ions, where spin-orbit and vibronic couplings are in action. Moreover, this work suggests that the mechanisms undertaken may depend on the multiply charged ion preparation by valence or inner-shell single photon photoionization, fast ion beam impact or ultrafast intense laser ionization.

A modified magnetic bottle electron spectrometer for the detection of multiply charged ions in coincidence with all correlated electrons: decay pathways to Xe3+ above xenon-4d ionization threshold.

Single-photon multiple photoionization results from electron correlations that make this process possible beyond the independent electron approximation. To study this phenomenon experimentally, the detection in coincidence of all emitted electrons is the most direct approach. It provides the relative contribution of all possible multiple ionization processes, the energy distribution between electrons that can reveal simultaneous or sequential mechanisms, and, if possible, the angular correlations between electrons. In the present work, we present a new magnet design of our magnetic bottle electron spectrometer that allows the detection of multiply charged Xen+ ions in coincidence with n electrons. This new coincidence detection allows more efficient extraction of minor channels that are otherwise masked by random coincidences. The proof of principle is provided for xenon triple ionization.

Double-core ionization photoelectron spectroscopy of C6H6: Breakdown of the "intuitive" ortho-meta-para binding energy ordering of K-1K-1 states.

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.

Evidence of Extreme Ultraviolet Superfluorescence in Xenon.

We present a comprehensive experimental and theoretical study on superfluorescence in the extreme ultraviolet wavelength regime. Focusing a free-electron laser pulse in a cell filled with Xe gas, the medium is quasi-instantaneously population inverted by 4d-shell ionization on the giant resonance followed by Auger decay. On the timescale of ∼10 ps to ∼100 ps (depending on parameters) a macroscopic polarization builds up in the medium, resulting in superfluorescent emission of several Xe lines in the forward direction. As the number of emitters in the system is increased by either raising the pressure or the pump-pulse energy, the emission yield grows exponentially over four orders of magnitude and reaches saturation. With increasing yield, we observe line broadening, a manifestation of superfluorescence in the spectral domain. Our novel theoretical approach, based on a full quantum treatment of the atomic system and the irradiated field, shows quantitative agreement with the experiment and supports our interpretation.

Ultrafast Charge Transfer Processes Accompanying KLL Auger Decay in Aqueous KCl Solution.

X-ray photoelectron and KLL Auger spectra were measured for the K^{+} and Cl^{-} ions in aqueous KCl solution. While the XPS spectra of these ions have similar structures, both exhibiting only weak satellites near the main line, the Auger spectra differ dramatically. Contrary to the chloride case, a very strong extra peak was found in the Auger spectrum of K^{+} at the low kinetic energy side of the ^{1}D state. Using the equivalent core model and ab initio calculations this spectral feature was assigned to electron transfer processes from solvent water molecules to the solvated cation. The observed charge transfer processes are suggested to play an important role in charge redistribution following single and multiple core-hole creation in atoms and molecules placed into environment.

Photoelectron Spectroscopy of Ions: Study of the Auger Decay of the 4d→nf (n=4,5) Resonances in Xe

We have studied, for the first time by electron spectroscopy, the Auger decay of the 4d→nf (n=4,5) resonances in Xe^{5+} ion. By detecting in coincidence the Auger electrons with the resulting Xe^{6+} ions, we unravel the contribution of the different final ionic states to the total cross section measured by ion spectroscopy. A strong intensity of 5s5p satellite lines has been observed, up to 4 times stronger than the 5s^{2} main lines. This unexpected behavior is confirmed by multiconfiguration Dirac-Fock calculations. This technique provides the most stringent test for theoretical models and allows us to disentangle the contribution of ions in the ground and metastable states in the target beam.

Single photon simultaneous K-shell ionization and K-shell excitation. I. Theoretical model applied to the interpretation of experimental results on H2O.

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.

Single photon simultaneous K-shell ionization and K-shell excitation. II. Specificities of hollow nitrogen molecular ions.

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.

Double photoionization in ring molecules: search of the cooper pair formation.

We provide a final state selective experimental study on the direct double photoionization of the valence states of benzene and pyrrole. The experiment is carried out using a magnetic-bottle electron time-of-flight coincidence setup at the incident photon energy region of 25-120 eV. We discuss on the recently discovered phenomenon of so-called Cooper pair formation [R. Wehlitz et al., Phys. Rev. Lett. 109, 193001 (2012)] and show that our experiment provides contradicting evidence on its existence in the proposed form. We confirm the finding of a new two-electron continuum resonance structure observed at about 30­70 eV above the double ionization threshold in benzene, provide further information from it, and suggest an alternative explanation.

