Integral Cross Sections Research Articles

FLASSH 1.0: Thermal Scattering Law Evaluation and Cross-Section Generation for Reactor Physics Applications

The Full Law Analysis Scattering System Hub (FLASSH) is a modern, advanced code that evaluates the thermal scattering law (TSL) along with accompanying cross sections. FLASSH features generalized methods that accommodate any material structure. Historical approximations including incoherent and cubic approximations have been removed. Instead, the latest release of FLASSH features advanced physics options including distinct effect corrections (one-phonon contributions) and noncubic formulations. Noncubic elastic and inelastic contributions are necessary to accurately evaluate one-phonon contributions. Both noncubic and one-phonon calculations require high-density sampling of the various scattering directions. Optimization and parallelization of these routines were therefore necessary to produce results in a reasonable computational time frame. With these notable improvements to the generalized TSL, FLASSH 1.0 can meet benchmark requirements by permitting realistic comparisons with experiments for both TSLs and the resulting integrated cross sections. Within FLASSH, these high-fidelity TSLs can be applied also to the resonance region to evaluate accurate, material structure-dependent Doppler broadening that captures the observed experimental behavior. Additional features including a graphical user interface (GUI), plotting diagnostics, and formatted output options including ACE files allow users to complete a TSL evaluation with minimal input and maximum flexibility. The user GUI creates input files for FLASSH, reducing user error and also providing built-in error checks. Autofill options and suggested input values help make TSL evaluation accessible to novice users. The FLASSH code is compiled to run on both Windows and Linux platforms with automatic parallelization.

Reaction dynamics of C(3P) + Si2(X $^{3}\Sigma ^-_g$ ) → Si(3P) + SiC(X 3Π) on a global CHIPR potential energy surface of the ground state Si2C(X 1A1)

ABSTRACT The dynamics of C(3P) + Si2(X $^{3}\Sigma ^-_g$ ) → Si(3P) + SiC(X 3Π) on its ground state Si2C(X 1A1) are of great significance in carbon-rich interstellar chemistry. Using the combined-hyperbolic-inverse-power-representation method, we construct the first global potential energy surface (PES) for the electronic ground state Si2C(X 1A1) based on a total of 4080 ab initio energy points, which are obtained at the Davidson-corrected internally contracted multireference configuration interaction level of theory. The topographical features of the newly constructed PES are examined in detail and show good agreement with previous theoretical and experimental studies. Finally, we investigate the C(3P) + Si2(X $^{3}\Sigma ^-_g$ ) → Si(3P) + SiC(X 3Π) reaction using the quasi-classical trajectory and time-dependent wave packet calculations, yielding reasonable integral cross sections and rate constants, which are expected to be useful for astrochemical modelling in carbon-rich interstellar environments.

Electron impact ionization cross-sections for CH3 and SiH3 radical targets

Single differential cross sections as a function of secondary electron energy at fixed incident electron energies 100 and 200 eV for the targeted molecule CH3 and SiH3 have been evaluated by using a modified Jain-Khare semi-empirical approach. The integral or total electron-impact ionization cross-sections and the averaged energy loss from differential ionization cross-sections are calculated for incident electron energy from the ionization threshold to the 5 keV regimes. The temperature-dependent ionization rate coefficient is also calculated from total ionization cross-sections and Maxwell-Boltzmann electron energy distribution.The calculated values of total ionization cross sections corresponding to (e, 2e) processes for CH3 and SiH3 molecules are in good accord with available theoretical and experimental results.

Oscillator strengths and cross sections of the valence-shell excitations of CH[formula omitted]I studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CH3I are important for studying molecular photodissociation and monitoring the destruction of ozone in the Earth’s atmosphere. Based on the relative flow technique, absolute generalized oscillator strengths of the valence-shell excitations in CH3I have been determined by using an angle-resolved electron energy loss spectrometer. The measurement was carried out at an incident electron energy of 1.5 keV and an energy resolution of 80 meV. Optical oscillator strengths for the dipole-allowed transitions have been obtained at the zero limit of the squared momentum transfer, providing an independent cross check with the previous experimental results. The integral cross sections from the excitation threshold to 5000 eV have also been obtained by means of the BE-scaling method. The present experimental oscillator strengths and cross sections can supplement the fundamental spectroscopic data of CH3I and will play an important role in the studies of molecular photodissociation dynamics.

