Vibrational Excitation Cross Sections Research Articles

Prediction of a complete and self-consistent collision cross-section set of C5F10O by data-driven method

The lack of a complete and consistent set of electron–molecule collision cross sections for the new eco-friendly gas C5F10O hinders the study of its microscopic discharge mechanisms. In this paper, we first calculate the elastic collision and electronic excitation cross-sections of C5F10O based on R-matrix theory and estimate its total attachment cross-sections through pulsed Townsend measurement. By combining cross sections from other literature, we are able to compile an initial set of collision cross-sections for C5F10O. However, this initial set remains incomplete and requires refinement. Therefore, this work establishes a neutral network to solve the inverse swarm problem of deriving cross-sections from swarm parameters. The inversion model is trained using a substantial amount of collision cross-sections from the LXCat project. Using the model and the initial set for C5F10O, a complete and self-consistent collision cross-section set for the C5F10O gas is predicted for the first time, based on the measured electron swarm parameters of C5F10O/Ar mixtures. The proposed method is capable of predicting C5F10O’s unknown vibrational excitation cross-section without any prior knowledge, thus enhancing its completeness. The refined set can reproduce the electron swarm parameters within an acceptable range of uncertainty, thus verifying its self-consistency. The set will be made available in the LXCat database, which is expected to be significant for fundamental studies of its discharge mechanisms as well as applications of C5F10O.

Integral Electron Scattering Cross Sections from N2O for Impact Energies Ranging from 1 to 1000 eV.

Accurate total cross sections (TCS), within 5%, for electron scattering by N2O molecules have been measured with a magnetically confined electron transmission apparatus for impact energies ranging from 1 to 200 eV. For higher energies, these measurements have been complemented with our independent atom-based screening corrected additivity rule, including interference (IAM-SCAR + I) method to determine a complete reference TCS data set in the energy range (1-1000 eV). After a critical discussion that includes our calculated integral elastic and ionization cross sections and the theoretical and experimental data available in the literature, a complete set of integral elastic and inelastic (rotational, vibrational, and electronic excitation, ionization and electron attachment) cross sections, consistent with the reference TCS data, have been derived. This update on the N2O collisional database may help to improve the accuracy of radiation-induced transport models.

Inelastic scattering of Li＋ with CO: New ab initio potential energy surface and quantum dynamics

The interaction potential between Li＋ and CO has been investigated using quantum dynamical calculations. The ab initio ground electronic potential energy surface has been computed using the aug-cc-pVQZ basis set at MRCI level of accuracy. Quantum dynamics results within the VCC-RIOSA framework have been presented at collision energy Ec.m. = 3.64 eV. The computed differential cross sections (DCS) for the inelastic vibrational excitations have been found to be in qualitative agreement with those obtained in the experiments of Böttner et al. (1976). Other attributes like rotationally-summed state-to-state DCS, and integral cross sections, are also calculated. The rotational excitations are studied using rigid-rotor coupled-states methodology at Ec.m. = 4.23 eV showing excitations up to j′ = 40. Rotational excitations rates at very low collision energy range (Ec.m. = 0–300 cm−1) using rigid-rotor full close-coupling method have also been calculated in view of their astrophysical significance.

Electron scattering cross sections from NH3: a comprehensive study based on R-matrix method

In this study, a comprehensive cross-sectional computation on the electron scattering from NH3 was performed using the frame of the R-matrix method. Cross section sets for total elastic, differential, momentum transfer, total ionization, and electronic excitation are presented. The electron-impact dissociation of NH3 to NHH and NH+H2 was considered by summing up the cross sections for the molecular dissociative excitations to the correlated dissociation channels. The reported cross sections for vibrational excitation of NH3 remain controversial and are unable to distinguish the N–H symmetric and asymmetric stretching modes of close frequencies. Here, the excitation cross section for each vibrational mode of NH3 was computed by applying the theoretical approach combining the fixed-nuclei R-matrix method, normal mode approximation and the vibrational frame transformation. The uncertainties of the cross section sets computed in this study were estimated for convenience of use in the plasma modeling.

