Electron Attachment Cross Sections Research Articles

Electron induced processes in CH: an R-matrix study of electronic excitation, dissociation and dissociative attachment

The CH radical is of considerable interest in both low temperature astrophysical plasmas as well as high temperature fusion plasmas. Although collision cross section data for different collision processes are required for modeling the relevant plasma, there are very few studies of electron collision with CH. To this end, we have used the R-matrix method to study electron collision with the CH molecule at low energies. CH target state energies, dipole and quadrupole moments were first determined at its equilibrium geometry R e = 2.116a 0. Scattering calculations were then performed for elastic scattering and electron impact excitation from the ground state to some of the low lying excited states of CH. The excitation cross sections were then used to estimate the cross section for the dissociation of CH under electron impact. An approximate calculation of the cross section for dissociative electron attachment of CH in the energetically lowest channel C−(4So) + H(2S), was also done via the lowest 5Σ− Feshbach resonance at R e = 2.116a 0.

Electron Scattering Cross Sections for Anthracene and Pyrene.

UK molecular R-matrix calculations have been carried out for electron scattering from anthracene and pyrene. These molecules belong to the family of polycyclic hydrocarbons (PAHs) and are found in a nebula known as the Red Rectangle. Static exchange (SE), static exchange plus polarization (SEP), and close coupling (CC) approximations are used for scattering calculations. Different elastic and inelastic cross sections are computed in the present work in the energy range of 0.1-15 eV. Dissociative electron attachment cross sections are also calculated for both the molecules. Various shape, Feshbach/core-excited, and mixed resonances are detected for these molecules below 10 eV. All of the resonances detected in the present study are in agreement with the existing experimental and theoretical results. Due to the complexity of the present targets, electron collision cross sections are essentially unknown and hence most of the results are presented for the first time.

Low energy electron interactions with Iodine molecule (I2)

A theoretical analysis is performed for electron interactions with the Iodine molecule (I2) for incident energies ranging from 0.1 eV to 20 eV. The calculations were carried out using Quantemol-N package, which uses the UK Molecular R-matrix Codes. Electron interactions with the I2 molecule have been studied with several target models in its equilibrium geometry, and the results are reported for the optimized target model. Scattering calculations are performed to provide resonance parameters along with Dissociative Electron Attachment (DEA) Cross-Sections. In addition, the study also focussed on the estimation of various cross-sections such as elastic, electronic excitation, differential, momentum transfer, ionization and total cross-sections. Many of these cross-sections reported here are for the first time for electron interaction with the iodine molecule to the best of our knowledge.

Charge accumulation characteristic on polymer insulator surface under AC voltage in air and C 4 F 7 N/CO 2 mixtures

Charge accumulation has always been a problem for the safe operation of gas insulated switchgear (GIS)/gas insulated transmission line (GIL) both under AC and DC. It is of great significance to investigate the behaviour of surface charge under high voltage. In this study, the charge distribution characteristics and accumulation mechanism on insulator surface in air and heptafluorobutyronitrile/carbon dioxide (C4F7N/CO2) mixtures under AC voltage are studied via the means of surface potential measurement and inversion algorithm, combined with the improved method of controlling the truncated phase of AC voltage. The results show that under the needle–plate electrode structure, charge distribution on the insulator surface presents a three-tier concentric circle structure both in air and C4F7N/CO2 gas mixtures, and the charge composition of the innermost circle is closely related to the truncated phase of AC voltage. Under the same amplitude of voltage, the range of charge distribution on insulator surface in C4F7N/CO2 mixtures is smaller than that in air, with the negative charges domination. It is suggested that the characteristic of charge distribution in C4F7N/CO2 mixtures is related to the larger electron attachment cross-section of C4F7N gas in a wide range of electron energy distribution.

Non-local complex potential theory of dissociative electron attachment: inclusion of two vibrational modes

SynopsisThe nonlocal complex potential theory is developed to treat the dissociative electron attachment to molecules when more than one vibrational mode is allowed in the neutral molecule. Temperature-averaged cross sections for electron attachment to CF3Cl are obtained assuming two vibrational degrees of freedom.

Formation of positive and negative ions in the gas phase of D-ribose by low-energy electrons

SynopsisUsing a hypocycloidal electron spectrometer the total dissociative electron attachment cross section and the ionization cross section for D-ribose molecule were studied in the E<30 eV energy range. A clear structure near zero energy and in the 6.0-9.5 eV range is observed in the dissociative electron attachment cross section. These features appear due to the formation of negative ion fragments of the molecule. The ionization potential of the D-ribose molecule was determined at 9.74 ± 0.02 eV. In the total ionization cross section weak features are observed, which most likely due to the formation of positive ion fragments of the initial molecule.

