Electron-ion Coincidence Research Articles

VUV photodissociation of thiazole molecule investigated by TOF-MS and photoelectron photoion coincidence spectroscopy.

Photoelectron photoion coincidence measurements have been performed for the thiazole (C3H3NS) molecule in gas phase, using time-of-flight mass spectrometry in the electron-ion coincidence mode and vacuum ultraviolet synchrotron radiation. photoelectron photoion coincidence spectra have been recorded as a function of the photon energy covering the valence range from 10 to 21 eV. The resulting photoionization products as well as the dissociation pathways leading to the ionic species were proposed and discussed. We have also performed density functional theory and ab initio calculations for the neutral molecule, its cation and the ion fragments produced in order to determine their electronic and structural parameters.

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.

A photoionization investigation of small, homochiral clusters of glycidol using circularly polarized radiation and velocity map electron–ion coincidence imaging

A detailed study of the valence photoionization of small homochiral glycidol (C3O2H6) clusters is carried out with the help of circularly-polarized VUV synchrotron radiation by recording photoionization-based spectroscopic data detected by velocity map electron imaging with coincidence ion selection. We show that information on the stability of cationic as well as neutral chiral clusters can be obtained with enhanced sensitivity by examining the chiral fingerprint encapsulated in Photoelectron Circular Dichroism (PECD) spectra. In particular, by varying the clustering conditions we demonstrate that the PECD signal effectively carries the signature of the neutral precursor species, prior to any fragmentation of the ion, as may be inferred from the below-threshold monomer measurements (including ion imaging). Here the monomer's direct ionization channel is closed and the monomer ion hence must result exclusively as a fragment from dissociative ionization of the dimer (or higher) clusters. At higher photon energies, the mass-selection on the electron spectroscopy data, achieved through filtering the electron images in coincidence with selected ion masses, evidently succeeds in providing a degree of size-selection on the neutral clusters being ionized with, in particular, a clear differentiation of monomer and dimer PECD, showing the strong sensitivity of this chiroptical effect to the non-local long-range molecular potential.

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.

Carrier-envelope phase stable few-cycle pulses at 400 kHz for electron-ion coincidence experiments

Coincident electron-ion detection after photoionization in a "reaction microscope" is a very powerful tool to study atomic and molecular dynamics. However, the implementation of this tool in the field of attosecond science has so far been rather limited, due to the lack of high repetition rate laser sources capable of delivering few-cycle pulses with sufficient energy per pulse. In this article, the development of a Non-collinear Optical Parametric Amplifier (NOPA) capable of delivering Carrier-Envelope Phase (CEP) stable pulses with sub-6 fs duration and pulse energies in the few-µJ range is presented. The potential of combining the high repetition rate source and a reaction microscope operating at this high frequency is demonstrated in a proof-of-principle experiment on strong field ionization of Ar atoms.

Nonsequential double ionization of N2in a near-single-cycle laser pulse

We present a comparative study of nonsequential double ionization (NSDI) of N${}_{2}$ and Ar exposed to near-single-cycle laser pulses. The NSDI process is investigated using carrier-envelope-phase-tagged electron-ion coincidence spectroscopy. The measured NSDI spectra of N${}_{2}$ and Ar exhibit a striking resemblance. In particular, the correlated two-electron momentum distribution arising from NSDI of N${}_{2}$ also displays a cross-shape very similar to that reported for Ar [Bergues et al., Nat. Commun. 3, 813 (2012)]. We interpret our results in terms of recollision-excitation with subcycle depletion and discuss how this mechanism accounts for the observed similarities and differences in the ionization behavior of the two species.

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.

Attempts to Improve the Sensitivity and the Energy Resolution of an Analyzer for Auger Photoelectron Coincidence Spectroscopy and Electron Ion Coincidence Spectroscopy

Sadanori Arae, Rui Kanemura, Kenta Hiraga, Yosuke Ogashiwa, Kohtaro Yanase, Noritsugu Kanayama, Shinya Ohno, Takuhiro Kakiuchi, Kazuhiko Mase, Koji K. Okudaira, Makoto Okusawa, and Masatoshi Tanaka 1 Yokohama National University, Yokohama 240-8501, Japan 2 Gunma University, Maebashi 371-8510, Japan Chiba University, Inage-ku 263-8522, Japan 4 Ehime University, Matsuyama 790-8577, Japan 5 Photon Factory, Tsukuba 305-0801, Japan The Graduate University for Advanced Studies, Tsukuba 305-0801, Japan

Ionization and Fragmentation of Methane Induced by 40 eV to 480 eV Synchrotron Radiation: From Valence to Beyond Core Electron Ionization

Photoionization and fragmentation of gaseous methane induced by tunable synchrotron radiation were investigated in a wide energy range, from 40 eV up to 480 eV. We report electron-ion coincidence experiments by measuring the relative partial-ion yields and precursor-specific relative yields for individual fragment ions and for ion fragment pairs as a function of photon energy. The fragmentation patterns are discussed with emphasis on the transition behavior of the bond breaking reactions and of the hydrogen rearrangements from valence to core electron ionization. Below the C 1s threshold, a comparison between photon induced dissociation and electron impact data showed that the ionic fragments formation depends for both projectiles on the same final electronic state reached upon ionization.

