Electron Excitation Cross Sections Research Articles

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.

Electron-impact excitations of highly charged tungsten ions and polarization study of their successive photon decay

Fully relativistic distorted wave theory has been utilized to study the electron-impact excitation of highly charged tungsten ions viz. Y-like (W35+), Sr-like (W36+), Rb-like (W37+) and Br-like (W39+). Electric dipole transitions pertaining to n = 4 → n′ = 4 states have been considered in these four ions. The electron excitation cross-sections are calculated for these ions in the wide range of electron-impact energies from excitation threshold to 20 keV. The required relativistic bound states wave functions for the initial and excited states of the ions are obtained using GRASP2k code. The accuracy of the computed wave functions is ascertained by comparing the calculated energies and oscillator strengths for the considered transitions with the available theoretical and experimental results. For the direct application of the cross-sections to the plasma modelling our calculated cross-sections are fitted to suitable analytical expressions. Further, the degree of linear polarization of the emitted photons due to the decay of the electron from the excited tungsten ions to their respective ground states is also calculated using density matrix theory. There are no previous calculations available for the cross-sections and polarizations to compare with our results of these ions.

Electron capture and excitation in He+–He(1s2s1,3S) collisions

State-selective single-electron capture and excitation processes in collisions of He+ ions with metastable He(1s2s1,3S) excited atoms are investigated by using the two-center atomic orbital close-coupling method in the energy range 0.5–100 keV/u. The expansion basis includes all the states with n ≤ 9, lmax = 4 on the projectile and all the states with n ≤ 5, lmax = 4 on the target, when treating the electron capture and vice versa, when treating the ionization. One-electron model potentials for the singlet and triplet series of states have been used that reproduce the exact energies within an accuracy of 1%–2%. Spin-resolved electron capture and excitation cross sections to nl final states with n ≤ 4 are presented. The origin of differences in the magnitude and energy behavior of spin-resolved cross sections is discussed. The reported cross section results should be useful in the studies of properties of various laboratory and astrophysical helium plasmas.

Note: Absolute electronic excitation cross sections for 8.5-17.5 eV electron scattering from condensed dimethyl phosphate (DMP).

Note: Absolute electronic excitation cross sections for 8.5-17.5 eV electron scattering from condensed dimethyl phosphate (DMP).

Computation of electron impact scattering studies on benzene

An elaborate theoretical study of electron scattering with benzene over a broad impact energy range is reported in this article. Integrated cross sections such as the electronic excitation cross section, momentum transfer cross section, ionization cross section and the total collision cross section along with the differential elastic cross section are computed at low energy (0.01–20 eV) using ab initio R-matrix employing Quantemol-N. Beyond ionization threshold, the calculations are also carried out using spherical complex optical potential (SCOP) formalism up to 5000 eV. A smooth crossover of the total cross section data is obtained through the two formalisms at the incident energy equal to 13 eV and hence we are able to study the cross sections over a wide energy range. The scattering rate coefficients using the computed elastic cross sections for this target are reported for the first time.

Electron impact elastic and excitation cross-sections of the isomers of C4F6 molecule for plasma modeling

We report the calculations of elastic (along with its symmetry components) and electronic excitation cross sections by electron impact of the three isomers of C4F6, namely, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c-C4F6) belonging to the point groups C2, D3d, and C2v, respectively, using the R-matrix approach. The electron energy range is from 0.01 eV to 12 eV. We have employed the cc-pVTZ basis set for C and F atoms to generate self-consistent field molecular orbitals to construct the target states for all the isomers included in our calculations. All the target states are constructed by including correlation effects in a configuration interaction (CI) approach. The target properties such as vertical excitation energies and dipole moment of all the isomers are in reasonable agreement with the literature values. Differences in the cross sections of these isomers are strongly influenced by the effect of correlation and polarization effects and their geometrical extent. We have included the ground state and many excited states of each isomer in the trial wave function of the entire scattering system. The resulting elastic cross sections are compared with the available experimental results. The agreement is reasonably good for energies above 5 eV. The shape resonances detected at 2.57, 2.95, and 3.20 eV for c-C4F6, 1,3-C4F6, and 2-C4F6 isomers are associated with the negative anion formation of C3F3- as observed in the mass spectrometry experiments. We have also performed 1-state CI calculation for all the isomers that include only the correlated ground state. The position of resonances shifts to lower energies as the number of target states is increased compared to 1-state calculation for all the isomers. The elastic cross section for 2-C4F6 isomer is larger than the other isomers because of its larger spatial extent. The present cross section data are important for plasma simulation and modeling, especially related to fluorocarbon plasma.