Positron Scattering from Pyrimidine

The positron impact cross-sections of pyrimidine molecules are reported from 1 eV to 5000 eV. These cross-sections include differential elastic, integral elastic, and direct ionisation. The elastic cross-sections are computed using the single-centre expansion scheme whereas the direct ionisation cross-sections are obtained using the binary-encounter-Bethe formula. The integral and differential cross-sections exhibit consistency with the experimental and other theoretical results. The direct ionisation cross-sections, which are reported for the first time, are compared with the experimental inelastic cross-sections (the sum of excitation and ionisation) to assess the trends in theoretically computed ionisation cross-sections and with the corresponding results for the electrons. The incoherently summed elastic and ionisation cross-sections match very well with the total cross-sections after 40 eV indicating the minimal impact of the positronium formation and electronic excitation processes. Based on this study, we recommend that the experimental data of the inelastic cross-sections reported by Palihawadana et al. be revisited.

A systematic analysis of deuteron breakup

Inclusive breakup cross sections for deuterons incident on a wide range of nuclei at different incident energies are studied. In our analysis, we use the post-form distorted-wave-Born approximation (DWBA) to calculate the elastic and nonelastic breakup cross sections. We also calculate exciton and statistical compound-nucleus model cross sections to account for pre-equilibrium and equilibrium emissions. The calculations are performed using the code EMPIRE and are compared to the available experimental data. The comparisons of the integrated cross sections reveal a generally good agreement between the calculations and the data. However, systematic discrepancies are observed between the calculated double differential neutron and proton cross sections and the data, especially for heavier targets. Possible reasons for the disagreements between the theoretical predictions and the experimental data are discussed.

Low-Energy Collisions of Zeeman-Decelerated NH Radicals with He Atoms.

We report an experimental study of state-to-state inelastic scattering of NH (X 3Σ-, N = 0, j = 1) radicals with He atoms. Using a crossed molecular beam apparatus that combines a Zeeman decelerator and velocity map imaging, we study both integral and differential cross sections in the N = 0, j = 1 → N = 2, j = 3 inelastic channel. We developed various new REMPI schemes to state-selectively detect NH radicals, and tested their performance in terms of sensitivity and ion recoil velocity. We found a 1 + 2' + 1' REMPI scheme using the A 3Π ← X 3Σ- resonant transition, which yields acceptable recoil velocities and is more than an order of magnitude more sensitive than conventional one-color REMPI schemes to detect NH. We used this REMPI scheme to probe state-to-state integral and differential cross sections around the channel opening at 97.7 cm-1, as well as at higher energies where structure in the scattering images could be resolved. The experimental results are in excellent agreement with the predictions from quantum scattering calculations which are based on an ab initio NH-He potential energy surface.

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

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

The elastic scattering cross sections of the N[formula omitted]-H and N-H[formula omitted] collisions for the transport theory

The transport cross sections and integrated cross sections are calculated for interactions of hydrogen and nitrogen atoms and ions (in the ground and excited states) for use in plasma science and hypersonic flows theory. The data for the transport cross sections are given in the energy range typically needed (or broader) for those applications: 10−6-37 hartree (around 2.7⋅10−5eV-1 keV). The reported cross sections are calculated from the high quality ab initio potential energy curves with additional extrapolation for highly repulsive region. The methods and energy range are discussed in the context of LXCat and IAEA (ALADDIN and CollisionDB) databases. The uncertainty of data is also estimated.

Testing the ϕ-nuclear potential in pion-induced ϕ meson production on nuclei near threshold

The near-threshold pion-induced ϕ production off nuclei has been studied in the kinematical conditions of the HADES experiment using a collision model based on the nuclear spectral function. Starting from the elementary reaction π−p→ϕn, absolute differential and integral cross sections for the production of ϕ mesons off a light carbon and a heavy tungsten target have been calculated and compared to the recently reported experimental cross sections. The absolute cross section values are shown to be sensitive to the effective nuclear scalar ϕ and neutron potentials while the transparency ratio is governed by the ϕN absorption cross section. The experimental data are found to be consistent with an attractive ϕ-nucleus potential of ≈ -(50–100) MeV at normal nuclear matter density ρ0 and an effective ϕN absorption cross section of ≈ 12–25 mb. The extracted ϕ-nucleus potential is deeper than previous theoretical and experimental findings as well as those obtained in the present work within the (tρ) approximation with accounting for Pauli correlations from the spin-averaged ϕ-p scattering length reported in a recent ALICE experiment.

