Low-energy Electron Collisions Research Articles

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Benzonitrile (BZN) and carbon tetrachloride (CCl4) are versatile solvents used as a precursor for the synthesis of many products. As multi-usage molecules, these compounds may be involved in sustainable chemistry processes such as the cold plasma techniques for which the generated electrons are known to be responsible for reactions. Therefore, it is desirable to explore the interaction of low energy electrons with the co-compounds in the gas phase. The production of chlorine and cyanine anions, initiated by the electron collision with CCl4 and BZN, respectively, undergo nucleophilic substitution SN2 reaction with the precursors molecules for the synthesis of chlorobenzene and tricholoacetonitrile. The mechanism of fragmentation of benzonitrile and the synthesis reactions are rationalized by DFT calculations. The yield of the cyanine anion produced from the ion reaction increases with the temperature of the admixture gas, probed in the 25-100 °C temperature range. The present work may contribute to a potential process for the production of chlorobenzene for instance via (cold) plasma techniques.

Low energy electron scattering cross sections of cyanotriacetylene (HC7N) based on ab-initio R-matrix method

Cyanotriacetylene (HC7N) is a linear unsaturated molecule detected in various regions of space. In this present study, we explored the quantum dynamics of low-energy electron collisions with linear cyanopolyyne molecule (HC7N) to identify the locations and structural features of their metastable negative ions (also known as transient negative ions). We employed the R-matrix method for handling low-energy electron scattering studies. To uncover trends among cyanopolyynes molecules, we have also incorporated results from Cyanoacetylene (HC3N) and Cyanodiacetylene (HC5N). Notably, we observed that the number of π* resonances increases and shifts to lower energies as the number of triple bonds increases in higher-order molecules. Our theoretical calculations contain elastic, differential, and momentum cross-sections in the elastic part. Additionally, we calculated excitation and ionization cross-sections in the inelastic regime. This comprehensive theoretical data on electron scattering will be valuable for understanding the molecular chemistry involving this interesting molecule.

Evolution of the resonances of small water cluster anions (H2O) n ⩽19 − in helium droplets upon attachment of electrons

Using a high-resolution electron monochromator, we studied the formation of (H2O) n ⩽19 − cluster ions upon collisions of free low-energy electrons with water clusters embedded in helium droplets. The anion efficiency curves as a function of the initial electron energy were measured for the cluster sizes n = 2–8, 10, 13, 16, and 19. The present experimental results show that the shape of the resonance yields is dependent on the size of the water cluster anion. The results are discussed in terms of the different electron states available for the excess electron from a linear cluster structure to three-dimensional cluster structures as the number of water molecules within the cluster increases.

Theoretical cross sections for electron collisions relevant for ammonia discharges part 1: NH3, NH2, and NH

Besides being the worlds’ most important fertilizer precursor, ammonia could play an important role as hydrogen carrier in a decarbonized future. The efficient production and decomposition (or cracking) of ammonia are essential to this end. An electricity-driven technology of interest for both these processes are non-thermal plasmas. Plasma processes have the advantage of activating—even inert—molecules and initiating chemical reactions through electron collisions, rather than through conventional heating. However, a complete set of low-energy cross section data is not available for the electron collisions with ammonia (NH3) and its radicals, amidogen (NH2) and imidogen (NH). Here, we used the ab initio R-matrix method to determine theoretical cross sections for the low-energy electron collision processes with NH3, NH2, and NH. Additionally, we explored the contribution of the different processes towards dissociation (especially from electronic excited states). Where possible, we compared our theoretical cross section data with experimental data and/or previous recommendations. Lastly, our own recommended cross section data for the electron collisions are presented. Use of this complete set of electron collision data should contribute to a more accurate description of and better insights into the plasma-chemical kinetics behind plasma-assisted ammonia production and decomposition processes.

Electron Scattering from Methyl Formate (HCOOCH3): A Joint Theoretical and Experimental Study.