Electron-impact excitation of diatomic hydride cations II: OH+ and SH+

R-matrix calculations combined with the adiabatic-nuclei-rotation and Coulomb-Born approximations are used to compute electron-impact rotational rate coefficients for two open-shell diatomic cations of astrophysical interest: the hydoxyl and sulphanyl ions, OH$^+$ and SH$^+$. Hyperfine resolved rate coefficients are deduced using the infinite-order-sudden approximation. The propensity rule $\Delta F=\Delta j=\Delta N=\pm 1$ is observed, as is expected for cations with a large dipole moment. A model for OH$^+$ excitation in the Orion Bar photon-dominated region (PDR) is presented which nicely reproduces Herschel observations for an electron fraction $x_e=10^{-4}$ and an OH$^+$ column density of $3\times 10^{13}$~cm$^{-2}$. Electron impact electronic excitation cross sections and rate coefficients for the ions are also presented.

Electronic excitation cross section in positron scattering by H2 molecules using distorted-wave method

In this work we have applied for the first time the distorted-wave approximation (DWA) combined with Schwinger Variational Iterative Method (SVIM) to describe electronic excitation of H2 molecules by positron collisions. The integral (ICS) and differential (DCS) excitation cross sections for X1Σg+ → B1Σu+ transition of H2 molecule, in the range from near threshold up to 45 eV of positron energies, were reported in static (ST) and static-correlation-polarization (STPOL) levels. Our two-state ICS in DWA-ST level have quantitative agreement with experimental measurement at energies from threshold up to 18 eV and the inclusion of polarization effects increases the cross sections. Comparison with 2-state close-coupling approximation (CCA), 2-state Schwinger Multichannel (SMC), 5-state SMC and 1013-state from Convergent Close-Coupling (CCC) methods are done and is encouraging. The relative steeper drop above 22 eV in experimental ICS was not observed by any theoretical calculations indicating that new measurements would be interesting for this transition in this energy range.

Low energy electron-impact study of AlO using the R-matrix method

This comprehensive study reports the electron-impact on the open shell AlO molecule at low energy (less than 10 eV) using the R-matrix method. We present the elastic (integrated and differential), momentum-transfer, electronic excitation and ionisation cross sections; along with effective collision frequency over a wide electron temperature range (1000–100 000 K). Correlations via a configuration interaction technique are used to represent the target states. Calculations are performed in the static-exchange and 24-target states close-coupling approximation at the experimental bond length of 1.6178 A. We have used different basis sets 6-311G*, double zeta, polarization (DZP), cc-pCVTZ to represent our target states. We have chosen the Gaussian Type Orbitals (GTOs) basis set DZP to represent the atomic orbitals which gave the best one-electron properties of the molecule. The calculated dipole moment (1.713 au), rotational constant (0.641399 cm-1) and the vertical excitation energies are in concurrence with the best available data. The continuum electron is also represented by GTOs and is placed at the center of mass of the molecule. Resonance analysis is carried out to assign the resonance parameters and the parentage of detected resonances by fitting the eigenphase sums to the Breit–Wigner profile. Our study has detected three core-excited shape resonances in the 24-state model. We detect a stable bound state of AlO- of 1 A 1 symmetry having configuration 1σ 2 … 7σ 21π 42π 4 with a vertical electronic affinity value of 2.59 eV which is in good accord with the experimental value of 2.6 ± (0.01) eV. The ionisation cross sections are calculated using the Binary-Encounter-Bethe Model in which Hartree–Fock molecular orbitals at self-consistent level are used to calculate kinetic and binding energies of the occupied molecular orbitals. We include partial waves up to g-wave beyond which Born closure method is employed to obtain converged cross sections.

Electron capture and excitation processes in collisions of H+ with H in finite-temperature quantum plasmas

Electron capture and excitation processes in H+-H collisions imbedded in warm dense quantum plasmas are studied by the two-center atomic orbital close-coupling (TC-AOCC) method in the energy range 1–200 keV. The screened potential which is used to describe the interaction between charged particles incorporates the quantum electron degeneracy, exchange-correlation and finite-temperature gradient correlation effects. The total and state-selective electron capture and excitation cross-sections for this collision system with this potential are calculated in a wide range of plasma densities and temperatures. It is demonstrated that the screening of Coulomb interactions affects the collision dynamics and the magnitude and energy behavior of the excitation and electron capture cross-sections. The results at very low and high temperatures are also compared with those obtained for zero-temperature quantum plasmas and Debye plasmas, respectively.

Cross sections for state-selective electron capture and excitation in He+–H collisions

Electron-capture and excitation processes in He+ +H collisions have been studied by using the full quantum-mechanical molecular orbital close-coupling (QMOCC) and the two-center atomic orbital close-coupling (TC-AOCC) methods in the energy ranges 10-2–5 keV/u and 0.1–100 keV/u, respectively. The spin-selected state-selective cross sections for these processes in the He+ +H collision system are reported for the first time. The spin-averaged total electron capture and excitation cross sections, as well as the emission cross section for Balmer α radiation, are compared with the results of available experimental and previous theoretical results in the overlapping energy ranges.