Complete collision data set for electrons scattering on molecular hydrogen and its isotopologues: III. Vibrational excitation via electronic excitation and radiative decay

We present state-resolved cross sections for vibrational excitation via electronic excitation followed by radiative decay (ERD), for electrons scattering on all bound vibrational levels of the ground electronic state (X1Σg+) of molecular hydrogen and its isotopologues (H2, HD, HT, D2, DT and T2). We consider excitation of the singlet n=2–3 states (where n refers to the united-atoms-limit principle quantum number) and account for all possible decay pathways back to the bound vibrational levels of the ground electronic state (X1Σg+) to produce an estimate for the ERD cross sections. A selection of the results are presented in graphical form and a full data set is provided as both numerical values and analytic fit functions in supplementary data files. The uncertainty in the cross sections is estimated to be 11%, except for scattering on the highest bound vibrational level of HD, HT, D2, and DT, where the uncertainty is 21%. The data can be downloaded from the MCCC database at mccc-db.org.

On the Role of Molecular Polarizability in Positron Coupling to Vibrations in Homonuclear Diatomics

In a previous work [Poveda, Varella, and Mohallem (Poveda et al., Atoms, 2021, 9: 64) it was shown that the bell-like shape of the 0 → 1 vibrational excitation cross section of H2 as a function of the incoming positron energy, with its characteristic sharp onset at threshold, can be accounted for by a simple model which couples the positron to the vibrational mode of the molecule, throught the behavior of the target polarizabitity with the internuclear bond distance. The study, carried out via time-dependent wave-packet dynamics propagation, relies on a two-dimensional potential energy surface involving just the scattering (positron-target) and vibrational (target) coordinates. Here the model is extended to the full three-dimensional configuration space of the positron-diatomic complex, with the cross sections computed within a time-independent close-coupling approach. The present results confirm the previous findings, shedding light on the mechanisms through which a low-energy positron couples to the molecular vibrations.

Vibrational excitation cross sections for non-equilibrium nitric oxide-containing plasma

A full set of vibrationally-resolved cross sections for electron impact excitation of NO(X2Π, v) molecules is calculated from ab initio molecular dynamics, in the framework of the local-complex-potential approach. Electron–vibration energy exchanges in non-equilibrium thermodynamic conditions are studied from a state-to-state model accounting for all electron impact excitation and de-excitation processes of the nitric oxide vibration manifold, and it is shown that the calculated vibration relaxation times are in good agreement with the experimental data. The new vibrational excitation cross sections are used in a complete electron impact cross section set in order to obtain non-equilibrium electron energy distributions functions and to calculate electron transport parameters in NO. It is verified that the new cross sections bring a significant improvement between simulations and experimental swarm data, providing an additional validation of the calculations, when used within the complete set of cross sections investigated in this work.

Cross sections for vibrational excitation and dissociative recombination of the ion in collisions with low-energy electrons

Cross sections for the vibrational excitation and dissociative recombination (DR) of the ion in collisions with electrons at low scattering energies are computed using a previously-developed approach combining the normal mode approximation for the vibrational states of the target ion and the UK R-matrix code for the evaluation of the scattering matrices at fixed geometries. The obtained cross section for the DR shows excellent agreement with the experimental data from the ASTRID storage ring. Thermally-averaged rate coefficients are obtained from the cross sections for temperatures 10–3000 K.

Third-order transport coefficients for electrons in N2 and CF4: effects of non-conservative collisions, concurrence with diffusion coefficients and contribution to the spatial profile of the swarm

Using a multi-term solution of the Boltzmann equation and Monte Carlo simulation technique we study behaviour of the third-order transport coefficients for electrons in model gases, including the ionisation model of Lucas and Saelee and modified Ness–Robson model of electron attachment, and in real gases, including N2 and CF4. We observe negative values in the E/n 0-profiles of the longitudinal and transverse third-order transport coefficients for electrons in CF4 (where E is the electric field and n 0 is the gas number density). While negative values of the longitudinal third-order transport coefficients are caused by the presence of rapidly increasing cross sections for vibrational excitations of CF4, the transverse third-order transport coefficient becomes negative over the E/n 0-values after the occurrence of negative differential conductivity. The discrepancy between the two-term approximation and the full multi-term solution of the Boltzmann equation is investigated for electrons in N2 and CF4. While the accuracy of the two-term approximation is sufficient to investigate the behaviour of the third-order transport coefficients in N2, it produces large errors and is not even qualitatively correct for electrons in CF4. The influence of implicit and explicit effects of electron attachment and ionisation on the third-order transport tensor is investigated. In particular, we discuss the effects of attachment heating and attachment cooling on the third-order transport coefficients for electrons in the modified Ness–Robson model, while the effects of ionisation are studied for electrons in the ionisation model of Lucas and Saelee, N2 and CF4. The concurrence between the third-order transport coefficients and the components of the diffusion tensor, and the contribution of the longitudinal component of the third-order transport tensor to the spatial profile of the swarm are also investigated. For electrons in CF4 and CH4, we found that the contribution of the component of the third-order transport tensor to the spatial profile of the swarm between approximately 50 Td and 700 Td, is almost identical to the corresponding contribution for electrons in N2. This suggests that the recent measurements of third-order transport coefficients for electrons in N2 may be extended and generalized to other gases, such as CF4 and CH4.