Elastic low-energy electron collisions with methyl formate have been studied theoretically at the level of various theories. The elastic integral cross section was calculated using Schwinger multichannel and R-matrix methods, in the static-exchange and static-exchange plus polarization levels of approximations for energies up to 15 eV. The absolute total cross section for electron scattering from methyl formate has been measured in a wide energy range (0.2-300 eV) using a 127° electron spectrometer working in the linear transmission configuration. The integral elastic and the absolute total cross sections display a π* shape resonance at around 1.70-1.84 eV, which can be related to the resonance visible for formic acid, and a broad structure located at 7-8 eV, which can be associated to a superposition of σ* shape resonances. Our results were compared with theoretical and experimental results available in the literature and with the results of electron collisions with formic acid. The additivity rule was used to estimate the total cross section of methyl formate and the results agree well with the experimental data.

Low-Energy Electron and Positron Scattering by para-Difluorobenzene.

The Schwinger multichannel method is employed in calculations of the elastic integral and differential cross sections for collisions of low-energy electrons and positrons with para-difluorobenzene (1,4-C6H4F2). The present calculations involving electron scattering have been performed using both static-exchange and static-exchange plus polarization levels of approximation. Our results indicate the presence of three resonances of π*-character in the low-energy region and a fourth resonance, of σ*-character, at higher energy levels. With respect to the positron scattering, the calculations were conducted using the static plus polarization approximation at three different polarization levels. The present calculated electron and positron cross sections show a reasonable agreement with the total cross sections measured by Makochekanwa et al. (J. Phys. B, 2004, 37, 1841). It is also important to highlight that our computed integral cross section for electron scattering indicates the presence of a Ramsauer-Townsend minimum, while the integral cross section for positron scattering indicates the presence of a virtual state.

Calculated cross sections for electron collisions with the BeN molecule

We report a low energy electron collision calculation with the BeN molecule in its X ground state using the molecular R-matrix method at a single geometry, namely, the equilibrium a 0 of the molecule. Using a suitable target model, we have calculated the elastic cross section and cross sections for electron impact excitation in several low-lying excited states of BeN. Cross sections for electron impact dissociation is derived from the electronic excitation cross sections. In addition, molecular data on bound sates of BeN and e-BeN negative ion resonances and widths at the target equilibrium Re that may be relevant for other studies are also provided.

Identifying molecules with high electrical strength

A search is conducted for possible gases with high electrical strength which could replace the widely used SF6 which has high global warming potential (GWP). The possible electrical strength of a molecule is assessed on the basis of low-energy electron collisions with low-energy resonances or weakly bound states taken as a possible indicator of high electrical strength. At the same time the energy of the highest occupied molecular orbital (HOMO) is used to assess the molecules’ GWP. A total of 62 small flourocarbon molecules are considered allowing the influence of different molecular structures (double bonded, triple bonded and cyclic) and the inclusion of different elements (hydrogen, nitrogen and oxygen) on the electrical strength to be assessed. Eight molecules show low-energy resonance and a further four have negative R-matrix poles implying that they support an anionic state. Our calculations suggest that molecules with double bonded structures, especially involving C = N, should have the best electrical strength, followed by cyclic and then triple bonded structures. Calculation on the C3F6−n H n (n = 0, 6) series suggest that introducing H atoms in selected positions can decrease GWP while retaining the electrical strength of pure fluorocarbon gases like C3F6.

Angular distribution of photons emitted in collision of low-energy electrons with noble gases

The angular distribution of photons emitted at interaction of low-energy electrons with atoms of noble gases is investigated. The angular asymmetry of the bremsstrahlung cross section is significant and strongly depends on the electron and photon energies. The effect of polarization radiation is considered. It is shown that this effect is noticeable even below the electroluminescence threshold, in contrast to the generally accepted point of view. In the case of argon, the account for the polarization radiation improves the agreement between the predictions and the available experimental data for the yield of bremsstrahlung photons.