Low-energy electron scattering by cyanamide: anion spectra and dissociation pathways.

The low-energy anion spectra of cyanamide and its rare tautomer carbodiimide were surveyed with elastic electron scattering calculations. Our assignments differ qualitatively and quantitatively from a previous theoretical report. We support that both tautomers present two π* and two shape resonances, while cyanamide should also display a dipole bound state and a shape resonance. Available dissociative electron attachment measurements have shown several structures for dehydrogenation below 4 eV, but no sharp peaks related to vibrational Feshbach resonances. The absence of these resonances is explained by the lack of a potential barrier for tunneling of the hydrogen atom, despite the coupling between dipole bound and states. We found that the π* resonances initiate the dynamics that lead to hydrogen loss at 1.5, 2.5 and 3 eV. The later two structures arise from the anion states of cyanamide, while carbodiimide should account for the lower-lying one. The rarity of the second tautomer would be offset by its larger dissociative electron attachment cross section, enough to leave a distinct signature in the measured ion yield spectra. Low-energy electrons should thus decompose carbodiimide much more efficiently than cyanamide.

On-the-fly dynamics simulations of transient anions.

A novel theoretical framework for describing the dynamics of transient anions is presented. An ensemble of classical trajectories is propagated on-the-fly, where resonance energies are computed with bound state techniques, and resonance widths are modeled with a combination of bound state and scattering calculations. The methodology was benchmarked against quantum dynamics results for model potential energy curves, and excellent agreement was attained. As a first application, we considered the electron induced dissociation of chloroethane. We found that electron attachment readily stretches the C-Cl bond, which stabilizes the transient anion within ∼10 fs and leads to the release of fast chloride ions. Both magnitude and shape of the computed dissociative electron attachment cross sections are very similar to the available experimental data, even though we found the results to be very sensitive on the accuracy of the underlying methods. These encouraging results place the proposed methodology as a promising approach for studies on transient anions' dynamics of medium sized molecules.

Dissociative Electron Attachment to 2,6- and 2,5-Dihydroxyacetophenone

Two model humic compounds, 2,6- and 2,5-dihydroxyacetophenone, were investigated by negative ion mass spectrometry. Dissociative electron attachment cross-sections and negative ion autodetachment lifetimes were measured in the electron energy region 0–10 eV. Adiabatic electron affinities of the compounds under investigation were evaluated. The results obtained were rationalized using DFT B3LYP/6-31G+(d) quantum-chemical calculations.

Assessment of the self-consistency of electron-THF cross sections using electron swarm techniques: Mixtures of THF–Ar and THF–N2

The pulsed Townsend technique has been used to measure transport coefficients in mixtures of tetrahydrofuran (THF) with N2 and argon. These measurements are the first investigations of swarm transport in gas mixtures with THF, with the drift velocity and effective Townsend ionization coefficient reported for a range of reduced electric fields between 0.23 and 800 Td (1 Td = 10−21 V m2). These transport coefficients are compared with those calculated using a multiterm kinetic theory, using the cross section set developed in our previous studies [N. A. Garland et al., Phys. Rev. A 88, 062712 (2013) and M. J. E. Casey et al., J. Chem. Phys. 147, 195103 (2017)]. The swarm technique of iteratively adjusting cross sections to reproduce experimental transport measurements is subsequently utilized in this study to address the deficiencies in the earlier cross section sets, exposed by the gas mixture measurements. Refinement of the low-energy extrapolation of the quasielastic database cross section and the low-energy extrapolation and magnitude of the dissociative electron attachment cross section are detailed, as well as the adjustments to the two previously proposed neutral dissociation cross sections. These refinements were necessary in order to minimize differences between our measured and calculated transport coefficients.

Calculated cross sections for low energy electron collision with OH

The hydroxyl radical, OH, is an important component of many natural and technological plasmas but there is little available information on processes involving its collisions with low-energy electrons. Low energy electron collisions with OH are studied in the framework of the R-matrix method. Potential energy curves of some of the low lying target states of doublet and quartet symmetry which go to the O(3P)+H(2S), O(1D)+H(2S) and O(1S)+H(2S) asymptotic limits are obtained for internuclear separations between . Scattering calculations are performed at the OH equilibrium geometry to yield cross sections for elastic scattering, electronic excitations from the ground state to the first three excited states of symmetry and for electron impact dissociation of OH. The positions and widths for negative ion resonances in the e–OH system are used to estimate the cross section for dissociative electron attachment to OH which is found to be significant at electron energies about 1.5 eV.

