R-matrix Approach Research Articles

Transport in a Two-Channel Nanotransistor Device with Lateral Resonant Tunneling.

We study field effect nanotransistor devices in the Si/SiO2 material system which are based on lateral resonant tunneling between two parallel conduction channels. After introducing a simple piecewise linear potential model, we calculate the quantum transport properties in the R-matrix approach. In the transfer characteristics, we find a narrow resonant tunneling peak around zero control voltage. Such a narrow resonant tunneling peak allows one to switch the drain current with small control voltages, thus opening the way to low-energy applications. In contrast to similar double electron layer tunneling transistors that have been studied previously in III-V material systems with much larger channel lengths, the resonant tunneling peak in the drain current is found to persist at room temperature. We employ the R-matrix method in an effective approximation for planar systems and compare the analytical results with full numerical calculations. This provides a basic understanding of the inner processes pertaining to lateral tunneling transport.

Low-Energy Electron Scattering from Ethyl Amine and Propyl Amine: A Comparative Study.

In this study, we investigate the low-energy electron (<20 eV) scattering cross sections for the linear ethyl amine and propyl amine using the R-matrix approach. A theoretical study is conducted, covering various cross sections such as elastic, symmetry-wise elastic, differential, momentum transfer, and electronic excitation. We obtained the ionization cross section using the CSP-ic and BEB methodologies. We unveil the intricate electron-amine interactions within these molecular systems. Additionally, our work extends to a comparative analysis among ethyl amine, propyl amine, and previously studied methyl amine systems. By comparing the obtained results, we discern subtle differences and similarities in the electron scattering behaviors across these structurally related amines. The findings presented herein shed light on fundamental electron-amine interactions, offering valuable insights for diverse fields ranging from chemical kinetics to astrophysics.

Photoionization of hydrogen halides using the R-matrix method

Abstract In this study, we use the UK Molecular R-matrix (UKRMol) codes in the close-coupling approximation to examine the photoionization of hydrogen halides (hydrogen fluoride, hydrogen chloride, and hydrogen bromide). This article reports the total and partial photoionization cross sections for the X2Π, A2Σ+, and B2Σ+ ionic states of these halides. The calculated cross sections are compared with the available literature, which does not accurately represent the effective cross sections near the threshold region, which is dominated by the Rydberg series autoionization resonances converging to the A2Σ+ ionic state. There seems to have been minimal effort to investigate the Rydberg-bound states of these halides. Meanwhile, the R-matrix approaches have traditionally excelled at characterizing such studies. This indicates the effectiveness of this method for molecular photoionization as well as for understanding the resonant contribution to the photoionization cross sections. The detailed cross sections calculated comprise the complex autoionizing resonance structures capable of significantly contributing to the computations of total photoionization rates, which are necessary to maintain a steady state of ionization in astrophysical plasmas. Comparisons with the experimental measurements and the theoretical data generally show reasonable agreement across the reported energy range.

Study of the direct 16O(p,γ)17F astrophysical capture reaction within a potential model approach

A potential model is applied for the analysis of the astrophysical direct nuclear capture process 16O(p,γ)17F. The phase-equivalent potentials of the Woods-Saxon form for the p−16O interaction are examined which reproduce the binding energies and the empirical values of ANC for the 17F(5/2+) ground and 17F(1/2+) (E⁎=0.495 MeV) excited bound states from different sources. The best description of the experimental data for the astrophysical S factor is obtained within the potential model which yields the ANC values of 1.043 fm−1/2 and 75.484 fm−1/2 for the 17F(5/2+) ground and 17F(1/2+) excited bound states, respectively. The zero-energy astrophysical factor S(0)=9.321 KeV b is obtained by using the asymptotic expansion method of D. Baye. The calculated reaction rates within the region up to 1010 K are in good agreement with those from the R-matrix approach and the hierarchical Bayesian model in both absolute values and temperature dependence.