Theoretical study of low-energy electron collision with normal-pentane using R-matrix method

Low-energy electron collisions with normal-pentane (n-C5H12) that initiate the plasma to assist combustion are critical in understanding the underlying physics and chemistries. In the present work, we studied this collisional process using the R-matrix method at the static-exchange plus polarization and close-coupling model levels. The scattering calculation was performed by running the UKRmol+ code using the Quantemol Electron Collision interface to obtain elastic differential, momentum transfer, integral, and electronic excitation cross sections up to 20 eV and ionization cross sections up to 1000 eV. Our computed cross-section data are in better agreement with the available experimental results both regarding the magnitude and shape. We also demonstrated the importance of using a diffuse basis set in describing the scattering due to the Rydberg nature of the lowest unoccupied molecular orbitals of n-C5H12.

Fragmentation dynamics and absolute dissociative electron attachment cross sections in the low energy electron collision with ethanol.

Dissociative electron attachment (DEA) to ethanol has been probed to study fragmentation dynamics using Time-of-Flight (ToF) mass spectrometric technique. Several fragment ions, namely, H-, O-, OH-, C2H3O- and C2H5O- have been observed. Extra effort has been made to detect low mass ions (here, H-). Absolute DEA cross sections for the formation of O- and OH- have been measured for the first time using relative flow technique (RFT). The threshold energy of different dissociation channels has been calculated using density functional theory (DFT) method. By combining the experimental and theoretical data, we found evidence of hydrogen migration in the production of O and C2H3O- ions.

Theoretical study of low energy electron collisions with the BeO molecule

An R-matrix calculation is reported for low energy electron collision with the beryllium oxide (BeO) molecule at its equilibrium bond length a 0 for incident energies below 15 eV. Using a suitably constructed model to represent the BeO target, detailed scattering calculations are performed to obtain cross sections for many collision processes, namely total cross sections for elastic scattering and electronic excitations to some of its low lying states and cross section for electron impact dissociation. Additionally, we have also calculated differential cross sections for electronic excitations from the X ground state to the first two excited states of the target BeO molecule. BeO is likely to be present as an impurity component in the plasma of fusion devices operating with beryllium as first wall material. We therefore expect the present study to be of substantial importance to the fusion community.

Electron impact studies on the imidogen (NH+) molecular ion

Low energy electron collision calculations have been performed on the imidogen molecular ion NH+ at its equilibrium geometry using the R-matrix method. A suitable model is first built to represent the NH+ target ion. With this target model we have performed scattering calculations to obtain cross sections for electronic excitation from the X2Π ground state of NH+ to few of its excited states. The excitation cross sections are then used to approximately obtain the cross section for dissociation for the production of N+ ions. We also report the cross sections for rotational excitation of NH+ in its ground state within the Coulomb–Born approximation.

R-Matrix Calculation of Electron Collisions with Molecular Oxygen in Its Electronically Excited States.

Low-energy electron collisions with the X3Σg- ground state and a1Δg and b1Σg+, the Herzberg states (c1Σu-, A'3Δu, and A3Σu+), and B3Σu- excited states of the O2 molecules are studied using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X3Σg- ground state overall agree well with the available experimental and theoretical results. The electronic (de-)excitation cross sections for the electron impact with the Herzberg states and the B3Σu- state are reported. The value of elastic cross sections for the six excited states decreases with the increment of electron energy, except for the resonance peaks. As the case of excitation from the X3Σg- ground state, the O2- 2Πu resonance makes a dominant contribution to the (de-)excitation cross sections from the a1Δg, b1Σg+, and the Herzberg states. The magnitude of the de-excitation cross sections at the location of the 2Πu resonance from the Herzberg states to the ground state is about 2 to 8 times those of the excitation cross sections for the corresponding excitation transitions. These results should be significant for models of oxygen plasma and the dynamics of the Herzberg states of molecular oxygen in the earth's atmosphere.

