eV Impact Energy Research Articles

Low-lying Negative Ion States Probed in Potassium - Ethanol Collisions.

Dissociative electron transfer in collisions between neutral potassium atoms and neutral ethanol molecules yields mainly OH-, followed by C2H5O-, O-, CH3 - and CH2 -. The dynamics of negative ions have been investigated by recording time-of-flight mass spectra in a wide range of collision energies from 17.5 to 350 eV in the lab frame, where the branching ratios show a relevant energy dependence for low/intermediate collision energies. The dominant fragmentation channel in the whole energy range investigated has been assigned to the hydroxyl anion in contrast to oxygen anion from dissociative electron attachment (DEA) experiments. This result shows the relevant role of the electron donor in the vicinity of the temporary negative ion formed allowing access to reactions which are not thermodynamically attained in DEA experiments. The electronic state spectroscopy of such negative ions, was obtained from potassium cation energy loss spectra in the forward scattering direction at 205 eV impact energy, showing a prevalent Feshbach resonance at 9.36±0.10 eV with character, while a less pronounced contribution assigned to a shape resonance has been obtained at 3.16±0.10 eV. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom have been performed to support the experimental findings.

Collisions of low- to intermediate-energy electrons with acetonitrile

A systematic investigation was carried out on the elastic scattering of low- to intermediate-energy electrons ( = 0.1–1000 eV eV) by acetonitrile (CH3CN). We calculated differential (DCS), integral (ICS), momentum-transfer (MTCS), and total absorption cross sections from 0.1 to 1000 eV energy range using an absorption-effects-implemented ePolyScat-E3 suite of codes, which applies the Schwinger variational method combined with the Padé-approximants technique. Also, the screen-corrected independent atom model was used to compute DCSs, ICSs, and MTCSs from 10–1000 eV impact energies. Present results are compared to other theoretical and experimental data when available. In general, our DCS and MTCS results show a good agreement with available data, however, the ICSs and the resonance features exhibit fair accordance. Our results show two shape-resonance structures: one at about 3.4 eV and a second at about 7 eV. They are labeled as and , and comprise the and scattering channels, respectively.

Methanol Negative Ion Fragmentation Probed in Electron Transfer Experiments.

In this contribution, we report a novel comprehensive investigation on negative ion formation from electron transfer processes mediated by neutral potassium atom collisions with neutral methanol molecules employing experimental and theoretical methodologies. Methanol collision-induced fragmentation yielding anion formation has been obtained by time-of-flight mass spectrometry in the wide energy range of 19 to 275 eV in the lab frame. The negative ions formed in such a collision process have been assigned to CH3O-, OH-, and O-, with a strong energy dependence especially at lower collision energies. The most intense fragment anions in the whole energy range investigated have been assigned to OH- and CH3O-. Additionally, the potassium cation energy loss spectrum in the forward scattering direction at 205 eV impact energy has revealed several features, where the two main electronic states accessible during the collision events have vertical electron affinities of -8.26 ± 0.20 and -10.36 ± 0.2 eV. Quantum chemical calculations have been performed for the lowest-lying unoccupied molecular orbitals of methanol in the presence of a potassium atom, lending strong support to the experimental findings.

Low energy electron scattering from pyridine using a Surko trap and beam

This paper presents measurements of low energy electron scattering from pyridine. The low energy positron beamline at the Australian National University was used for these measurements, with a change in operational parameters allowing for the measurement of electron scattering processes. We have collected data for the low energy total cross section for electron scattering, as well as measurements of the differential cross sections for electrons up to 3 eV impact energy. The operation of the beamline will be briefly outlined and data are compared to R-matrix and Schwinger multichannel theoretical calculations, as well as previous experimental data.

Sputtering of surfaces by ion irradiation: A comparison of molecular dynamics and binary collision approximation models to laboratory measurements

We compare various sputtering simulation methods to experimental results in both the low energy (<1 keV) and high energy (≥1 keV) impact regimes for argon ions impacting a pure copper substrate at normal incidence. Our results indicate that for high energy impacts, both binary collision approximation (BCA) and molecular dynamics methods can be used to generate reasonable predictions for the yield and energy distribution of the sputtered atoms. We also find reasonable agreement between the theoretical and experimental results down to impact energies of 600 eV. However, at 200 eV impact energies, significant discrepancies appear between the experimental and theoretical ejecta energy distributions in the peak position, the width of the energy distribution, and the magnitude of the high energy tail. These discrepancies appear to arise from the experimental results being only for atoms sputtered normal to the substrate surface, whereas the theoretical results are integrated over all 2π solid angles above the surface. Using the BCA code SDTrimSP and limiting the results to only atoms emitted within ±15° of the surface normal brings theory and experiment into reasonable agreement. These results suggest that for low energy impacts, the energy distribution of sputtered atoms is highly dependent on the emission angle of the ejecta.

