L. Sigaud

Absolute total and partial dissociative cross sections of pyrimidine at electron and proton intermediate impact velocities.

Absolute total non-dissociative and partial dissociative cross sections of pyrimidine were measured for electron impact energies ranging from 70 to 400 eV and for proton impact energies from 125 up to 2500 keV. MOs ionization induced by coulomb interaction were studied by measuring both ionization and partial dissociative cross sections through time of flight mass spectrometry and by obtaining the branching ratios for fragment formation via a model calculation based on the Born approximation. The partial yields and the absolute cross sections measured as a function of the energy combined with the model calculation proved to be a useful tool to determine the vacancy population of the valence MOs from which several sets of fragment ions are produced. It was also a key point to distinguish the dissociation regimes induced by both particles. A comparison with previous experimental results is also presented.

Strong Electronic Selectivity in the Shallow Core Excitation of the CH2Cl2 Molecule.

The photoexcitation and multiphotoionization of the dichloromethane molecule have been studied for photons with energies from 100 eV to the Cl 2p edge, using the time-of-flight multicoincidence technique and synchrotron radiation. The electronic de-excitation gives rise to one to three electrons and an ionic molecule that decays onto smaller moieties through several fragmentation channels. To discern the channels, sets of fragments have been dispersed in time, measured in coincidence, and recorded as a function of incident photon energy. The chlorine ion, Cl(+), has the highest intensity around and above the Cl 2p edge, while the CHnCl(+) ion, corresponding to the loss of one neutral chlorine atom, dominates the mass spectra in the valence region. In addition, strong electronic selectivity has been observed for the core-excited molecule.

Absolute cross sections for O2 dication production by electron impact.

Direct detection of homonuclear diatomic dications using mass spectrometry has the intrinsic inability to distinguish between fragments with the same mass-to-charge ratio, as is the case of the oxygen molecule. In this work, absolute cross sections for the double ionization of the homoisotopic (16)O2 molecule by electron impact, in the 30-400 eV energy range, is reported for the first time, and show significant discrepancies with previous results, obtained with the heteroisotopic (16)O(17)O. The measurements suggest that O2 (++) is mainly produced through post-collisional Auger-like deexcitation.

Kinetic Energy Release of the Singly and Doubly Charged Methylene Chloride Molecule: The Role of Fast Dissociation

The center of mass kinetic energy release distribution (KERD) spectra of selected ionic fragments, formed through dissociative single and double photoionization of CH2Cl2 at photon energies around the Cl 2p edge, were extracted from the shape and width of the experimentally obtained time-of-flight (TOF) distributions. The KERD spectra exhibit either smooth profiles or structures, depending on the moiety and photon energy. In general, the heavier the ionic fragments, the lower their average KERDs are. In contrast, the light H(+) fragments are observed with kinetic energies centered around 4.5-5.5 eV, depending on the photon energy. It was observed that the change in the photon energy involves a change in the KERDs, indicating different processes or transitions taking place in the breakup process. In the particular case of double ionization with the ejection of two charged fragments, the KERDs present have characteristics compatible with the Coulombic fragmentation model. Intending to interpret the experimental data, singlet and triplet states at Cl 2p edge of the CH2Cl2 molecule, corresponding to the Cl (2p → 10a1*) and Cl (2p → 4b1*) transitions, were calculated at multiconfigurational self-consistent field (MCSCF) level and multireference configuration interaction (MRCI). These states were selected to form the spin-orbit coupling matrix elements, which after diagonalization result in a spin-orbit manifold. Minimum energy pathways for dissociation of the molecule were additionally calculated aiming to give support to the presence of the ultrafast dissociation mechanism in the molecular breakup.

Molecular fragmentation by electron impact investigated using a time delayed spectroscopic technique

A new technique based on time-of-flight mass spectrometry with delayed extraction, in which it is possible to determine the amount of ions with a given energy distribution, is presented. Unlike most techniques designed to measure the energy distribution of molecular fragments, its sensitivity is higher in the low range of the energy distribution, for ions with kinetic energy from thermal energy up to 3 eV. In this progress report we show that this technique is able to open new perspectives for mass spectrometry based on the well established TOF technique, presenting some selected results for collisions of electrons with energy between 15 eV and 400 eV with N2, O2 and CHClF2, which have interesting features involving their low energy fragments.

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.

A novel double-focusing time-of-flight mass spectrometer for absolute recoil ion cross sections measurements

In this work, the inclusion of an Einzel-like lens inside the time-of-flight drift tube of a standard mass spectrometer coupled to a gas cell-to study ionization of atoms and molecules by electron impact-is described. Both this lens and a conical collimator are responsible for further focalization of the ions and charged molecular fragments inside the spectrometer, allowing a much better resolution at the time-of-flight spectra, leading to a separation of a single mass-to-charge unit up to 100 a.m.u. The procedure to obtain the overall absolute efficiency of the spectrometer and micro-channel plate detector is also discussed.

Three-Body Fragmentation from Single Ionization of Water by Electron Impact: The Role of Satellite States

Ionization of water with energy transfers close to the 2a1 inner valence orbital is accompanied by a fast electronic rearrangement driven by electron relaxation and electron-electron correlations. Quasi-degenerate single vacancy and satellites one-electron-two-vacancies excited states in outer shells are created and leave their fingerprints in the three-body fragmentation pattern. Single vacancy states have been associated with Auger decay and double ionization. Satellite excited states are here convincingly assigned to single ionization. We focus on the H0 + O+ + H0 fragmentation by electron impact taking advantage of the high sensitivity of the delayed extraction time-of-flight technique to uncover kinematic attributes of suprathermal O+ ions. It is found that the H0 ejections occur under a large angular rearrangement, in an approximate linear geometry, and leaving the O+ near at rest, with ∼50% of the O+ produced in water fragmentation by swift electrons presenting this feature.

Ionization and fragmentation of pyrimidine and pyridazine induced by proton and electron impact

The ionization and fragmentation of pyrimidine and pyridazine have been studied experimentally, by recording the time of flight mass spectra, and theoretically, by calculating the valence shell MOs ionization cross sections. The absolute total non-dissociative and dissociative cross section has been measured between 70 to 400 eV for electron impact on pyrimidine and from 250 to 2500 keV for proton impact on both molecules. The partial yields and the absolute cross sections measured as a function of the energy combined with the model calculation allows us to suggest the vacancy population of the valence MOs from which several sets of fragment ions are produced.

Energy distribution of slow fragments, OH+ and O+, in water dissociation by electron impact

In this work, we present measurements of the energy distributions of OH+ and O+ produced in water molecule fragmentation by electron impact. We scanned the electron beam energy between 28 eV and 800 eV, examining the processes that occur from low energy transfer, below the double ionization threshold, up to processes occurring at the Oxygen K edge. The results show very different behaviors for the OH+ and O+ energy distributions for different electron energies.