Electron Scattering Cross-Section Calculations for Atomic and Molecular Iodine

Cross sections for electron scattering from atomic and molecular iodine are calculated based on the R-matrix (close-coupling) method. Elastic and electronic excitation cross sections are presented for both I and I2. The dissociative electron attachment and vibrational excitation cross sections of the iodine molecule are obtained using the local complex potential approximation. Ionization cross sections are also computed for I2 using the BEB model.

Vibrational Excitation Cross-Section by Positron Impact: A Wave-Packet Dynamics Study

The vibrational excitation cross-section of a diatomic molecule by positron impact is obtained using wave-packet propagation techniques. The dynamics study was carried on a two-dimensional potential energy surface, which couples a hydrogenlike harmonic oscillator to a positron via a spherically symmetric correlation polarization potential. The cross-section for the excitation of the first vibrational mode is in good agreement with previous reports. Our model suggests that a positron couples to the target vibration by responding instantly to an interaction potential, which depends on the target vibrational coordinate.

Resonant collisions of electrons with O2 via the lowest-lying 2Πg state of O2−

We report the results of an ab initio study of resonant vibrational excitation of molecular oxygen by electron impact at low energies where the lowest-lying ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{g}$ resonant state of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ dominantly contributes to the cross sections. The contribution of this resonance to the dissociative electron attachment cross section is also determined and the origin of its unusual oscillations is discussed. Calculations were performed within the nonlocal resonance model describing the nuclear dynamics of ${\mathrm{O}}_{2}^{\ensuremath{-}}$ after electron capture into the resonant state. The model was constructed using potential-energy curves obtained with standard quantum-chemical methods and eigenphase sums from the fixed-nuclei $R$-matrix calculations of electron scattering off ${\mathrm{O}}_{2}$. The effect of the spin-orbit interaction is taken into account when determining the potential-energy curves of the molecular negative ion. The vibrational excitation cross sections are compared with other available theoretical and experimental results.

Ramsauer\u2013Townsend minimum in electron scattering from CF_4: modified effective range analysis

Elastic cross sections for electron scattering on tetrafluoromethane (CF_4) from 0 up to 5 eV energy are analyzed using semi-analytical approach to the modified effective range theory (MERT). It is shown that energy and angular variations of differential, integral and momentum transfer cross sections can be parameterized accurately by six MERT coefficients up to the energy region of the resonant scattering. In particular, the model is used to determine the depth and the position of the Ramsauer–Townsend minimum as well as the s-wave scattering length. Moreover, we investigate the influence of the dipole polarizability value on the predictions of present model. To further validate our approach, the elastic data are combined with the Born-dipole cross sections for vibrational excitations as the input data for Monte Carlo simulation of electron swarm coefficients.Graphic

Resonance scattering of positronium atoms by nitrogen molecules

We study theoretically the resonance scattering of positronium (Ps) atoms by nitrogen molecules and compare it with resonance e–N2 scattering in the Πg symmetry. The velocity positions of the Ps–N2 resonances in Σu, Πu, and Δg symmetries are very close to the velocity position of the e–N2 resonance which leads to the total Ps–N2 cross section being very similar to the e–N2 cross section when both are plotted as functions of the projectile velocity. However, in the Ps–N2 case the resonances are much wider and their positions vary much slower with increasing internuclear separation than in the e–N2 case. This makes the Ps-impact vibrational excitation cross section much smaller than the electron-impact vibrational excitation cross section, and no boomerang oscillations are observed in Ps–N2 scattering. We analyze the role of the static potential in the e–N2 case and compare exchange potentials for e–N2 and Ps–N2 interactions. This analysis allows us to explain the difference in the vibrational dynamics in these two cases.