Sputtering of the beryllium tungsten alloy Be2W by deuterium atoms: molecular dynamics simulations using machine learned forces

Material erosion and fuel retention will limit the life and the performance of thermonuclear fusion reactors. In this work, sputtering, reflection and retention processes are atomistically modeled by simulating the non-cumulative sputtering by deuterium projectiles on a beryllium–tungsten alloy surface. The forces for the molecular dynamics trajectories were machine learned from density functional theory with a neural network architecture. Our data confirms and supplements previous results for simulated sputtering rates. In the non-cumulative scenario we simulate, we did not observe reaction mechanisms leading to swift chemical sputtering. Thus, our sputtering rates at low impact energies are smaller than in comparable non-cumulative studies. The sputtering yields of the Be2W alloy are generally lower than those of pure beryllium. We found a strong dependence of the sputtering yield on the incident angle with an increase by about a factor of 3 for larger incident angles at 100 eV impact energy. In the pristine surface, a large majority of the impacting hydrogen projectiles at perpendicular impact remains in the surface.

Evolution of a Molecular Shape Resonance along a Stretching Chemical Bond.

We report experiments on laser-assisted electron recollisions that result from strong-field ionization of photoexcited I_{2} molecules in the regime of low-energy electron scattering (<25 eV impact energy). By comparing differential scattering cross sections extracted from the angle-resolved photoelectron spectra to differential scattering cross sections from quantum-scattering calculations, we demonstrate that the electron-scattering dynamics is dominated by a shape resonance. When the molecular bond stretches during the evolution of a vibrational wave packet this shape resonance shifts to lower energies, both in experiment and theory. We explain this behavior by the nature of the resonance wave function, which closely resembles an antibonding molecular orbital of I_{2}.

Experimental and theoretical analysis for total electron scattering cross sections of benzene.

Measurements of the total electron scattering cross sections (TCSs) from benzene, in the impact energy range of 1-1000 eV, are presented here by combining two different experimental systems. The first utilizes a magnetically confined electron transmission beam for the lower energies (1-300 eV), while the second utilizes a linear transmission beam apparatus for the higher energies (100-1000 eV). These cross sections have also been calculated by means of two different theoretical methods, the Schwinger Multichannel with Pseudo Potential (SMCPP) procedure, employing two different approaches to account for the polarization of the target for impact energies between 0.1 and 15 eV, and the Independent Atom Model with the Screening Corrected Additivity Rule including Interference effect (IAM-SCAR+I) paradigm to cover the 10-10 000 eV impact energy range. The present results are compared with available theoretical and experimental data, with the level of accord being good in some cases and less satisfactory in others, and some predicted resonances have been identified. In particular, we found a π* shape resonance at 1.4 eV and another feature in the energy region 4.6-4.9 eV interpreted as a π* resonance (2B2g symmetry), which is a mixture of shape and a core excited resonance, as well as a Feshbach resonance at 5.87 eV associated with the 3s (a1g) Rydberg state. A Born-type formula to extrapolate TCS values for energies above 10 000 eV is also given. This study provides a complete set of TCS data, with uncertainty limits within 10%, ready to be used for modeling electron transport applications.

Total electron scattering cross sections from thiophene for the (1-300 eV) impact energy range.

Experimental electron scattering cross sections for thiophene in the impact energy range from 1 to 300 eV have been measured with a magnetically confined electron transmission-beam apparatus. Random uncertainty limits have been estimated to be less than 5%, and systematic errors derived from acceptance angle limitations have also been identified and evaluated. Experimental values are compared with our previous low energy (1-15 eV) R-matrix and intermediate/high energy (15-300 eV) IAM-SCAR+I calculations finding reasonable agreement, within the combined uncertainty limits. Some of the low energy shape and core-excited resonances predicted by previous calculations are experimentally confirmed in this study.

Interactions of low-energy electrons with the FEBID precursor chromium hexacarbonyl (Cr(CO)6).