Electron elastic scattering from biomolecular targets: 1,3-dihydroxytetrahydrofuran, deoxyribose, THFA monophosphate and deoxyribose monophosphate

We numerically calculated doubly differential and integrated cross sections for electron elastic scattering on molecules composing the deoxyribonucleic acid (DNA), namely, the sugar 2-deoxyribose (C5H10O4) and its analogue 1,3-dihydroxytetrahydrofuran (C4H8O3) as well as molecules containing the phosphate group such as deoxyribose monophosphate (C5H11O7P) and THFA monophosphate (C5H11O5P). The model of independent atoms (IAM), taking into account the different potentials at short and long ranges as well as the effects of multiple-scattering terms, is here used. Then doubly differential cross sections are calculated for various impact energies (50, 80, 100, 300, 500, 800 and 1000 eV) and the integrated ones for energy ranging from 20 eV to 100 keV. The obtained results are compared and discussed with the available data for molecules similar to deoxyribose such as THF and THFA. Good agreements are generally found particularly in shape, while small discrepancies are observed in terms of amplitude that may be explained by the effect of the potentials used in the current approach.

Elastic Electron scattering by thermal mixture of glycine conformers in gas phase

A theoretical study of electron scattering by a thermal mixture of glycine molecules in the energy range from 1 to 10 eV is performed using the UK-RMol codes which are based on the R-Matrix method. The six lowest relative Gibbs free energies glycine conformers considered, Ip, IIp, IIn, IIIp, IIIn and IVn, are significantly populated in thermal mixtures. All these conformers present similar resonance structures in the eigenphase sums: a lower-energy resonance state near 1.8 eV and another at higher-energy above 7 eV. For the six conformers the lowest resonance lies between 1.75 eV and 2.21 eV. The very large dipole moments of 6.32 D and 5.67 D for IIp and IIn, respectively, makes the magnitude of their cross sections significantly larger than other conformers, which increases the average cross sections in thermal mixtures compared with the cross sections of the lowest energy Ip conformer. Three conformer population sets are used to calculate the averaged differential and integral cross sections: two theoretical sets based on the relative Gibbs free energies and another set that aims to mimic experiment based on the observed populations. The averaged cross sections are similar for all population sets, but differ from the Ip conformer cross section. This suggests that, for large and flexible molecules, the computed average cross sections should be used when comparing with experimental data.

Oscillator strengths and cross sections of the valence shell excitations in nitrous oxide studied by high-energy electron scattering

Generalized oscillator strengths (GOSs) of the valence shell excitations to the A1Σ−+B1Δ, C1Π, D1Σ+ and 21Π states in nitrous oxide have been determined by employing an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. Detailed comparisons with the previous data indicate that the higher-order Born terms play a significant role in the GOSs of C1Π and D1Σ+ states at either low impact energies or large momentum transfers, and the GOSs of A1Σ−+B1Δ and C1Π states measured with the gas cell may suffer from the pressure effect. The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths (OOSs) and calculate the integral cross sections (ICSs) based on the BE-scaling method. The oscillator strengths and cross sections in this work provide a cross check on the previous data and can serve as a benchmark for testing the developed theoretical models and computational codes.