Theoretical study of the low-energy electron-collision cross sections of isomers HOOCl, HOClO and HClOO in gas phase

Cross sections for the elastic electron scattering by HOOCl, HOClO and HClOO isomers are calculated using the R-matrix method. A systematic study considering basis set, polarisation and the Born closure technique is performed. Low-energy resonances are found for both HOOCl and HOClO, near 1.7 and 1.2 eV respectively, but not for HClOO. The lowest-energy resonance for HClOO is observed at 4.5 eV. Basis sets and polarisation effects on the differential cross sections are found to be important for scattering energies below 5 eV for HOOCl and HOClO due to the presence of these resonances. The dependence of the molecular dipole moment on target basis set used affects the integral cross sections (ICS) results. The ICS for HOClO is larger than other two isomers due to its larger dipole moment, around 3.2 D, while the ICS for HOOCl and HClOO have similar magnitude as both their dipole moments are near 1.9 D. Estimated dissociative electron attachment (DEA) cross sections suggest that HOOCl and HOClO will undergo DEA with low-energy electrons but that HClOO requires higher-energy electrons to undergo DEA.

Vibrationally resolved excitation, dissociation, and rotational cross sections of NH radical by electron-impact using the R-matrix method

The ab initio R-matrix method is employed to study the electronic excitation of low-lying target states, the dissociation cross sections, and the rotational cross sections of the NH radical by electron impact for energies up to 15 eV. We have included dynamic interaction in our scattering model that uses close coupling formalism. There are 24 target states whose vertical excitation energies are below 15 eV, which are included in the trial wavefunction of the entire scattering system. In our scattering model, two electrons are frozen in the 1σ2 configuration and the remaining six electrons are free to move in the molecular orbitals 2σ 3σ 4σ 5σ 6σ 1π and 2π. The cc-pVTZ Dunning basis sets are used to optimize the target structure and electron scattering. The vibrationally resolved cross sections for transition from the ground state X 3Σ− to excited state c 1Π are computed by scaling the corresponding electronic excitation cross section by Franck-Condon factors. These factors are evaluated by using accurate potential energy curves of the ground state and the c excited state. We have compared the excitation cross section for the optically allowed transition X 3Σ− to A 3Π computed using our scattering model with the corresponding cross section evaluated in Born approximation, and the agreement is quite good indicating that for a reasonably high transition moment and a low threshold of the excited state, the effects of correlation and polarization are not very significant. The dissociative excitation cross section by electron impact into atomic fragments N and H is also shown which are caused mainly by c 1Π and a purely repulsive state 1 5Σ−. We have also shown that the vibrational cross section evaluated in Born approximation for the fundamental transition 0–1 of the ground state is quite small as compared to the rotational cross sections. The rotational cross sections are computed for j = 0 to j′ = 0–5, and it is shown that the dipole induced transition is predominant with an order of magnitude higher than the pure rotational 0–0 cross section. We have detected many low-lying core-excited shape resonances and provided their resonance parameters which form an important input to estimate dissociative electron attachment cross sections.

Single-center approach for photodetachment and radiative electron attachment: Comparison with other theoretical approaches and with experimental photodetachment data

A single-center method for calculating photodetachment cross section for anions and radiative electron attachment cross section for neutral molecules by microreversibility is presented. It uses the integral equation method to calculate the ejected electron's continuum wave function while the single-electron bound function of the anion is described by the Dyson orbital. It is compared with related theoretical approaches and benchmarked to the experimental photodetachment cross sections of ${{\mathrm{O}}_{2}}^{\ensuremath{-}}$, ${\mathrm{OH}}^{\ensuremath{-}}$, and ${\mathrm{CN}}^{\ensuremath{-}}$. The use of the plane-wave approximation of the ejected electron wave function combined with the Hartree-Fock frozen-core approximation of the Dyson orbital is also considered and its results are compared with those of our methods and with experiment. A good agreement between the calculated photodetachment cross sections and the experimental data is obtained for ${{\mathrm{O}}_{2}}^{\ensuremath{-}}$ and ${\mathrm{CN}}^{\ensuremath{-}}$ when using the three methods. For ${\mathrm{OH}}^{\ensuremath{-}}$, the calculated scattering-wave electron photodetachment cross sections agree well with two most recent sets of experimental data among the three available while the plane-wave results disagree with all the experimental and theoretical data. The different approaches to calculate the Dyson orbital are also discussed as well as the convergence of the calculations with respect to the choice of the one-electron basis set. The approximation of Dyson orbitals by Kohn-Sham orbitals appears to overestimate the photodetachment cross section.