Interactions of low-energy electrons with the FEBID precursor Cr(CO)6 have been investigated in a crossed electron–molecular beam setup coupled with a double focusing mass spectrometer with reverse geometry. Dissociative electron attachment leads to the formation of a series of anions by the loss of CO ligand units. The bare chromium anion is formed by electron capture at an electron energy of about 9 eV. Metastable decays of Cr(CO)5− into Cr(CO)4−, Cr(CO)4− into Cr(CO)3− and Cr(CO)3− into Cr(CO)2− are discussed. Electron-induced dissociation at 70 eV impact energy was found to be in agreement with previous studies. A series of Cr(CO)nC+ (0 ≤ n ≤ 3) cations formed by C–O cleavage is described for the first time. The metastable decay of Cr(CO)6+ into Cr(CO)5+ and collision-induced dissociation leading to bare Cr+, are discussed. In addition, doubly charged cations were identified and the ration between doubly and singly charged fragments was determined and compared with previous studies, showing considerable differences.

On e− + Mn elastic scattering at ε = 20 eV impact energy

SynopsisCurrent situation relating to 20-eV electron elastic scattering off Mn is discussed and complemented by the author’s calculation. Noticeable discrepancies between available calculated and experimental data for large scattering angles are highlighted. Existing understanding of electron scattering off Mn is, thus, inconclusive. Further probes into the matter are urged.

Postcollision multifragmentation in fullerene-surface impact: Microscopic insights via molecular dynamics simulations.

Postcollision multifragmentation which we have recently observed experimentally in C60 (-)-surface impact is the phenomenon of a delayed multiparticle breakup of a highly collisionally vibrationally excited large molecule/cluster (the precursor species) into several polyatomic fragments, after leaving the surface. In this paper, we show that the molecular dynamics simulations of near-grazing C60 collisions with a gold surface at 300 eV impact energy (very similar to the experimental conditions) successfully reproduce the experimentally observed characteristics of the postcollision multifragmentation process. The calculated mass resolved kinetic energy distributions and the time dependent yield curves of the Cn fragments revealed a precursor mediated, velocity correlated, delayed fragmentation event along the outgoing trajectory, far away from the surface. Most of the large fragments (n ≥ 5) are formed within a time window of 2-20 ps after leaving the surface, corresponding to the vertical distances of 3-30 nm from the surface. Analysis of delay times and actual time duration for multifragmentation reveal that a large part can be described as simultaneous postcollision (delayed) multifragmentation events. The delayed nature of the event seems to be due to an early sequence of structural transformations of the precursor.

SPECTRA, EMISSION YIELDS, CROSS SECTIONS, AND KINETIC ENERGY DISTRIBUTIONS OF HYDROGEN ATOMS FROM H2 – EXCITATION BY ELECTRON IMPACT

ABSTRACT Electron-impact excitation of H2 triplet states plays an important role in the heating of outer planet upper thermospheres. The d 3 &Pgr; u ?> state is the third ungerade triplet state, and the d 3 &Pgr; u ?> – a 3 &Sgr; g &plus; ?> emission is the largest cascade channel for the a 3 &Sgr; g &plus; ?> state. Accurate energies of the d 3 &Pgr; u &minus; ?> (v, J) levels are calculated from an ab initio potential energy curve. Radiative lifetimes of the d 3 &Pgr; u ?> (v, J) levels are obtained by an accurate evaluation of the d 3 &Pgr; u ?> – a 3 &Sgr; g &plus; ?> transition probabilities. The emission yields are determined from experimental lifetimes and calculated radiative lifetimes and are further verified by comparing experimental and synthetic d 3 &Pgr; u ?> – a 3 &Sgr; g &plus; ?> spectra at 20 eV impact energy. Spectral analysis revealed that multipolar components beyond the dipolar term are required to model the X 1 &Sgr; g &plus; ?> – d 3 &Pgr; u ?> excitation, and significant cascade excitation occurs at the d 3 &Pgr; u ?> (v = 0,1) levels. Kinetic energy (E k ) distributions of H atoms produced via predissociation of the d 3 &Pgr; u ?> state and the d 3 &Pgr; u ?> − a 3 &Sgr; g &plus; ?> − b 3 &Sgr; u &plus; ?> cascade dissociative emission are obtained. Predissociation of the d 3 &Pgr; u ?> state produces H atoms with an average E k of 2.3 ± 0.4 eV/atom, while the E k distribution of the d 3 &Pgr; u ?> − a 3 &Sgr; g &plus; ?> − b 3 &Sgr; u &plus; ?> channel is similar to that of the X 1 &Sgr; g &plus; ?> – a 3 &Sgr; g &plus; ?> − b 3 &Sgr; u &plus; ?> channel and produces H(1s) atoms with an average E k of 1.15 ± 0.05 eV/atom. On average, each H2 excited to the d 3 &Pgr; u ?> state in an H2-dominated atmosphere deposits 3.3 ± 0.4 eV into the atmosphere, while each H2 directly excited to the a 3 &Sgr; g &plus; ?> state gives 2.2–2.3 eV to the atmosphere. The spectral distribution of the calculated a 3 &Sgr; g &plus; ?> – b 3 &Sgr; u &plus; ?> continuum emission due to the X 1 &Sgr; g &plus; ?> – d 3 &Pgr; u ?> excitation is significantly different from that of direct a 3 &Sgr; g &plus; ?> excitation.

