P. Limão-Vieira

Elastic cross sections for electron scattering from GeF4: Predominance of atomic-F in the high-energy collision dynamics

We report absolute differential cross sections (DCSs) for elastic electron scattering from GeF(4). The incident electron energy range was 3-200 eV, while the scattered electron angular range was typically 15°-150°. In addition, corresponding independent atom model (IAM) calculations, within the screened additivity rule (SCAR) formulation, were also performed. Those results, particularly for electron energies above about 10 eV, were found to be in good quantitative agreement with the present experimental data. Furthermore, we compare our GeF(4) elastic DCSs to similar data for scattering from CF(4) and SiF(4). All these three species possess T(d) symmetry, and at each specific energy considered above about 50 eV their DCSs are observed to be almost identical. These indistinguishable features suggest that high-energy elastic scattering from these targets is virtually dominated by the atomic-F species of the molecules. Finally, estimates for the measured GeF(4) elastic integral cross sections are derived and compared to our IAM-SCAR computations and with independent total cross section values.

NCO – , a Key Fragment Upon Dissociative Electron Attachment and Electron Transfer to Pyrimidine Bases: Site Selectivity for a Slow Decay Process

AbstractWe report gas phase studies on NCO– fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO– production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO– upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO– formation in DEA. Site selectivity in the NCO– yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO– ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation. Graphical abstractᅟ

Electron interaction cross sections for CF3I, C2F4, and CFx (x=1-3) radicals

The supply of absolute electron-impact cross sections for molecular targets and radicals is extremely important for developing plasma reactors and testing different types of etching gases. Current demand for such models is high as the industry aims to replace traditional plasma processing gases with less polluting species. New theoretical electron impact cross sections at typical etching plasma energies (sub 10 eV) are presented for the CFx (x=1-3) active radical species in a form suitable for plasma modeling. The available experimental and theoretical data are summarized for two potential feed gases, CF3I and C2F4. This data cover recommended cross sections for electron scattering (total, excitation, momentum transfer, and elastic integral), electron impact dissociation, and dissociative electron attachment, wherever possible. Numerical values are given as tables in the paper and are also placed in the electronic archive. (c) 2006 American Institute of Physics.

The role of pyrimidine and water as underlying molecular constituents for describing radiation damage in living tissue: A comparative study

Sem PDF. This research was supported by the Australian Research Council (ARC) through its Centres of Excellence Program. D.B.J. thanks the ARC for provision of a Discovery Early Career Researcher Award. We also acknowledge the support of the Spanish Ministerio de Economia y Competitivad under Project No. FIS 2012-31230 and the European Union COST Actions (MP1002 and CM1301). P.L.V. acknowledges the Portuguese Foundation for Science and Technology (FCT-MEC) through research grants PTDC/FIS-ATO/1832/2012, UID/FIS/00068/2013, and SFRH/BSAB/105792/2014. P.L.V. also acknowledges his Visiting Professor position at Flinders University, Adelaide, South Australia.

Electron collisions with phenol: Total, integral, differential, and momentum transfer cross sections and the role of multichannel coupling effects on the elastic channel

We report theoretical and experimental total cross sections for electron scattering by phenol (C6H5OH). The experimental data were obtained with an apparatus based in Madrid and the calculated cross sections with two different methodologies, the independent atom method with screening corrected additivity rule (IAM-SCAR), and the Schwinger multichannel method with pseudopotentials (SMCPP). The SMCPP method in the Nopen-channel coupling scheme, at the static-exchange-plus-polarization approximation, is employed to calculate the scattering amplitudes at impact energies ranging from 5.0 eV to 50 eV. We discuss the multichannel coupling effects in the calculated cross sections, in particular how the number of excited states included in the open-channel space impacts upon the convergence of the elastic cross sections at higher collision energies. The IAM-SCAR approach was also used to obtain the elastic differential cross sections (DCSs) and for correcting the experimental total cross sections for the so-called forward angle scattering effect. We found a very good agreement between our SMCPP theoretical differential, integral, and momentum transfer cross sections and experimental data for benzene (a molecule differing from phenol by replacing a hydrogen atom in benzene with a hydroxyl group). Although some discrepancies were found for lower energies, the agreement between the SMCPP data and the DCSs obtained with the IAM-SCAR method improves, as expected, as the impact energy increases. We also have a good agreement among the present SMCPP calculated total cross section (which includes elastic, 32 inelastic electronic excitation processes and ionization contributions, the latter estimated with the binary-encounter-Bethe model), the IAM-SCAR total cross section, and the experimental data when the latter is corrected for the forward angle scattering effect [Fuss et al., Phys. Rev. A 88, 042702 (2013)].