Dissociation dynamics of doubly charged CO2 and CS2 molecules as studied by electron-ion coincidence spectroscopy and simulations

Auger decay following C 1s core ionization leads to dissociating doubly ionized molecular states of CO2 and CS2. Detailed dynamics of these three- and two-body processes is investigated using electron-ion multicoincidence spectroscopy and Monte-Carlo simulations of various dissociation scenarios.

Doubly excited states of ammonia by scattered electron–ion coincidence measurements

To obtain information on the optically forbidden doubly excited states of ammonia (NH3), we performed scattered electron–ion coincidence measurements. First, we observed scattered electrons using electron energy-loss spectroscopy and determined the generalized oscillator strength distribution (GOSD) under 200 eV incident electron energy at a scattering angle of 8°. Ionic GOSDs were also determined by combination with the coincidence signal, which was observed by the time-of-flight mass spectrometer at each energy-loss value, for each ion. The total and partial ionic GOSDs were compared with the experimental results of both photon and fast electron impact. Moreover, the neutral GOSD determined by subtracting the total ionic GOSD from the total was compared with previous results. In addition to the optically forbidden doubly excited states, which were identified by Kato et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 3541) and Ishikawa et al (2008 J. Phys. B: At. Mol. Opt. Phys. 41 195204), we found a new optically forbidden doubly excited state at around 35 eV.

Experimental and theoretical investigation of the parabanic acid molecule following VUV excitation and photodissociation

Photodissociation experiments have been performed for the parabanic acid (C 3H 2N 2O 3) molecule in vapor phase using time-of-flight mass spectrometry and synchrotron radiation in the VUV photon energy range. Electron ion coincidence (PEPICO) spectra and partial ion yields have been recorded as a function of the photon energy covering the 11–21 eV valence range region. The resulting photoionization products as well as proposed fragmentation pathways leading to those species are presented and discussed. Electronic structure computations for the neutral and ionic species were also carried out at the B3LYP/ aug-cc-pVTZ level of theory.

Electron-ion coincidence study of photofragmentation of the CdCl2 molecule

In this work, the photofragmentation subsequent to valence and Cd4d photoionization of cadmium dichloride (CdCl(2)) were studied using He I and synchrotron excitation. The measurements were performed with a photoelectron-photoion coincidence (PEPICO) setup, and the connection between the singly ionized electronic states and cationic fragments was investigated. The valence-ionized states were found to lead to CdCl(2)(+), Cd(+) and CdCl(+). The Cd4d(- 1) states were found to lead only to Cl(+) ions. The observed charge transfer effect between Cd and Cl was concluded to take place due to internal conversion or fluorescence decay to dissociating valence states either directly or through consecutive fragmentation. The fragmentation energetics were investigated with molecular ab initio calculations, and the calculated energies were found to agree with the detected fragment appearances.

Auger electron spectra of hydrogenated Si(111)-1×1 surface obtained from Si-L23VV Auger electron Si-2p photoelectron coincidence measurements

The Si-L23VV Auger electron Si-2p photoelectron coincidence spectra (Si-L23VV-Si-2p APECS) of a hydrogenated Si(111)-1×1 surface are measured with an analyzer that is used for electron-electron and electron-ion coincidence measurements of surfaces. The Si-L23VV-Si-2p APECS show a sharp peak, a small shoulder, and a broad large peak at electron kinetic energies of about 85, about 80.5, and about 70 eV, respectively. The clarity of the last two peaks is lower in the Si-L23VV-Si-2p APECS of a clean Si(111)-7×7 surface. We attribute the three peaks to the Si-L23V3pV3p, Si-L23V3pV3s, and Si-L23V3sV3s Auger lines, respectively. The three-peak structure indicates that the local density of states of the 3s + 3p band is reduced in the case of the hydrogenated Si(111)-1×1 surface. This finding is consistent with that of a previous study on valence electronic states of hydrogenated Si(111) using ultraviolet photoemission spectroscopy.