Single photon K(-2) and K(-1)K(-1) double core ionization in C(2)H(2n) (n=1-3), CO, and N(2) as a potential new tool for chemical analysis.

We have observed single photon double K-shell photoionization in the C(2)H(2n) (n=1-3) hydrocarbon sequence and in N(2) and CO, using synchrotron radiation and electron coincidence spectroscopy. Our previous observations of the K(-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(-1)K(-1) process). In the C(2)H(2n) sequence, the spectroscopy of K(-1)K(-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(-1)K(-1) double core ionization in the C(2)H(2n) sequence and in the isoelectronic C(2)H(2n), 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.

A local chemical environment effect in site-specific Auger spectra of ethyl trifluoroacetate.

We have investigated a local chemical environment effect on Auger spectra of ethyl trifluoroacetate (C(4)H(5)F(3)O(2)), using multi-electron coincidence spectroscopy and high-resolution electron spectroscopy. Site-specific KVV Auger spectra for each carbon atom, and for the fluorine and oxygen atoms are presented. The extent of hole localization in the final dicationic states was investigated with the help of theoretical calculations based on a two-hole population analysis. The Auger spectra have been simulated using a statistical approach. It is found that all Auger decays populate mainly localized dicationic states, with the two holes located either on the same fluorine atom or on adjacent fluorine atoms. While the decay of the F 1s hole populates exclusively the former states, the latter class of states is also populated by the decay of the C and O 1s holes.

Decay of a 2p inner-shell hole in an Ar(+) ion.

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 Ar(+) ion. Observations compare well with theoretical predictions obtained within multiconfigurational Dirac-Fock formalism.

Evidence of single-photon two-site core double ionization of C2H2 molecules.

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.

Energy correlation among three photoelectrons emitted in core-valence-valence triple photoionization of Ne.

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.

Triple ionization of CO2 by valence and inner shell photoionization.

Spectra of triply ionized CO(2) have been obtained from photoionization of the molecule using soft x-ray synchrotron light and an efficient multi-electron coincidence technique. Although all states of the CO(2) (+++) trication are unstable, the ionization energy for formation of molecular ions at a geometry similar to that of the neutral molecule is determined as 74 ± 0.5 eV.

Unveiling residual molecular binding in triply charged hydrogen bromide.

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 HBr3+. 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 HBr3+ ionic states and the other, a cascade process involving double Auger decay characterized by the autoionization of Br*+ ion subsequent to the HBr2+ fragmentation.

Properties of hollow molecules probed by single-photon double ionization.

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.

A rotatable electron spectrometer for multicoincidence experiments.

We have developed a rotatable hemispherical spectrometer with good energy and angular resolution, which can be positioned with the lens axis arbitrarily within a solid angle of 1 pi. The collection angle of the emitted electrons with respect to the polarization axis of the light is set by means of a three-axes goniometer, operating under vacuum. An important requirement for this setup was the possibility to perform coincidences between the electron analyzed by the spectrometer and one or several other particles, such as ions, electrons, or photons. The lens system and the hemispheres have been designed to accommodate such experimental demands, regarding parameters such as the resolving power, the acceptance angle, or the width of the kinetic energy window which can be recorded for a given pass energy. We have chosen to detect the impact position of the electron at the focal plane of the hemispherical analyzer with a delay line detector and a time-to-digital converter as acquisition card rather than using a conventional charge-coupled device camera.

Theoretical spectroscopy of acetylene dication and its deuterated species.

We mapped the six-dimensional potential energy surface of the electronic ground state of HCCH(++)(X (3)Sigma(g) (-)) dication using the coupled cluster approach. This potential energy surface is incorporated later into perturbative and full variational treatments to solve the nuclear motions. We derived a set of spectroscopic data for HCCH(++), HCCD(++), and DCCD(++). Our calculations reveal the presence of anharmonic resonances even at low energies, which complicates their assignment by vibrational quantum numbers. In light of our theoretical vibrational spectra, we propose an assignment of the experimental vibrationally resolved valence double ionization spectra of HCCH, HCCD, and DCCD. These spectra are viewed to be mostly composed by a pure vibrational progression involving the CC stretching mode together with a second progression involving both the CC stretching and the bendings.