Electron-ion Coincidence Technique Research Articles

Resonant Inner-Shell Photofragmentation of Adamantane (C10H16).

Adamantane, the smallest diamondoid molecule with a symmetrical cage, contains two distinct carbon sites, CH and CH2. The ionization/excitation of the molecule leads to the cage opening and strong structural reorganization. While theoretical predictions suggest that the carbon site CH primarily causes the cage opening, the role of the other CH2 site remains unclear. In this study, we used advanced experimental Auger electron-ion coincidence techniques and theoretical calculations to investigate the fragmentation dynamics of adamantane after resonant inner-shell photoexcitation. Our results demonstrate that some fragmentation channels exhibit site-sensitivity of the initial core-hole location, indicating that different carbon site excitations could lead to unique cage opening mechanisms.

FlexPES: a versatile soft X-ray beamline at MAX IV Laboratory.

FlexPES is a soft X-ray beamline on the 1.5 GeV storage ring at MAX IV Laboratory, Sweden, providing horizontally polarized radiation in the 40-1500 eV photon energy range and specializing in high-resolution photoelectron spectroscopy, fast X-ray absorption spectroscopy and electron-ion/ion-ion coincidence techniques. The beamline is split into two branches currently serving three endstations, with a possibility of adding a fourth station at a free port. The refocusing optics provides two focal points on each branch, and enables either focused or defocused beam on the sample. The endstation EA01 at branch A (Surface and Materials Science) is dedicated to surface- and materials-science experiments on solid samples at ultra-high vacuum. It is well suited not only to all flavours of photoelectron spectroscopy but also to fast (down to sub-minute) high-resolution X-ray absorption measurements with various detectors. Branch B (Low-Density Matter Science) has the possibility to study gas-phase/liquid samples at elevated pressures. The first endstation of this branch, EB01, is a mobile setup for various ion-ion and electron-ion coincidence techniques. It houses a versatile reaction microscope, which can be used for experiments during single-bunch or multi-bunch delivery. The second endstation, EB02, is based on a rotatable chamber with an electron spectrometer for photoelectron spectroscopy studies on primarily volatile targets, and a number of peripheral setups for sample delivery, such as molecular/cluster beams, metal/semiconductor nanoparticle beams and liquid jets. This station can also be used for non-UHV photoemission studies on solid samples. In this paper, the optical layout and the present performance of the beamline and all its endstations are reported.

Relation between the spatial shape of excited molecular orbital and molecular-frame electron scattering cross section

SynopsisWe report the molecular-frame electron scattering cross sections of the 2σg→1πg excitation in O2 and N2, obtained using an electron-ion coincidence technique. The vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV. The results exhibit clear molecular-orientation dependence reflecting the nodal structure of the 1πg orbital. It strongly suggests that the electron-ion coincidence technique can provide a powerful tool to investigate spatial shapes of unoccupied molecular orbitals.

Revealing the role of electron-electron correlations by mapping dissociation of highly excited D2+ using ultrashort XUV pulses

Author(s): Martin, L; Bello, RY; Hogle, CW; Palacios, A; Tong, XM; Sanz-Vicario, JL; Jahnke, T; Schoffler, M; Dorner, R; Weber, T; Martin, F; Kapteyn, HC; Murnane, MM; Ranitovic, P | Abstract: Understanding electron-electron correlations in matter ranging from atoms to solids represents a grand challenge for both experiment and theory. These correlations occur on attosecond timescales and have only recently become experimentally accessible. In the case of highly excited systems, the task of understanding and probing correlated interactions is even greater. In this work, we combine state-of-the-art light sources and advanced detection techniques with ab initio calculations to unravel the role of electron-electron correlation in D2 photoionization by mapping the dissociation of a highly excited D2+ molecule. Correlations between the two electrons dictate the pathways along which the molecule dissociates and lead to a superposition of excited ionic states. Using 3D Coulomb explosion imaging and electron-ion coincidence techniques, we assess the relative contribution of competing parent ion states to the dissociation process for different orientations of the molecule with respect to the laser polarization, which is consistent with a shake-up ionization process. As a step toward observing coherent superposition experimentally, we map the relevant nuclear potentials using Coulomb explosion imaging and show theoretically that such an experiment could confirm this coherence via two-path interference.