A many-body theory of the electron–ion coincidence spectroscopy spectra of the O+ ion desorption from TiO2(1 1 0) surface upon the Ti or O core-electron photoemission

AbstractUsing the charge transfer (CT) core-hole screening model we analyzed the photoelectron–photoion coincidence spectroscopy (PEPICS) spectra of the O+ ion desorption from TiO2(110) surface measured in coincidence with the Ti or O core-electron photoemission. The present theory predicts well the ratio of the PEPICS spectral intensity measured in coincidence with the Ti core photoelectron spectroscopy (PES) satellite to that with the Ti core PES main line. In contrast to the Knotek and Feibelman (KF) mechanism the three localized O 2p holes created by CT Ti core-hole screening outlast the desorption so that the O+ ion desorbs from TiO2(110) surface upon the Ti core-electron ionization. The O 1s PES satellite creates three localized O holes by the O KV-L1VV spectator Auger decay, while the O 1s PES main line creates three localized O holes by the O K-VVV and K-L1VV Auger shakeup decay. As the Auger shakeup decay is very small compared to the spectator Auger decay, the O+ ion desorption yield in coincidence with the O 1s PES shakeup satellite is much enhanced compared to that with the O 1s PES main line.

VUV-induced dissociation of methylchlorosilanes, studied by electron–ion coincidence spectroscopy

Fragmentation patterns of gas-phase trimethylchlorosilane and methyltrichlorosilane molecules were investigated using the electron-energy-resolved photoelectron–photoion coincidence technique. The production of parent ions and the dominant fragments by 30 eV synchrotron radiation was followed as a function of the photoelectron binding energy. Ion appearance energies were determined from these coincident ion yield curves. The fragmentation patterns and their development are discussed in terms of available internal excess energy as well as the molecular orbital character of the parent molecules. A common dissociation scenario is proposed for both molecules, producing methyl- and chlorine-loss fragments at lower excess energies and smaller fragments through second-step processes at sufficiently high energies.

Description and performance of an electron-ion coincidence TOF spectrometer used at the Brazilian synchrotron facility LNLS

This paper reports the characteristics and performance of a Time-of-Flight Mass Spectrometer (TOF-MS) for coincidence measurements between electrons and ions that has been developed jointly in Sweden and Brazil. The spectrometer, used for studies of inner-shell photoexcitation of molecules in the gas-phase, has been optimized by implementing ion and electron lenses to allow the use of relatively small diameter detectors. Simulations were performed to understand the lens performance and they show that ions (electrons) could be collected without angular discrimination with a maximum kinetic energy up to ten (two) times higher than without the lens actions. A rotary vacuum chamber allows the spectrometer axis to be positioned at different angles relative to the polarization vector of the excitation beam. An important characteristic of the apparatus is that the acquisition setup allows a multi-hit capability with 1 ns resolution. Hereby, Photoelectron–Photoion–Photoion Coincidence (PEPIPICO) measurements can be performed on molecules containing two or more atoms of equal mass. A method to obtain experimental detection efficiencies of a single ion and one of one, two or three electrons has been developed. A systematic study of the interaction region has been performed to determine the shape of the photon and gas beams. Measurements on molecular nitrogen demonstrate the spectrometer's ability to resolve fragments with the same charge to mass ratio arriving within only a few ns. Simulations and experimental results of fragmentation of two singly charged cation nitrogen atoms agree, confirming that the spectrometer performance is well understood.

Inner shell excitation, ionization and fragmentation of pyrimidine

The inner shell excitation and ionisation of pyrimidine have been studied at the carbon K edge by near-edge X ray absorption fine structure (NEXAFS) and X ray photoelectron (XPS) spectroscopies. The theoretical predictions of density functional theory (DFT) provide a satisfactory assignment of the complex spectra of this polyatomic molecule. The fragmentation following the C(1s−1)π* excitation has been investigated by resonant Auger electron-ion coincidence spectroscopy, which allows a site and state selective study.

Molecular-frame angular distribution of normal and resonant Auger electrons

Molecular-frame angular distributions (MFADs) of resonant and non-resonant ("normal") Auger electrons were determined using angle-resolved electron-ion coincidence spectroscopy. Here, we concentrate on the MFADs of normal N2 KVV Auger electrons. We conclude that their MFADs are independent of both photon energy and light polarization direction, thus confirming that the two-step model is a good approximation. All threshold and shape-resonance phenomena can be attributed to the absorption process and are irrelevant for the Auger decay in the molecular frame. Distinct differences in the MFADs are found as a function of the Auger final state.

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.