N-Dimensional Lattice Integrable Systems and Their bi-Hamiltonian Structure on the Time Scale Using the R-Matrix Approach

A time scale is a special measure chain that can unify continuous and discrete spaces, enabling the construction of integrable equations. In this paper, with the Lax operator generated by the displacement operator, N-dimensional lattice integrable systems on the time scale are given by the R-matrix approach. The recursion operators of the lattice systems are derived on the time scale. Finally, two integrable hierarchies of the discrete chain with a bi-Hamiltonian structure are obtained. In particular, we give the structure of two-field and four-field systems.

Development of alpha cluster structure in nuclei of the same mass number

The resonant 14N+α particle scattering was studied in the 18F excitation region from 6.5 to 9 MeV at Astana cyclotron using the TTIK approach. The excitation functions for the elastic 14N+α scattering were analyzed in the framework of R-matrix approach. The observed strong alpha cluster structure in 18F is compared with that in 18O.

Electron-impact Excitation of Pt i–iii: The Importance of Metastables and Collision Processes in Neutron Star Merger and Laboratory Plasmas

The detection of a gravitational-wave signal and subsequent electromagnetic transient from a neutron star merger in 2017 is consistent with expectations of neutron star mergers as an r-process element production site. Within the first few days post-merger, the kilonova spectra are consistent with a blackbody illuminating a mix of heavy, r-process elements. With increasing time, the kilonova transitions to the non-LTE regime where the level populations and ionization balance are determined by both collisional and photoprocesses. Detailed cross section data for electron-impact processes involving the relevant species are often not available. In such circumstances, it is reasonable to use approximate methods as baseline data for use in spectral modeling, and it is useful to evaluate the accuracy of such methods against more sophisticated collision calculations when possible. We describe new calculations of the electron-impact excitation cross sections of Pt i–iIi using the DARC R-matrix codes. Using collisional-radiative models, we show that, at plasma conditions expected in kilonovae, the expressions of van Regemorter and Axelrod are insufficient for producing electron-impact excitation data for complex, heavy species such as the low charge states of Pt. Through comparisons with data generated with the relativistic distorted wave approach, as implemented in the Flexible Atomic Code, we show the distorted wave method produces cross section data that, when incorporated into spectral models, predicts strong spectral feature distributions similar in intensity to those from models built on data computed with the R-matrix approach for the considered ions and plasma conditions.

L-Shell Photoionization of Magnesium-like Ions with New Results for Cl5+

This study reports on the absolute photoionization cross sections for the magnesium-like Cl5+ ion over the 190–370 eV photon energy range, corresponding to the L-shell (2s and 2p subshells) excitation regime. The experiments were performed using the Multi-Analysis Ion Apparatus (MAIA) on the PLéIADES beamline at the SOLEIL synchrotron radiation storage ring facility. Single and double ionization ion yields, produced by photoionization of the 2p subshell of the Cl5+ ion from the 2p63s2 1S0 ground state and the 2p63s3p 3P0,1,2 metastable levels, were observed, as well as 2s excitations. Theoretical calculations of the photoionization cross sections using the Multi-Configuration Dirac-Fock and R-matrix approaches were carried out, and the results were compared with the experimental data. The Cl5+ results were examined within the overall evolution of L-shell excitation for the early members of the Mg-like isoelectronic sequence (Mg, Al+, Si2+, S4+, Cl5+). Characteristic photon energies for P3+ were estimated by interpolation.

Study of bound and resonant states of NS molecule in the R-matrix approach

The bound and resonance states along with corresponding autoionization widths for nitrogen sulphide (NS) molecule are determined using electron NS+ cation scattering calculations. The calculations are performed for 2Σ+, 2Π and 2Δ total symmetries using the ab initio R-matrix method for both bound and continuum states. Calculations are performed on a grid of 106 points for internuclear separations between 1.32 and 3 Å. The resonance states yield dissociative potential curves which, when considered together with their widths, provide input for models of different electron-cation collision processes including dissociative recombination (DR), and rotational and vibrational excitation. Curves and couplings which will lead directly to DR are identified.