Formation, stability and dissociation dynamics of cations (n = 1 − 2) in 3.5 keV e−-N2 collisions studied using the energy resolved electron–ion coincidence technique

Coincidences between energy selected electrons and ions produced in the decay of a core hole ionized (excited) state in a free nitrogen molecule have been measured at three specified energies of emitted electrons to reveal the individual pathways produced in 3.5 keV electron-induced fragmentation processes. From these measurements, it has been possible to show, for the first time, that in addition to the normal Auger decay, the resonant Auger excitation channels also share their appreciable contributions in producing singly charged parent ions in an electron-induced collision system. The correlations between ion fragmentation products and electronic structures with a hole configuration in singly-, doubly- and possibly in triply charged molecular electronic states populated in the electronic decay of the initial core hole have been studied and discussed. KER values obtained from our experiments are found to be consistent with the previous results of photo absorption experiments for fragmentation channel → N+ + N+; however, N2+ fragment ions are found to arise mainly from the fragmentation channel → N2+ + N and to possess relatively low kinetic energies in the considered region of binding energies.

Electron-ion momentum vector coincidences in electron collisions with atoms and molecules

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.

Study of multiply charged Argon-ions formed by decay of 2p-hole state under e-- Ar collision employing energy selected ion coincidence technique

SynopsisIn this presented work we have performed the energy selected electron-ion coincidence technique to investigate the formation of multiply charged argon ions and determined correlation probabilities for individual charge state of formed ions and several other features of the employed technique is presented and discussed.

Molecular-frame EELS experiment on inner-valence ionization of N2

SynopsisWe report molecular-frame electron scattering cross sections of N2, obtained using an electron-ion coincidence technique. The vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV for inner-valence ionization of N2. The angular distributions of the electron scattering cross sections thus obtained exhibit clear momentum transfer dependence, indicating the significant change of the ionization dynamics due to contributions from electric multipole transitions.

Molecular-frame electron-scattering experiment on the dipole-forbidden 2σg→1πg transition of N2

We report molecular-frame electron-scattering cross sections of ${\mathrm{N}}_{2}$, obtained using an electron-ion coincidence technique. Vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV for the dipole-forbidden $2{\ensuremath{\sigma}}_{g}\ensuremath{\rightarrow}1{\ensuremath{\pi}}_{g}$ transition of ${\mathrm{N}}_{2}$ as well as for the inner-valence ionization. The generalized oscillator strength thus obtained exhibits characteristic molecular-orientation dependence due to anisotropic shapes of the molecular orbitals participating in the transition. The present method enables one to explore stereodynamics of dipole-forbidden transitions to autoionization states, which are inaccessible by photoabsorption experiments.

Electron excited multiply charged argon ions studied by means of an energy resolved electron-ion coincidence technique

Multiply charged argon ions produced from decay of L-shell hole states by impact of a continuous beam of 3.5 keV electrons are studied for the first time using an energy resolved electron-ion coincidence technique. The TOF spectra of argon ions are measured in coincidence with 18-energy selected electrons emitted in a wide energy range (126–242 eV). The coincidence measurement between the energy selected electrons and the correlated ions specifies the individual decay channel for various multiply charged ions. New experimental data are obtained and reported on the correlation probability for production of argon ions with charge states 1+ to 4+ as a function of ejected electrons in the considered energy range. The relative correlation probability of producing different charge state ions and corresponding physical processes involved in their production are presented and discussed. It has been found that the maximum probability for production of Ar2+ ions correlated to ejected Auger electrons in the energy range of 205–209 eV is 100%. No theoretical predictions are available to compare with these results. The present study shows further that not only the auto-ionization and normal Auger transitions but also several other decay processes including Coster-Kronig transitions followed by Auger cascades with a fraction of shake process play important role in producing ions with charge states 1+ to 4+.

