E. C. Montenegro

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.

CDW-EIS calculations for multiple ionization of Ne, Ar, Kr and Xe by the impact of H+ and He+, including post-collisional electron emission

We present theoretical single to quintuple ionization cross sections for Ne, Ar, Kr and Xe bombarded by H+ and He+. Post-collisional contributions due to Auger-like processes are taken into account using recent photoionization data. The present continuum distorted wave-eikonal initial state (CDW-EIS) and first Born approximation results are compared with the experimental data available in the energy range of 50–10 000 keV amu−1 for H+ on Ne and Ar, and 50–1000 keV amu−1 for the other cases. In general, the combination of the CDW-EIS with the post-collisional branching ratios describes well the multiple ionization data above 300 keV amu−1, showing a clear tendency to coalesce with the first Born approximation at high energies. The surprising result of this work is the good performance of the first Born approximation which describes rather well the experimental data of double and triple ionization, even in the intermediate energy range (50–300 keV amu−1), where direct ionization is the dominant contribution.

Cross-section measurements for the fragmentation of CHClF2 by electron impact

CFC compounds present in the upper atmosphere have a significant effect on the environment, strongly contributing to the increase of the hole in the ozone layer. Recent studies show that low-energy electron impact is an important process in the dissociation of these molecules, creating atomic chlorine, which breaks down ozone molecules. In this work, the CHClF2 fragmentation by electron impact in the 40–400 eV energy range is measured. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which amounts to around 60% of the total yield. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Multiple ionization of neon induced by Li3+ and C3+ projectiles: influence of projectile screening in the ionization and electron capture channels

Neq + (q = 1,2,3,4) ionization and charge exchange cross sections (total electron capture, single electron capture and transfer ionization) in the collisions with Li3+, with energies between 100 and 900 keV amu−1, and C3+, with energies between 250 and 500 keV amu−1 are reported. Bare Li3+ projectiles give a key benchmark to study the role of projectile screening in collisions involving dressed projectile ions, and the measurements have shown a strong screening effect for all n-fold recoil ion charge states in the ionization channel which, unexpectedly, does not appear for transfer ionization.

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.

Pathways for the release of atomic chlorine from CHClF2 fragmentation by electron impact

Chlorine atoms released in the upper atmosphere from CFC and HCFC compounds when impacted by energetic particles are recognized as the main mechanism for the enlargement of the hole in the ozone layer. A recent study showed a strong indication that the chlorine atom could be released from CHClF2 due to the ionization of either chlorine or fluorine present in the molecule. Investigation of the possible pathways for the dissociation of chlorine from CHClF2 by electron impact has been performed by scanning its relative contribution to Cl production along the 2p fluorine threshold and by measuring the energy distributions of the fragments produced by this collision for energies below and above the 2p fluorine threshold using the new delayed extraction time-of-flight technique. The data presented here provide the kinetic energy distribution of the fragments emerging from the collision and show direct experimental evidence that the release of chlorine can be triggered by the removal either of a chlorine electron or of a non-bonding fluorine electron, a result that should also apply to photoionization.

Time-of-flight spectrometer for absolute measurements of multiply charged ions and ionic molecular fragments in the gas phase

A time-of-flight spectrometer using an extended target gas cell was built to perform absolute measurements of the yields of ionic species in the gas phase produced by charged particles. A detailed description of the spectrometer design and of an independent, absolute calibration procedure is given. The performance of the spectrometer was verified through absolute measurements of multiple ionization of noble gases by charged particles.

Absolute measurements of electron capture cross sections of C3+ from atomic and molecular hydrogen

Absolute measurements of single- and double-electron-capture cross sections by C3+ projectiles on atomic and molecular hydrogen targets were performed for projectile energies between 1.0 and 3.5 MeV for the single- and 1.0 and 2.0 MeV for the double-capture processes. The H / H2 cross section ratios were measured using an absolutely calibrated tungsten-tube furnace for the production of atomic hydrogen. The single-capture data are compared with calculations based on the boundary-corrected first Born approximation, the eikonal approximation and a semiclassical model, presenting a good overall agreement. Calculations for the double capture using an analytical expression, obtained within the independent electron approximation and based on the same semiclassical model, give a reasonable qualitative description of the data.

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.