An experimental and theoretical investigation into the electronically excited states of para-benzoquinone

We report on a combination of experimental and theoretical investigations into the structure of electronically excited para-benzoquinone (pBQ). Here synchrotron photoabsorption measurements are reported over the 4.0–10.8 eV range. The higher resolution obtained reveals previously unresolved pBQ spectral features. Time-dependent density functional theory calculations are used to interpret the spectrum and resolve discrepancies relating to the interpretation of the Rydberg progressions. Electron-impact energy loss experiments are also reported. These are combined with elastic electron scattering cross section calculations performed within the framework of the independent atom model–screening corrected additivity rule plus interference (IAM-SCAR + I) method to derive differential cross sections for electronic excitation of key spectral bands. A generalized oscillator strength analysis is also performed, with the obtained results demonstrating that a cohesive and reliable quantum chemical structure and cross section framework has been established. Within this context, we also discuss some issues associated with the development of a minimal orbital basis for the single configuration interaction strategy to be used for our high-level low-energy electron scattering calculations that will be carried out as a subsequent step in this joint experimental and theoretical investigation.

Electron scattering studies of NO2 radical using R-matrix method

Nitrogen dioxide molecule plays an important role in modeling atmospheric process. It is a toxic gas and considered as an atmospheric pollutant due to its involvement in reactions that produce ground-level ozone. The electron scattering of NO2 molecule has been extensively studied, specifically at intermediate and high energies. The discrepancies between previous theoretical studies and experimental data at low impact energies (below 4 eV) suggest that the in-depth research should be carried out. The target optimized equilibrium geometry is computed at the highly accurate coupled cluster singles, doubles and perturbative triples[CCSD(T)] level in this study. The ab initio R-matrix method is employed to study the integral and momentum transfer cross sections of low-energy electron scattering from NO2 radical up to 10 eV. Two models including static-exchange and close-coupling approximation are used to reveal the dynamic interaction. The electronic excitation cross sections are computed from ground state to seven electronically allowed excited states. All target states whose vertical excitation energies are below 20 eV are included in the close-coupling expansions of the scattering system. In our CC model, six electrons are in the core orbitals 1a1, 2a1 and 1b2, and the remaining 17 electrons are free to occupy the 4a1, 5a1, 6a1, 7a1, 1b1, 2b1, 3b2, 4b2, and 1a2 orbitals. The aug-cc-pVTZ dunning basis sets are used to optimize the target structure and electron scattering. A Born closure procedure is used to account for the contribution of partial waves higher than l=4 to obtain cross sections. Two shape resonances found at 0.76 eV and 1.82 eV in this study are lower than previous theoretical calculations, but the comparisons with other theoretical calculations and experimental data show that the present R-matrix study not only agrees well with the experiments but also corrects the overestimations of total cross sections of some other theoretical data in the very low energy regions. To study the influence of electron correlations, 21, 82 and 107 target electronic configurations are used in the close coupling model calculations, respectively. The comparisons of integrated cross sections indicate that it is very important to include more target electronic configurations to obtain the converged scattering cross sections, which reveals the importance of electron correlations.

Theoretical study of low-energy electron scattering with GeH2

We present a comprehensive study of electron collisions with germylene (GeH2 using the UK molecular R-matrix codes for electron energies ranging from 0.01 to 10 eV. The calculations are performed within the static-exchange, static-exchange-polarization, and 17-state close-coupling approximations. The elastic integral, differential, momentum transfer cross sections and the excitation cross sections from the ground state to the six low-lying electron excited states are presented. We found three Feshbach resonances and one Core-excited resonance. These resonances reveal the probability of anion formation by an electron attachment process and further decay to neutral and negative ion fragments. The electronic and rotational excitation cross sections for e-GeH2 scattering are reported for the first time. The cross-section dataset obtained from the present calculations are expected to be sufficiently accurate and comprehensive for most current modeling applications involving neutral GeH2.

Electron impact excitation with the cascade population of the 4p 5 5s levels of the krypton atom

Data are presented on the direct and effective, i.e., including the cascade population, electronic excitation cross sections of the 4p55s levels of the krypton atom from its ground state by the electron impact. The data are made up of the results available in the literature on this process and the results of the measurements performed by the authors.