An experimental setup for studying the core-excited atoms and molecules by electron impact using energy analysed electron–ion coincidence technique

Operation and performance of an apparatus for studying the decay dynamics relevant to core–hole decay processes in atoms and molecules excited by energetic electrons using an energy analysed electron–ion coincidence technique are described in some detail. The setup consists of a time- and position sensitive double-field linear TOF mass spectrometer coupled with a dual MCP detector and a single-pass CMA to select the energy of detected electrons. Details of different components involved in the setup are presented and discussed. To demonstrate the performance and capability of the apparatus, we present some typical results extracted from the TOF argon ion-mass spectra observed in coincidence with 18-energy selected electrons emitted from interaction of a continuous beam of 3.5 keV electrons with a dilute gaseous target of argon atoms. Specifically, the variation of relative correlation probability for the final ion-charge states Ar1+ to Ar4+ produced in the considered collision reactions as a function of energy of emitted electrons is determined and discussed.

A merged-beam setup at SOLEIL dedicated to photoelectron–photoion coincidence studies on ionic species

We describe the merged-beam setup permanently installed on a dedicated optical branch of the PLEIADES beamline at SOLEIL, the French synchrotron radiation facility in St-Aubin, delivering photons in the 10–1000eV photon energy range. The setup is designed both for photoion and photoelectron spectroscopy experiments on atomic and molecular ions. Ion spectrometry is dedicated to the determination of absolute single and multiple photoionization cross sections. Electron spectroscopy brings additional information on the non-radiative decay of inner-vacancies produced in the photoionization processes and allows for the determination of partial cross sections. Efficient reduction of the background in the electron spectra is achieved by the use of the electron-ion coincidence technique. Examples of photoion and photoelectron spectra are given for the Xe5+ ion.

Site- and state-selected photofragmentation of 2Br-pyrimidine.

The fragmentation of the 2Br-pyrimidine molecule following direct valence photoionization or inner shell excitation has been studied by electron-ion coincidence experiments. 2Br-pyrimidine has been chosen as a model for the class of pyrimidinic building blocks of three nucleic acids and several radiosensitizers. It is known that the site- and state-localization of energy deposition, typical of inner shell excitation, results in the enhancement of the total ion yield as well as in changes in the relative intensity of the different fragmentation channels. Here we address the question of the origin of this selective fragmentation by using electron-ion coincidence techniques. The results show that the fragmentation is strongly selective in the final singly charged ion state, independently of the process that leads to the population of that state, and the dominant fragmentation patterns correlate with the nearest appearance potential.

Chiral asymmetry in the multiphoton ionization of methyloxirane using femtosecond electron-ion coincidence imaging.

Multiphoton photoelectron circular dichroism (MP-PECD) has been observed as an asymmetry in the angular distribution of photoelectrons emitted in the ionization of pure enantiomers of the small chiral molecule methyloxirane using a femtosecond laser operated at 420 nm. Energetically, this requires the uptake of four photons. By switching the laser between left- and right-circular polarization, and observing the differences in the full three-dimensional electron momentum distribution recorded in an electron-ion coincidence technique, the chiral (odd) terms in the angular distribution expression can be isolated. Electron events can additionally be filtered by coincident ion mass, providing mass-tagged electron distributions and quantitative measures of the MP-PECD asymmetry that help characterize the different ionization channels. For the production of ground state parent cation, the magnitude of the mean chiral asymmetry is measured to be 4.7%, with a peak magnitude exceeding 10%

Doubly excited states of molecular hydrogen by scattered electron-ion coincidence measurements

Electron energy-loss spectroscopy and electron-ion coincidence techniques were used to determine the total generalized oscillator strength distribution (GOSD) of molecular hydrogen (H2) under 200 eV incident electron energy at a scattering angle of 6 degree. Ionic GOSDs were obtained for each formed ion at each energy-loss value and were determined by scattered electron-ion coincidence measurements. The total and partial ionic GOSDs of H2 were compared with the OOSDs from the experimental results of both photon and fast-electron impacts. We observed auto-ionization induced by excitation to the optically forbidden Q 1 1Σ+ g (1) state near 28 eV. Additionally, the non-ionic GOSD, defined as the difference between the total GOSD and the total ionic GOSD, was determined. From the non-ionic GOSD, neutral dissociation from excitation to the forbidden Q 1 1Σ+ g (1) state near 28 eV and the Q 2 series near 35 eV were identified.