Inclusive quarkonium production in pp collisions at sqrt{s} = 5.02 TeV

This article reports on the inclusive production cross section of several quarkonium states, textrm{J}/psi , psi mathrm{(2S)}, Upsilon mathrm (1S), Upsilon mathrm{(2S)}, and Upsilon mathrm{(3S)}, measured with the ALICE detector at the LHC, in pp collisions at sqrt{s} = 5.02 TeV. The analysis is performed in the dimuon decay channel at forward rapidity (2.5< y < 4). The integrated cross sections and transverse-momentum (p_{textrm{T}}) and rapidity (y) differential cross sections for textrm{J}/psi , psi mathrm{(2S)}, Upsilon mathrm (1S), and the psi mathrm{(2S)}-to-textrm{J}/psi cross section ratios are presented. The integrated cross sections, assuming unpolarized quarkonia, are: sigma _{textrm{J}/psi } (p_{textrm{T}} <20 GeV/c) = 5.88 ± 0.03 ± 0.34 ~mu b, sigma _{psi mathrm{(2S)}} (p_{textrm{T}} <12 GeV/c) = 0.87 ± 0.06 ± 0.10~mu b, sigma _{Upsilon mathrm (1S)} (p_{textrm{T}} <15 GeV/c) = 45.5 ± 3.9 ± 3.5 nb, sigma _{Upsilon mathrm{(2S)}} (p_{textrm{T}} <15 GeV/c) = 22.4 ± 3.2 ± 2.7 nb, and sigma _{Upsilon mathrm{(3S)}} (p_{textrm{T}} <15 GeV/c) = 4.9 ± 2.2 ± 1.0 nb, where the first (second) uncertainty is the statistical (systematic) one. For the first time, the cross sections of the three Upsilon states, as well as the psi mathrm{(2S)} one as a function of p_{textrm{T}} and y, are measured at sqrt{s} = 5.02 TeV at forward rapidity. These measurements also significantly extend the textrm{J}/psi p_{textrm{T}} reach and supersede previously published results. A comparison with ALICE measurements in pp collisions at sqrt{s} = 2.76, 7, 8, and 13 TeV is presented and the energy dependence of quarkonium production cross sections is discussed. Finally, the results are compared with the predictions from several production models.

The state-to-state dynamics of the N + NH(3Σ−) → N2(X1 Σg +) + H reaction: based on a new global potential energy surface

A new global potential energy surface (PES) of the ground state of the HN2 system is built using the neural network method. The aug-cc-pVQZ basis set for the H atom, cc-pwCVQZ basis set for the N atom and multi-reference configuration interaction method are employed in the ab initio calculations. The topographic features of the PES are discussed and compared with available theoretical studies. In addition, the initial state (v 0 = 0, j 0 = 0) dynamic studies of the N + NH(3Σ−) → N2(X1 ) + H reaction are performed using a time-dependent wave packet method, based on the newly constructed PES. The dynamics properties, such as reaction probabilities, integral cross-sections, differential cross-sections, the rovibrational state distribution of product and rate constants, are calculated at the state-to-state level of theory and compared with available theoretical and experimental results. The differential cross-section indicates that the forward abstraction mechanism dominates the reaction with the increase of collision energy. Highlights A new global potential energy surface of the ground state of the HN2 system was constructed using the neural network method. Quantum state-resolved dynamical calculations for the N + NH(3Σ−)(v 0 = 0, j 0 = 0) → N2(X1 ) + H reaction were carried out using a time-dependent wave packet method based on the newly constructed potential energy surface. Some meaningful dynamic properties were reported and compared with available theoretical and experimental data.

Excitations of H2 by positron impact

In this paper, we present a theoretical study of scattering cross sections for positron impact of electronic states of hydrogen molecule (H2) using the scaling Born positron (SBP) approach. Cross sections to low-lying electronic states, B1∑+u, C1Пu, B´1 ∑+u, and D1Пu are investigated. In an earlier theoretical effort [J.L.S.Lino, Rev. Mex. Fisica 62 (2016) 596], an application of the SBP approach for the X1∑+g - &gt; B1∑+u electronic excitation of the H2 molecule, gave cross sections with reasonable qualitative agreement with experimental data [J.P.Sullivan et. al., Phys.Rev.Lett.87,073201 (2001)]. However, recent studies for the excitation of the B1∑+u state by positron impact showed that this electronic state still demand of a refined degree of description of the quality of the cross sections (Weiss et. al., Eur. Phys. J. D (2018) 72). The purpose of this work was to reexamine the B1∑+u state and verify the quality of the numerical convergence of SBP method and extend the investigation to other states. The possibility to estimate a indirect contribution of multichannel effects ( three states considering the ground X1, B1∑+u and E,F1∑+u state) are introduced within the SBP context. For the first time, integral cross sections to these new states using the SBP approach are reported. In the absence of the experimental data and theoretical developments, comparisons are made with analogous electron scattering.

