Cross Sections For Rotational Excitation Research Articles

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.

An improved artificial neural network fit of the ab initio potential energy surface points for HeH+ + H2 and its ensuing rigid rotors quantum dynamics

Artificial neural networks (ANN) have been shown for the last several years to be a versatile tool for fitting ab initio potential energy surfaces. We have demonstrated recently how a 60-neuron ANN could successfully fit a four-dimensional ab initio potential energy surface for the rigid rotor HeH+ - rigid rotor H2 system with a root-mean-squared deviation (RMSD) of 35 cm−1. We show in the present study how a (40, 40) neural network with two hidden layers could achieve a better fit with an RMSD of 5 cm−1. Through a follow-up quantum dynamical study of HeH+(j1)-H2(j2) collisions, it is shown that the two fits lead to slightly different rotational excitation and de-excitation cross sections but are comparable to each other in terms of magnitude and dependence on the relative translational energy of the collision partners. When averaged over relative translational energy, the two sets of results lead to rate coefficients that are nearly indistinguishable at higher temperatures thus demonstrating the reliability of the ANN method for fitting ab initio potential energy surfaces. On the other hand, we also find that the de-excitation rate coefficients obtained using the two different ANN fits differ significantly from each other at low temperatures. The consequences of these findings are discussed in our conclusions.

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.

Electron-impact rotational excitation cross-sections and rate coefficients of molecular ion: HCO+

ABSTRACT HCO+ is one of the most abundant polyatomic ion in the Universe, also its presence in the planetary atmospheres and interstellar medium (ISM) makes it an interesting case to study. Low energy e−-HCO+ collisions result in rotational excitation and other molecular processes. In this study, the cross-sections for rotational transitions in HCO+ are evaluated for four rotational levels (j = 0, 1, 2, and 3) using the UK R-matrix method and the multichannel quantum defect theory (MQDT). The UK R-matrix method used to generate scattering matrices, through which cross-sections are computed. In past two decades, after developments and implementations of different theoretical approaches, the improved channel elimination procedure applied here is more promising to evaluate more accurate results near rotational thresholds important for the analysis of intensities of microwave region in plasma modelling. The rate coefficients are deduced from the computed cross-sections and studied over a wide electron temperature range.

Collisional excitation of interstellar PN by H2: New interaction potential and scattering calculations.

Rotational excitation of interstellar PN molecules induced by collisions with H2 is investigated. We present the first ab initio four-dimensional potential energy surface (PES) for the PN-H2 van der Waals system. The PES was obtained using an explicitly correlated coupled cluster approach with single, double, and perturbative triple excitations [CCSD(T)-F12b]. The method of interpolating moving least squares was used to construct an analytical PES from these data. The equilibrium structure of the complex was found to be linear, with H2 aligned at the N end of the PN molecule, at an intermolecular separation of 4.2 Å. The corresponding well-depth is 224.3 cm-1. The dissociation energies were found to be 40.19 cm-1 and 75.05 cm-1 for complexes of PN with ortho-H2 and para-H2, respectively. Integral cross sections for rotational excitation in PN-H2 collisions were calculated using the new PES and were found to be strongly dependent on the rotational level of the H2 molecule. These new collisional data will be crucial to improve the estimation of PN abundance in the interstellar medium from observational spectra.

Sodium isocyanide–Helium potential energy surface and astrophysical applications

In this work, we generated the two-dimensional potential energy surface (2D-PES) of the l-NaNC–He van der Waals interacting system. The electronic computations are done at the CCSD(T)-F12/aug-cc-pVTZ level. This 2D-PES exhibits a unique shallow minimum for helium approaching l-NaNC from the Na atom side. Afterward, we derived an analytical form for this ab initio 2D-PES, which is incorporated into quantum scattering code to compute the rotational (de-)excitation cross sections (rate coefficients) of l-NaNC by collision with He atom for total energies up to 2000 cm−1 (from 5 to 300 K). The propensity rules that favor Δj = − 1 transitions are less pronounced than in the case of t-NaCN isomer. Our data may be used for the determination of the abundances, in astrophysical media, of the candidate isomer l-NaNC with respect to those of the already detected t-NaCN isomer. We also performed comparison with the astrophysical relevant Al- and Mg-bearing (iso)cyanide systems interacting with He.