Low-energy electron collisions with cubane

Low-energy electron collisions with cubane

Elastic and Inelastic Cross Sections for Low-Energy Electron Collisions with ClF Molecule Using the R-Matrix Method

The ClF molecule belongs to an interhalogen family and is important in laser physics and condensed phase molecular dynamics. The elastic and excitation scattering cross sections are obtained in a fixed nuclei approximation using the UKRmol+ codes based on R-matrix formalism. The scattering calculations were performed in the static-exchange (SE), static-exchange-plus-polarisation (SEP), and close-coupling (CC) models. Three CC models with different target states were employed, namely, the 1-state, 5-states, and 12-states. In the CC model, the target states were represented by configuration interaction (CI) wavefunctions. A good agreement of dipole and quadrupole moments of the ground state was obtained with the experimental values, which indicates a good representation of the target modelling. The study predicted the existence of a shape resonance in the SE, SEP, and 5-states CC models. This resonance vanished in the 12-states CC model. The excitation cross sections from ground to the lowest two excited states were also reported. The elastic differential and momentum transfer cross sections were obtained in the 12-states CC models. The contribution of long-range interactions to elastic scattering was included via Born closure approach. The quantities like collision frequencies and rate coefficients were also presented over a wide range of electron temperatures. The ionization cross sections were computed using the binary-encounter-Bethe (BEB) model. The results were reported in C2v point group representation.

Relativistic B-Spline R-Matrix Calculations for Electron Scattering from Thallium Atoms

The Dirac B-spline R-matrix (DBSR) method is employed to treat low-energy electron collisions with thallium atoms. Special emphasis is placed on spin polarization phenomena that are investigated through calculations of the differential cross-section and the spin asymmetry function. Overall, good agreement between the present calculations and the available experimental measurements is found. The contributions of electron exchange to the spin asymmetry cannot be ignored at low impact energies, while the spin–orbit interaction plays an increasingly significant role as the impact energy rises.

Multi-Basis-Set (TD-)DFT Methods for Predicting Electron Attachment Energies.

The interaction of low-energy electron collisions with molecules may lead to temporary anions via resonant processes. While experimental measurements, e.g., electron transmission spectroscopy or dissociation electron attachment spectroscopy, are efficient to characterize the temporary anions, simulating the electron attachment is still very challenging. Here, we propose a methodology to calculate the resonance energies of the electron attachment using ab initio (TD)-DFT calculations together with two different basis sets: a large basis set with diffuse functions to compute the vertical electron affinity and a smaller one to calculate the excitation energy of the anion. To demonstrate the capabilities and the reliability of this computational approach, 53 resonance energies from 18 molecules are calculated and compared to experimental data.

Formation of negative and positive ions in the radiosensitizer nimorazole upon low-energy electron collisions.

A comprehensive investigation of low-energy electron attachment and electron ionization of the nimorazole radiosensitizer used in cancer radiation therapy is reported by means of a gas-phase crossed beam experiment in an electron energy range from 0 eV to 70 eV. Regarding negative ion formation, we discuss the formation of fifteen fragment anions in the electron energy range of 0 eV-10 eV, where the most intense signal is assigned to the nitrogen dioxide anion NO2 -. The other fragment anions have been assigned to form predominantly from a common temporary negative ion state close to 3 eV of the nitroimidazole moiety, while the morpholine moiety seems to act only as a spectator in the dissociative electron attachment event to nimorazole. Quantum chemical calculations have been performed to help interpreting the experimental data with thermochemical thresholds, electron affinities, and geometries of some of the neutral molecules. As far as positive ion formation is concerned, the mass spectrum at the electron energy of 70 eV shows a weakly abundant parent ion and C5H10NO+ as the most abundant fragment cation. We report appearance energy (AE) measurements for six cations. For the intact nimorazole molecular cation, the AE of 8.16 ± 0.05 eV was obtained, which is near the presently calculated adiabatic ionization energy.

Core-excited resonances initiated by unusually low energy electrons observed in dissociative electron attachment to Ni(II) (bis)acetylacetonate.

Dissociative electron attachment is a mechanism found in a large area of research and modern applications. This process is initiated by a resonant capture of a scattered electron to form a transitory anion via the shape or the core-excited resonance that usually lies at energies above the former (i.e., >3 eV). By studying experimentally and theoretically the interaction of nickel(II) (bis)acetylacetonate, Ni(II)(acac)2, with low energy electrons, we show that core-excited resonances are responsible for the molecular dissociation at unusually low electron energies, i.e., below 3 eV. These findings may contribute to a better description of the collision of low energy electrons with large molecular systems.