Theoretical studies of the spectral characteristics and electron impact dynamics of Ti XXI placed in the hot dense regimes

We describe a theoretical approach to calculate the electron impact dynamics of atoms/ions placed in a dense plasma. The model takes into consideration that the continuous electron and the guest atom/ion are affected by a recently proposed (dense) plasma shielding parameter potential. As a test case, the present numerical method is employed to study the electron-impact excitation of He-like Ti XXI by using the distorted-wave wavefunction in the framework of the fully relativistic theory. The finite temperature and density effects as well as solid density plasma effects on the excitation energies, transition properties, and integrated cross sections are discussed in detail, which shed light on the shielding parameter potential in resolving practical problems. Results are compared with the available measurements and previous computations. The present work is not only providing a new implementation of the finite-temperature method by a self-consistent methodology, but also of great interest for the high energy density physics community, i.e., our results are of interest to the fusion researches.

Full-dimensional automated potential energy surface development and detailed dynamics for the CH2OO + NH3 reaction.

With the help of the ROBOSURFER program package, a global full-dimensional potential energy surface (PES) for the reaction of the Criegee intermediate, CH2OO, with the NH3 molecule is developed iteratively using different ab initio methods and the monomial symmetrization fitting approach. The final permutationally-invariant analytical PES is constructed based on 23447 geometries and the corresponding ManyHF-based CCSD(T)-F12b/cc-pVTZ-F12 energies. The accuracy of the PES is confirmed by the excellent agreement of its stationary-point properties and one-dimensional potential energy curves compared with the corresponding ab initio data. The reaction probabilities and integral cross sections are calculated for the ground-state and several vibrationally excited-state reactions by quasi-classical trajectory simulations. Remarkable is that the maximum impact parameter b where reactivity vanishes is almost independent of collision energy ranging from 1 to 40 kcal mol-1, and the reaction probability increases with increasing collision energy for this negative-barrier reaction. At the same time, a slight mode-specificity effect is observed. In addition, the deuterium effect is investigated and the sudden vector projection is discussed.

Feshbach resonances in the F + CHD3 → HF + CD3 reaction.

The signature of dynamics resonances was observed in the benchmark polyatomic F + CH4/CHD3 reactions more than a decade ago; however, the dynamical origin of the resonances is still not clear due to the lack of reliable quantum dynamics studies on accurate potential energy surfaces. Here, we report a six-dimensional state-to-state quantum dynamics study on the F + CHD3 → HF + CD3 reaction on a highly accurate potential energy surface. Pronounced oscillatory structures are observed in the total and product rovibrational-state-resolved reaction probabilities. Detailed analysis reveals that these oscillating features originate from the Feshbach resonance states trapped in the peculiar well on the HF(v' = 3)-CD3 vibrationally adiabatic potential caused by HF chemical bond softening. Most of the resonance structures on the reaction probabilities are washed out in the well converged integral cross sections (ICS), leaving only one distinct peak at low collision energy. The calculated HF vibrational state-resolved ICS for CD3(v = 0) agrees quantitatively with the experimental results, especially the branching ratio, but the theoretical CD3 umbrella vibration state distribution is found to be much hotter than the experiment.

The impact of non-adiabatic effects on reaction dynamics: a study based on the adiabatic and non-adiabatic potential energy surfaces of CaH2.

The two-state non-adiabatic potential energy matrices of the CaH2+ system are calculated via a diabatization approach by using a neural network model. Subsequently, the adiabatic and non-adiabatic potential energy surfaces (PESs) are constructed based on these non-adiabatic potential energy matrices. Furthermore, based on the adiabatic and non-adiabatic PESs, the Ca+(4s2S) + H2(X1Σ+g) → H(2S) + CaH+(X1Σ+) reaction is studied using the time-dependent wave packet method. Comparative analysis of the experimental and theoretical integral reaction cross-sections (ICSs) indicates that the maximum deviation between the results obtained from the adiabatic PES and the corresponding experimental value is 12.7 bohr2; in contrast, the maximum discrepancy between the theoretical result derived from the non-adiabatic PES and the experimental value is merely 0.42 bohr2. The potential well along the reaction path acts as a 'filter', selectively guiding intermediates with longer lifetimes in the potential well back to the reactant channel. This phenomenon indicates that the non-adiabatic effects significantly influence the reaction dynamics of the CaH2+ system.