Quantum dynamical study of rotational excitation of CO+ (X2Σ+) by He collisions

The knowledge of accurate rate coefficients for collisional excitation of molecules by the abundant species like He, H2 and H are required for modeling the atmospheric conditions of interstellar medium. The present paper aims to study the transitions in the lowest twelve rotational levels in the ground vibrational state of CO+(X2Σ+) by collisions with helium atoms. We computed ab initio two-dimensional potential energy surface of 2Σ+/2A' symmetry for the He-CO+ van der Waals complex asymptotically correlating to He(1S) + CO+(X2Σ+) using restricted coupled-cluster method with perturbative treatment of triples employing augmented correlation consistent polarized valence quadruple-ζ basis set. Rovibrational bound states of He-CO+ complex for total angular momentum, J = 0 have been computed. The state-to-state rotational excitation cross sections are performed by close-coupling method up to center of mass collision energy of 2000 cm−1. The corresponding rate coefficients are obtained up to a temperature of 300 K.

Rotational de-excitations of C3H+ (1Σ+) by collision with He: new ab initio potential energy surface and scattering calculations

ABSTRACT Molecular ions play an important role in the astrochemistry of interstellar and circumstellar media. C3H+ has been identified in the interstellar medium recently. A new potential energy surface of the C3H+–He van der Waals complex is computed using the ab initio explicitly correlated coupled cluster with the single, double and perturbative triple excitation [CCSD(T)-F12] method and the augmented correlation consistent polarized valence triple zeta (aug-cc-pVTZ) basis set. The potential presents a well of 174.6 cm−1 in linear geometry towards the H end. Calculations of pure rotational excitation cross-sections of C3H+ by He are carried out using the exact quantum mechanical close-coupling approach. Cross-sections for transitions among the rotational levels of C3H+ are computed for energies up to 600 cm−1. The cross-sections are used to obtain the collisional rate coefficients for temperatures T ≤ 100 K. Along with laboratory experiments, the results obtained in this work may be very useful for astrophysical applications to understand hydrocarbon chemistry.

Rotational excitation of cyanogen ion, CN+ (X1Σ+) by He collisions

Modeling the physical conditions of interstellar medium requires knowledge of accurate rate coefficients for collisional excitation of molecules by the abundant chemical species like He and H2. The present paper aims to study transitions in the low rotational levels in the ground vibrational state of CN+(X1Σ+) by collisions with helium atoms. We computed ab initio two-dimensional (rigid-rotor) potential energy surface for the He-CN+ van der Waals collision complex using multi-reference configuration interaction method employing augmented correlation consistent polarized valence quadruple-ζ basis set. The bound-states of the van der Waals complex are obtained by coupled channel approach. The state-to-state rotational excitation cross sections are computed by exact close-coupling quantum mechanical formalism up to center of mass collision energy of 2000 cm−1. The corresponding rate coefficients are obtained up to 300 K temperature.

Positron scattering by the Ar2 and Xe2 dimers

Positron scattering by Ar2 and Xe2 dimers are studied for low incident energies. The Zero Range Potential approximation is used and the dimer is modeled as two independent atoms under an internuclear distance constraint. The scattering calculations are performed within the fixed nuclei and the rigid rotor formulations. For the fixed nuclei approximation, analytical expressions are obtained for the positron-dimer scattering length and for the elastic cross section. In the rigid rotor treatment, an analytical expression is obtained for the rotational excitation cross sections. The main results obtained in this investigation are that the short-range interaction between the positron and the molecular dimer is the dominant mechanism for rotational excitation in this energy range. Such effect is explained by the small values of the quadrupole moments of the Ar2 and Xe2 dimers.