A study of electron interactions with silicon tetrafluoride: elastic scattering and vibrational excitation cross sections

We report absolute differential cross sections for elastic electron scattering from SiF4. The incident electron energy range is 1.5–200 eV, while the scattered electron angular range was from 15° to 150°. The absolute scale of the differential cross sections was set using the relative flow technique with helium as the reference species. Corresponding integral elastic and momentum transfer data have been derived from our differential measurements. As a part of this study, independent atom model calculations were also performed. Those computations were found to be in generally quite good accord with the experimental results at both the differential and integral levels. In addition, excitation function measurements at a scattering angle of 60° and in the energy range 1–14 eV for the ν3 (d-stretch) and ν4 (d-deformation) fundamental vibrational modes are reported. Finally, differential cross sections for all four fundamental modes (ν1, ν2, ν3, ν4) at the single electron energy of 7 eV are given. These vibrational excitation measurements, when coupled with the elastic angular distributions in the 5–8 eV energy range, demonstrate the presence of a t2-symmetry shape resonance.

The electronic states of pyrimidine studied by VUV photoabsorption and electron energy-loss spectroscopy

The electronic state spectroscopy of pyrimidine C(4)H(4)N(2) has been investigated using both high resolution VUV photoabsorption in the energy range 3.7 to 10.8 eV (335 to 115 nm) and lower resolution electron energy loss in the range 2 to 15 eV. The low energy absorption band, assigned to the (pi*) <-- 7b(2)(n(N)) (1(1)B(1)<-- 1(1)A(1)) transition, at 3.85(4) eV and the vibrational progressions superimposed upon it have been observed for the first time, due to the availability of a high-resolution photon beam (0.075 nm), corresponding to 3 meV at the midpoint of the energy range studied. Vibronic coupling has been shown to play an important role dictating the nature of the observed excited states, especially for the lowest (1)B(1) state. The 2(1)B(1) state is proposed to have its origin at 7.026 eV according to the vibrational excitation reported in this energy region (7.8-8.4 eV). New experimental evidence of 4(1)A(1) state with a maximum cross section at 8.800 eV is supported by previous ab initio quantum chemical calculations. Rydberg series have been assigned converging to the three lowest ionisation energy limits, 9.32 eV ((2)B(2)), 10.41 eV ((2)B(1)) and 11.1 eV ((2)A(1) + (2)A(2)) with new members reported for the first time and classified according to the magnitude of the quantum defects (delta). Additionally, the absolute differential cross section for inelastic electron scattering has been measured for the most intense band from 6.9 to 7.8 eV assigned to (1)pipi* (3(1)A(1) + 2(1)B(2)).

Negative ion formation in potassium–nitromethane collisions

Ion-pair formation in gaseous nitromethane (CH(3)NO(2)) induced by electron transfer has been studied by investigating the products of collisions between fast potassium atoms and nitromethane molecules using a crossed molecular-beam technique. The negative ions formed in such collisions were analysed using time-of-flight mass spectroscopy. The six most dominant product anions are NO(2)(-), O(-), CH(3)NO(2)(-), OH(-), CH(2)NO(2)(-) and CNO(-). By using nitromethane-d(3) (CD(3)NO(2)), we found that previous mass 17 amu assignment to O(-) delayed fragment, is in the present experiment may be unambiguously assigned to OH(-). The formation of CH(2)NO(2)(-) may be explained in terms of dissociative electron attachment to highly vibrationally excited molecules.

Differential cross sections for the electron impact excitation of pyrimidine

We report on differential cross section (DCS) measurements for the electron-impact excitation of the electronic states of pyrimidine. The energy range of the present measurements was 15-50 eV with the angular range of the measurements being 10°-90°. All measured DCSs displayed forward-peaked angular distributions, consistent with the relatively large magnitudes for the dipole moment and dipole polarizability of pyrimidine. Excitations to triplet states were found to be particularly important in some energy loss features at the lower incident electron energies. To the best of our knowledge there are no other experimental data or theoretical computations against which we can compare the present results.

Electron transfer-induced fragmentation of thymine and uracil in atom-molecule collisions

Ion-pair formation has been studied in hyperthermal (30-100 eV) neutral potassium collisions with gas phase thymine (C(5)H(6)N(2)O(2)) and uracil (C(4)H(4)N(2)O(2)). Negative ions formed by electron transfer from the alkali atom to the target molecule were analysed by time-of-flight (TOF) mass spectrometry. The most abundant product anions are assigned to CNO(-) and (U-H)(-)/(T-H)(-) and the associated electron transfer mechanisms are discussed. Special emphasis is given to the enhancement of ring breaking pathways in the present experiments, notably CNO(-) formation, compared with free electron attachment measurements.