Imaging photoelectron circular dichroism of chiral molecules by femtosecond multiphoton coincidence detection

Here, we provide a detailed account of novel experiments employing electron-ion coincidence imaging to discriminate chiral molecules. The full three-dimensional angular scattering distribution of electrons is measured after photoexcitation with either left or right circular polarized light. The experiment is performed using a simplified photoelectron-photoion coincidence imaging setup employing only a single particle imaging detector. Results are reported applying this technique to enantiomers of the chiral molecule camphor after three-photon ionization by circularly polarized femtosecond laser pulses at 400 nm and 380 nm. The electron-ion coincidence imaging provides the photoelectron spectrum of mass-selected ions that are observed in the time-of-flight mass spectra. The coincident photoelectron spectra of the parent camphor ion and the various fragment ions are the same, so it can be concluded that fragmentation of camphor happens after ionization. We discuss the forward-backward asymmetry in the photoelectron angular distribution which is expressed in Legendre polynomials with moments up to order six. Furthermore, we present a method, similar to one-photon electron circular dichroism, to quantify the strength of the chiral electron asymmetry in a single parameter. The circular dichroism in the photoelectron angular distribution of camphor is measured to be 8% at 400 nm. The electron circular dichroism using femtosecond multiphoton excitation is of opposite sign and about 60% larger than the electron dichroism observed before in near-threshold one-photon ionization with synchrotron excitation. We interpret our multiphoton ionization as being resonant at the two-photon level with the 3s and 3p Rydberg states of camphor. Theoretical calculations are presented that model the photoelectron angular distribution from a prealigned camphor molecule using density functional theory and continuum multiple scattering X alpha photoelectron scattering calculations. Qualitative agreement is observed between the experimental results and the theoretical calculations of the Legendre moments representing the angular distribution for the two enantiomers. The electron-ion coincidence technique using multiphoton ionization opens new directions in table-top analytical mass-spectrometric applications of mixtures of chiral molecules.

Valence photodissociation of trifluoroethyl iodide investigated by photoelectron photoion coincidence spectroscopy

Dissociative photoionization of trifluoroethyl iodide (C2H2F3I) molecule has been investigated by electron-ion coincidence technique (PEPICO) and VUV synchrotron radiation. Mass spectra have been recorded for this molecule in the photon energy range of 10–22eV. The molecular ion as well as cationic fragments have been detected and analyzed as a function of photon energy and the main dissociation pathways are proposed. We also performed ab initio calculations for the neutral molecule, its cation and the ion fragments in order to determine electronic and structural parameters.

A multi-coincidence study of the multiple photoionization of CH4 and NH3 molecules

The multiple photoionization of CH4 and NH3 molecules was investigated using monochromated synchrotron radiation and applying time-of-flight mass spectrometry in a multi-coincidence mode. Ion mass spectra were recorded in the photon energy range of 200 to 600 eV, including the C 1s and N 1s edges. Three electron-ion coincidence techniques (PEPICO, PE2PICO, and PE3PICO) were used to in order understand the fragmentation mechanisms of highly excited molecular ions produced following the absorption of a X-Ray photon.

Enhanced charge transfer in doped nano-systems measured via electron-ion coincidence technique

Interatomic energy and charge transfer in a Xe doped Ar cluster have been investigated. Besides interatomic Coulombic decay (ICD), electron transfer mediated decay (ETMD) seems to play a significant role in the relaxation process of heterogeneous systems which is in strong contrast to pristine clusters. Ar 2p electrons have been measured in coincidences with cluster fragments. The high resolution in time and energy of the detectors enabled us to determine the site of ionization on the cluster and the kind of the resulting fragments.

Valence photoionization and the following fragmentation pathways inSb4clusters

The valence photoelectron spectrum of Sb{sub 4} clusters and the following fragmentation patterns have been studied using synchrotron radiation and electron-ion coincidence technique. An experimental photoelectron spectrum of the 5t{sub 2} ionization of Sb{sub 4} is presented. Theoretical molecular calculations, together with the existing data on the noble gas xenon, were used to describe the experimental results. The bonding properties of the molecular orbitals involved are used to qualitatively describe the dissociation process.