Rotational-state-changing collisions between N2+ and Rb at low energies

We present a theoretical study of rotationally elastic and inelastic collisions between molecular nitrogen ions and Rb atoms in the sub-Kelvin temperature regime prevalent in ion-atom hybrid trapping experiments. The cross sections for rotational excitation and de-excitation collisions were calculated using quantum-scattering methods on ab-initio potential energy surfaces for the energetically lowest singlet electronic channel of the system. We find that the rotationally inelastic collision rates are at least an order of magnitude smaller than the charge-exchange rates found in this system, rendering inelastic processes a minor channel under the conditions of typical hybrid trapping experiments.

Cross Sections for Electron Collisions with NO, N2O, and NO2

Cross section data are compiled from the literature for electron collisions with oxides of nitrogen (NxOy) molecules: the species nitric oxide (NO), nitrous oxide (N2O), and nitrogen dioxide (NO2) are explicitly considered. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, and dissociative attachment. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed up to the end of 2017. These results are supplemented by a reanalysis of the swarm measurements for NO and newly calculated cross sections for rotational excitation of N2O and for rotational excitation and electronic excitation of NO2.

Rotational excitation of tetrahedral and octahedral molecules caused by electron and positron impact

Rotational excitation cross sections of tetrahedral and octahedral molecules caused by low energy positron/electron impact are the subject of this work. We developed a scattering model considering the Born approximation and the asymptotic multipole potential. These assumptions permitted us to achieve analytical expressions for rotational integral, momentum transfer and differential cross sections. We have found good agreement for positron and marginal agreement for electron interactions. The model can be improved with the inclusion of exchange effects and the distortion of the scattering wave function.

Collisional Excitation of CF+ by H2: Potential Energy Surface and Rotational Cross Sections

The CF+ molecule is considered one of the key species for the study of fluorine chemistry in the interstellar medium (ISM). Its recent detection, as well as its potential use as a tracer for atomic fluorine in the ISM, has increased the interest in the study of the physical and chemical properties of this cation. Accurate determination of the CF+ abundance in the ISM requires detailed modeling of its excitation from both radiation and collisions with the most dominant species, which are usually atomic and molecular hydrogen. Here, we report a new highly accurate potential energy surface (PES) describing the interaction between CF+ and the H2 molecule. Exact quantum time-independent calculations of the rotational excitation cross sections for collisions of CF+ with both para- and ortho-H2 are reported. Results obtained for collisions with para-H2 are compared to previous calculations performed using an approximate PES averaged over H2 rotation. Excitation data for collisions with ortho-H2 are provided for the first time.

Vibrationally-resolved excitation and dissociation collision strengths of AlO+ by electron-impact using the R-matrix method

The electron-impact calculations are reported for excitation and dissociation of AlO+ ion using the R-matrix method. Calculations are performed in the static-exchange (SE) and close-coupling (CC) approximation. Each target state in CC approximation is represented by a configuration interaction (CI) wavefunction that takes into account the correlation and polarisation effects. In CC approximation 14-target states are included in the trial wavefunction of the entire scattering system. Potential energy curves (PECs) for the first four low-lying states are generated using the basis functions 6-311G* wherein we obtain X1Σ+ as the ground state contrary to a3Π as stated elsewhere in literature. Scattering calculations are then performed to yield vibrationally-resolved electronic excitation collision strengths to the first three lowest excited states a3Π, A1Π and b3Σ+. Using more accurate PECs we calculated the Franck–Condon factors which were then employed to get the vibrationally-resolved electronic excitations and dissociation collision strengths for the fragment channel Al++O of the lowest three excited states a3Π, A1Π and b3Σ+. All scattering calculations are performed at the experimental bond length 1.6178 A of AlO+. Rotational excitation cross sections (0→j, j = 1, 2 … 5) have also been calculated and the corresponding rate coefficients have been evaluated for excitation and de-excitation by using the Maxwellian distribution function for electron temperature upto 5000 K. There are many Feshbach resonances detected in this work. We have analysed only the low-lying resonances below the excitation threshold of A1Π excited state. Beyond this threshold the resonance structure is too complex to analyse due to many overlapping resonances.