Momentum imaging of dissociative electron attachment in biologically relevant molecules

SynopsisWe measured dissociative electron attachment in biologically relevant molecules using momentum imaging for negative ions in an apparatus that combines high resolutions of impact energy, fragment mass and fragment momentum. First investigations of the production of NH2−-ions around the A1 resonance at 5.7 eV impact energy show a clear dependence of the distribution of fragment dissociation angles on the projectile energy.

Three-body fragmentation dynamics of CO2q+ (q = 3, 4) induced by electron collision at 500 eV impact energy

SynopsisElectron impact induced three-body fragmentation of CO2q+ (q = 3, 4) is investigated. For CO23+, non-sequential and sequential fragmentation processes are observed. For CO4+2, however, only non-sequential fragmentation processes are observed. The momentum correlations of each dissociative channel is analyzed by Newton diagram.

Molecular alignment resolved (e,2e) cross sections for H2 at 38 eV impact energy

SynopsisFully differential cross sections for electron impact ionization of H2 are determined experimentally using a reaction microscope. For dissociation of the final H2+ ion in its electronic ground state the measurement of the proton momentum allows to conclude on the molecular alignment during the collision. The cross sections obtained are compared to published theoretical data.

Comparison of Experimental and Theoretical Double Differential Cross Sections of CH4at 250 eV Impact Energy

Comparison of Experimental and Theoretical Double Differential Cross Sections of CH₄at 250 eV Impact Energy

The 4p6 electron energy-loss spectrum of Rb atoms

The scattered electron energy-loss spectrum due to the excitation of the 4p5n1l1n2l2 states in rubidium atoms was measured at 25.6 eV impact energy. New optically forbidden non-autoionizing states located between 16.5 and 16.8 eV were observed.

The 1sn1l1n2l2 electron spectra of lithium atoms

The energy-loss spectrum of scattered electrons due to the excitation of the 1sn1l1n2l2 autoionizing states and the ejected-electron spectrum arising from the decay of these states in lithium atoms were measured at scattering angle 54.7° and 80 eV impact energy. A comparative analysis of excitation and decay channels of autoionizing states was performed.

Excitation-autoionization cross section of alkali atoms by electron impact

The excitation-autoionization (EA) cross section of Na atoms was obtained experimentally in the form of normalized intensities of autoionizing spectra measured in the ejected-electron energy region between 25.5 and 33.7 eV for electron impact energies from the 2p6 excitation threshold at 30.77 up to 400 eV. The cross section exhibits the strong resonance behaviour in the near-threshold region and reaches the maximum value of 1.2 × 10−17 cm2 at 200 eV. By assigning the lines in ejected-electron spectra, the relative role of the 2p6 and 2s2 subshells as well as quartet and doublet levels in formation of the EA cross section was estimated. In particular, the autoionizing doublets in 2p53sns(n = 3 − 6), np(n = 3 − 6), nd(n = 3 − 4) configurations provide approximately 80% of the EA cross section at 200 eV impact energy. A comparative analysis of the EA cross sections for whole alkali group Li–Cs indicates that the near-threshold resonance behaviour of all cross sections is caused exclusively by the negative-ion resonances formed on the basis of the levels 1sn1l1n2l2 in Li and np5n1l1n2l2 in Na–Cs atoms. The relative EA contribution to the total single ionization cross section does not exceed 2% in Li and 5% in Na, and reaches 26% in K, 32% in Rb and 39% in Cs atoms.