Calculation of fully relativistic cross sections for electron excitation of cesium atom and its application to the diagnostics of hydrogen-cesium plasma

Electron impact excitation cross-sections and rate coefficients have been calculated using fully relativistic distorted wave theory for several fine-structure transitions from the ground as well as excited states of cesium atom in the wide range of incident electron energy. These processes play dominant role in low pressure hydrogen-cesium plasma, which is relevant to the negative ion based neutral beam injectors for the ITER project. As an application, the calculated detailed cross-sections are used to construct a reliable collisional radiative (CR) model to characterize the hydrogen-cesium plasma. Other processes such as radiative population transfer, electron impact ionization and mutual neutralization of Cs+ ion with negative hydrogen ion along with their reverse processes are also taken into account. The calculated cross-sections and the extracted plasma parameters from the present model are compared with the available experimental and theoretical results.

Absolute cross sections for electronic excitation of condensed tetrahydrofuran (THF) by 11-16 eV electrons.

Absolute cross section (CS) data on the interaction of low energy electrons with DNA and its molecular constituents are required as input parameters in Monte-Carlo type simulations, for several radiobiological applications. Previously [V. Lemelin et al., J. Chem. Phys. 144, 074701 (2016)], we measured absolute vibrational CSs for low-energy electron scattering from condensed tetrahydrofuran, a convenient surrogate for the deoxyribose. Here we report absolute electronic CSs for energy losses of between 6 and 11.5 eV, by electrons with energies between 11 and 16 eV. The variation of these CSs with incident electron energy shows no evidence of transient anion states, consistent with theoretical and other experimental results, indicating that initial electron capture leading to DNA strand breaks occurs primarily on DNA bases or the phosphate group.

Cross-section studies of cyanoacetylene by electron impact

The present article provides the first comprehensive study of electron scattering from the cyanoacetylene molecule. The results reported here include integral elastic cross-sections over a wide energy range (0.5 to 5000 eV) and differential, momentum transfer and electronic excitation cross-sections at low energies (<10 eV). For the computation of cross-sections over such a wide energy range, two formalisms are employed: the ab initio R-matrix method and the spherical complex-optical-potential method operating at low and high incident energies, respectively. The total ionization cross-section has also been calculated using the complex scattering potential-ionization contribution method and shows excellent agreement with previously reported values. The target parameters and resonance positions predicted by the R-matrix method are in reasonable agreement with, albeit limited, previous data. The resonances identified at low energies are associated to several dissociative electron-attachment channels.

A CF4 based positron trap

All buffer-gas positron traps in use today rely on N2 as the primary trapping gas due to its conveniently placed electronic excitation cross-section. The energy loss per excitation in this process is 8.5 eV, which is sufficient to capture positrons from low-energy moderated beams into a Penning-trap configuration of electric and magnetic fields. However, the energy range over which this cross-section is accessible overlaps with that for positronium (Ps) formation, resulting in inevitable losses and setting an intrinsic upper limit on the overall trapping efficiency of ∼25%. In this paper we present a numerical simulation of a device that uses CF4 as the primary trapping gas, exploiting vibrational excitation as the main inelastic capture process. The threshold for such excitations is far below that for Ps formation and hence, in principle, a CF4 trap can be highly efficient; our simulations indicate that it may be possible to achieve trapping efficiencies as high as 90%. We also report the results of an attempt to re-purpose an existing two-stage N2-based buffer-gas positron trap. Operating the device using CF4 proved unsuccessful, which we attribute to back scattering and expansion of the positron beam following interactions with the CF4 gas, and an unfavourably broad longitudinal beam energy spread arising from the magnetic field differential between the source and trap regions. The observed performance was broadly consistent with subsequent simulations that included parameters specific to the test system, and we outline the modifications that would be required to realise efficient positron trapping with CF4. However, additional losses appear to be present which require further investigation through both simulation and experiment.

Electron capture and excitation processes in H+−H collisions in dense quantum plasmas

Electron capture and excitation processes in proton–hydrogen atom collisions taking place in dense quantum plasmas are studied by employing the two-centre atomic orbital close-coupling (TC-AOCC) method. The Debye–Hückel cosine (DHC) potential is used to describe the plasma screening effects on the Coulomb interaction between charged particles. The properties of a hydrogen atom with DHC potential are investigated as a function of the screening strength of the potential. It is found that the decrease in binding energy of nl levels with increasing screening strength is considerably faster than in the case of the Debye–Hückel (DH) screening potential, appropriate for description of charged particle interactions in weakly coupled classical plasmas. This results in a reduction in the number of bound states in the DHC potential with respect to that in the DH potential for the same plasma screening strength, and is reflected in the dynamics of excitation and electron capture processes for the two screened potentials. The TC-AOCC cross sections for total and state-selective electron capture and excitation cross sections with the DHC potential are calculated for a number of representative screening strengths in the 1–300 keV energy range and compared with those for the DH and pure Coulomb potential. The total capture cross sections for a selected number of screening strengths are compared with the available results from classical trajectory Monte Carlo calculations.