R-matrix study for electron scattering of beryllium dihydride for fusion plasma

We report the integral elastic, differential, momentum transfer, dissociative electron attachment and electronic- and rotational excitation cross sections for low-energy electron impact on beryllium dihydride computed using the Quantemol-N interface for driving the UK molecular R-matrix code. The energy of the projectile electron is in the range 0.1–10 eV. The effect of multichannel coupling is investigated by calculating the cross sections with various target models by increasing the number of target states in the trial wave function of the entire scattering system. The cross sections converge for the calculation with more than 15-target states. The vertical excitation energies calculated with the present model give excellent agreement with the EOM-CCSD calculations. Collisional frequencies are determined using the momentum transfer cross section for a Maxwell–Boltzmann distribution. From the collisional frequency, the transport properties such as mean free path, diffusivity and mobility are calculated for the temperature range of 100 to 500 000 K. The cross section data and transport properties reported in this article are important for fusion plasma.

Rotational excitation of N2 by positron impact in the adiabatic rotational approximation

In this article we present the rotational excitation cross sections of N2 by positron impact for energies between 1 and 10 eV. The cross sections were computed in the adiabatic rotational approximation within the many-body formalism of the Schwinger multichannel method. Our results show fortuitous agreement with previous rovibrational closed-coupling calculation and are higher in magnitude when compared to other similar calculation performed within the rigid rotor approximation. The convergence of static and static plus polarisation treatments suggests that dependence of polarisation for hexadecapole transitions may be neglected.

Cross Sections and Rate Coefficients for Rotational Excitation of HeH+ Molecule by Electron Impact

Cross sections for rotational excitation and de-excitation of the HeH+ ion by an electron impact are computed using a theoretical approach that combines the UK R-matrix code and the multi-channel quantum defect theory. The thermally-averaged rate coefficients derived from the obtained cross sections are fitted to an analytical formula valid for a wide range of temperatures.

Electron and positron scattering cross sections for propene

The molecular R-matrix calculation is performed for the low energy electron scattering from propene to obtain total and rotational excitation cross sections. The rotational excitation cross sections are calculated for the transitions involving levels from j = 0 to j = 5. These cross sections are used for calculating their corresponding rate coefficients for the temperature range 30–5000 K. The rate coefficients at such temperatures have relevance in the interstellar medium as well as in the plasma environment. The spherical complex optical potential and complex scattering potential- ionization contribution methods are used for high energy calculation of the total cross section (Qtot) and ionization cross section (Qion), respectively, for both electron and positron scattering. The present Qtot results give reasonable agreement with previous experimental data. We have detected a π* shape resonance for electron scattering at 3.71 eV due to 2A″ scattering state, which is in accordance with the experimental results. However, no such resonance is observed in the positron scattering calculations. The cross sections reported here show good comparison with existing data, validating present calculations and its reliability.

Elastic scattering and rotational excitation of Li2 by positron impact

We report an ab initio and model potential investigation of elastic and rotational excitation cross sections for positron impact with Li2 molecule in the energy range from 0.1 to 10.0 eV. The ab initio cross sections were calculated with the Schwinger Multichannel Method, while the model potential calculations were performed with the Method of Continued Fractions, applying the correlation–polarization interaction known as PCOP. The converged elastic cross sections suggest a spherical polarizability for Li2 between 150 and 160 a03, a value considerably lower than the experimental one (≈216 a03). Rotational excitation cross sections obtained corroborate previous results available, with some discrepancy for the quadrupolar transition for energies below 3 eV. Our results indicate that such divergence comes from the consideration of a correlation-potential properly designed to treat positron-molecule scattering.