Vibrationally resolved electron impact electronic excitation of BeH

Beryllium is being adopted for plasma facing walls in fusion reactors. This has led to the observation of emissions from the A 2Π state of beryllium hydride. Use of these emissions to monitor Be erosion requires electron impact excitation rates. Cross sections for electron impact vibrational excitation within the X 2Σ+ state and vibrationally resolved electronic excitation to the A 2Π state are reported for BeH, BeD and BeT. Electron collisions are studied at a range of internuclear separations using the UK molecular R-matrix (UKRmol+) codes. Electronic excitation is studied both within the Franck–Condon approximation and by explicit averaging of the T-matrix elements. It is found that (a) inclusion of the effect of higher partial waves using the Born approximation leads to significant increases in the cross sections and (b) the Franck–Condon approximation underestimates the importance of collisions for which the vibrational state changes during electronic excitation.

Nonlocal treatment of resonant vibrational excitation in electron-dioxygen collision

SynopsisThe cross sections for vibrational excitations of O2 through collisions with electron are calculated using the nonlocal discrete-state-in-continuum model and its local approximation and compared to experimental results.

Absolute vibrational and electronic cross sections for low-energy electron scattering from condensed Thymidine

SynopsisAbsolute vibrational and electronic excitation cross sections (CSs) for low-energy electrons (1-18 eV) scattering from condensed thymidine (dT) are measured with High-Resolution Electron Energy Loss Spectros-copy (HREELS). EEL spectra are recorded and the vibrational and electronic modes of dT identified. Absolute CSs for these modes are extracted from each spectrum by a fitting procedure. Resonances are observed in the incident energy dependence of the CSs, below 3 eV and near 4, 8 and 10 eV. The CSs are compared to those of its constituents: thymine and tetrahydrofuran.

Theoretical study of electron-induced vibrational excitation of NO2

In this study, we compute cross sections for vibrational excitation of NO2 by electron impact. Calculations are performed using a theoretical approach based on a combination of the normal mode approximation for vibrational states of the target molecule, fixed-nuclei electron-NO2 scattering matrices and the vibrational frame transformation employed to evaluate the scattering matrix for vibrational transitions. Thermally-averaged rate coefficients are derived from the obtained cross sections for temperatures in the 10–10000 K interval for excitation of each normal mode of the target molecule. Analytical fits for the rate coefficients for singlets and triplets are provided. In addition, a comprehensive set of calculations are performed for assessing the uncertainty of the present calculations. The uncertainty assessments indicate that the computed observables for vibrational (de-)excitation is reasonable for later use in NO2-containing plasma kinetics modeling.

Coupling of electronic and nuclear motion in a negative ion resonance: Experimental and theoretical study of benzene

We present calculated and measured elastic and vibrational excitation cross sections in benzene with the objective to assess the reliability of the theoretical method and to shed more light on how the electronic motion of the incoming electron is coupled with the nuclear motion of the vibrations. The calculation employed the discrete momentum representation method which involves solving the two-channel Lippmann-Schwinger equation in the momentum space. The electron-molecule interaction was described by the exact static-exchange potential extended by a density-functional theory correlation-polarization interaction that models the molecular response in the field of the incoming electron. Cross sections were calculated for all 20 vibrational modes from near threshold until 20 eV. They were convoluted with a simulated instrumental profile for comparison with electron energy-loss spectra or appropriately summed for overlapping vibrations for comparison with measured cross sections plotted as a function of electron energy. An electron spectrometer with hemispherical analyzers was employed for the measurements. Good agreement of theory with experiment was obtained for the spectral profiles at 8 eV, and a nearly quantitative agreement was obtained at 3 and 4.8 eV. The theoretical results provided new insight into the excitation process, and it showed that more modes are excited than predicted by simple symmetry rules. Spectra showing the details of boomerang structure in the 1.15 eV π* resonance were recorded and are presented, although this aspect of experiment cannot be compared with the current theory.

Dissociative recombination of CH2NH2+: a crucial link with interstellar methanimine and Titan ammonia

Cross sections for dissociative recombination (DR) and vibrational excitation of the CH2NH + 2 ion in collisions with electrons are determined theoretically. The corresponding thermally averaged rate coefficients are computed and fitted to analytical formulas. The obtained DR rate coefficient is significantly smaller than the values recently employed in the photochemical models of the upper atmosphere of Titan, which has an important impact on the models that aim to reproduce the Titan ammonia abundance. On the other hand, the present results support the astrophysical models reproducing the abundance of the methanimine (CH2NH) detected in massive star formation regions. In these models, the CH2NH + 2 DR is a major route of formation of this molecule with a high prebiotic potential.