Соударения медленных электронов с молекулами тимина

AbstractUsing a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them ( E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E &lt; 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)^–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

Collisions of Slow Electrons with Thymine Molecules

Using a hypocycloidal electron spectrometer, the total scattering cross section of slow (0–9 eV) electrons and the dissociative electron attachment cross section for thymine molecules in the gas phase were measured. The ionization cross section for a thymine molecule was studied in the energy range of 9–32 eV. Some features were found in the scattering cross section, caused by the formation and decay of short-lived states of the molecular negative ion. Three of them (E = 0.32, 1.71, and 4.03 eV) relate to shape resonances; the others, which are observed for the first time, refer to the Feshbach resonances (or core-excited resonances). In the total dissociative attachment cross section in the energy range of E < 4 eV, a clear structure is observed due to the formation of a negative ion (T–H)–, and a less intense structure associated with the total contribution of fragment thymine ions is found above 4 eV. The correlation of the features found in the total scattering cross section and in the dissociative attachment cross section is assessed. The absolute total scattering cross section was obtained by normalizing the measured curve to the theoretical calculation. In the total ionization cross section, features are observed that are associated both with the effect of the formation of fr-agment ions and with ionization due to the ejection of electrons from the orbitals of the outer shell of the molecule.

Electron collisions with cyanoacetylene HC3N : Vibrational excitation and dissociative electron attachment

We experimentally probe electron collisions with HC$_3$N in the energy range from 0 to 10 eV with the focus on vibrational excitation and dissociative electron attachment. The vibrational excitation cross sections show a number of resonances which are mode specific: the two dominant $\pi^*$ resonances are visible in the excitation of all the vibrational modes, however, broad $\sigma^*$ resonances are visible only in certain bond-stretching vibrational modes. The lower $\pi^*$ resonance shows a pronounced boomerang structure. Since it overlaps with the threshold peak originating from a long-range electron-molecule interaction, the interference pattern is rather unusual. Somewhat surprisingly, the boomerang structure is visible also in the elastic scattering cross section. The dissociative electron attachment cross sections agree qualitatively with the data of Gilmore and Field [J. Phys. B 48 (2015) 035201], however, approximately a factor of two difference is found in the absolute values.

Electron attachment to hexafluoropropylene oxide (HFPO).

We probe the electron attachment in hexafluoropropylene oxide (HFPO), C3F6O, a gas widely used in plasma technologies. We determine the absolute electron attachment cross section using two completely different experimental approaches: (i) a crossed-beam experiment at single collision conditions (local pressures of 5 × 10-4 mbar) and (ii) a pulsed Townsend experiment at pressures of 20-100 mbar. In the latter method, the cross sections are unfolded from the electron attachment rate coefficients. The cross sections derived independently by the two methods are in very good agreement. We additionally discuss the dissociative electron attachment fragmentation patterns and their role in the radical production in industrial HFPO plasmas.

Perfluoro-1,3-dioxolane and perfluoro-oxetane: promising gases for electrical insulation

Perfluorinated cyclic ethers represent an interesting class of molecules to use as dielectric gases for electrical insulation. Measurements of the two compounds 4,4,5,5-tetrachloro-2,2-difluoro-1,3-dioxolane (, CAS 87075-01-2) and octafluoro-1,4-dioxane (, CAS 32981-22-9) in a pulsed Townsend experiment are presented, their ionization and attachment rate coefficients are obtained and their electron attachment cross sections are estimated. Both gases have a high critical electric field strength of about and times the one of SF, respectively, which is presumably due to very high electron attachment cross sections in the range from 2–4 eV. In particular, for the values appear to be very close to the S-wave scattering limit around 3 eV. Based on these results, we recommend the two analogous compounds perfluoro-1,3-dioxolane (, CAS 21297-65-4) and perfluoro-oxetane (, CAS 425-82-1) for further research for the use as insulation and refrigerant gases, since they are chemically comparable, presumably environmentally friendly, and have high vapor pressures.

Electrical insulation properties of the perfluoronitrile C4F7N

The electrical insulation properties of pure C4F7N and of C4F7N/N2 and C4F7N/CO2 mixtures are investigated in a pulsed Townsend setup. The electron rate and transport coefficients and the density-reduced critical electric field of these mixtures are obtained, and a synergy effect is observed in C4F7N/N2 and C4F7N/CO2 mixtures. The total electron attachment cross section of C4F7N is estimated based on the attachment rate to C4F7N in diluted C4F7N/N2, C4F7N/CO2 and C4F7N/O2/CO2 mixtures. Measurements in pure C4F7N at pressures of a few hundred pascal show that ion kinetics play a major role in C4F7N discharges and that further modelling is required to assess the electric strength of C4F7N at high pressures.