Spectroscopic Peculiarities at Ionization of Excited 2p5(2PJf)3s[K]0,1,2 States of Ne: Cooper Minima and Autoionizing Resonances

An extensive study of photoionization from neon excited states was performed. The R-matrix approach was applied to calculate a photoionization cross-section from the metastable 2p5(2PJf)3s[K]0,2 and dipole-allowed 2p5(2PJf)3s[K]1 states. The resonance structures and Cooper minimum accessible in photoionization from the excited states by the photons with energy below 30 eV were analyzed. The parameters of the lowest autoionizing states (AISs) of even parity were extracted by fitting of the photoionization cross-section. For the dipole-allowed states, calculations are presented for unpolarized, linearly and circularly polarized radiation.

Generalizations of the R-Matrix Method to the Treatment of the Interaction of Short-Pulse Electromagnetic Radiation with Atoms

Since its initial development in the 1970s by Phil Burke and his collaborators, the R-matrix theory and associated computer codes have become the method of choice for the calculation of accurate data for general electron–atom/ion/molecule collision and photoionization processes. The use of a non-orthogonal set of orbitals based on B-splines, now called the B-spline R-matrix (BSR) approach, was pioneered by Zatsarinny. It has considerably extended the flexibility of the approach and improved particularly the treatment of complex many-electron atomic and ionic targets, for which accurate data are needed in many modelling applications for processes involving low-temperature plasmas. Both the original R-matrix approach and the BSR method have been extended to the interaction of short, intense electromagnetic (EM) radiation with atoms and molecules. Here, we provide an overview of the theoretical tools that were required to facilitate the extension of the theory to the time domain. As an example of a practical application, we show results for two-photon ionization of argon by intense short-pulse extreme ultraviolet radiation.

Radiative capture of proton by 9Be(p, γ)10B at low energy

Radiative capture p + 9Be → 10B + γ at energies bearing astrophysical importance is a key process for the spectroscopic study of 10B. In this work, we consider the radiative capture cross-section for the 9Be(p, γ)10B within the framework of the potential model and the R-matrix method for the multi-entrance channel cases. In certain cases, when the potential fails, therefore, the R-matrix approach is better to use for the description of partial components of the cross-section that have sharp or broad resonances. For all possible electric and magnetic dipole transitions, partial components of the astrophysical S-factor are computed. The computed value of the total S-factor at zero energy is consistent with the reported results.

Positron scattering from pyrazine

The present paper provides a joint experimental and theoretical study of positron scattering from pyrazine. Experimental data were measured employing a low-energy positron beamline, and covered an energy range from 1 to 79 eV. Cross sections were measured for total scattering, total elastic scattering, positronium formation, direct ionization, and the sum of the total discrete electronic-state excitation processes. In addition, measured total electronic excitation cross sections for the individual ${1}^{1}{B}_{3u},$ ${1}^{1}{B}_{2u},$ ${1}^{1}{B}_{1u},$ and ${2}^{1}{B}_{2u}$ states are also reported. Finally, experimental elastic differential cross sections (DCSs), here in the energy range $1--20$ eV, are also presented. Results from two theoretical approaches are given: the R-matrix approach and the independent atom method with screening corrected additivity rule and interference terms. Where a comparison between our experimental and theoretical results could be made, the level of accord varied from being quite good to marginal depending on the actual scattering process under consideration. Last, a comparison between the present elastic DCS results and those from our earlier study on positron scattering from pyrimidine [Palihawadana et al., Phys. Rev. A 88, 012717 (2013)] showed a somewhat unexpected level of agreement.

Photoionisation of the CH radical using the B-spline R-matrix method

Aims. The primary motivation for this paper is to provide accurate data for the photoionisation of the CH radical, including the absolute total photoionisation cross-section, partial cross-sections, and photoelectron angular distribution. In addition, the near threshold features in the photoionisation curve (which are absent in previous studies) are produced with high precision. Methods. A multichannel wavefunction based on the R-matrix approach, which uses the configuration interaction (CI) method to describe electronic correlation, is carried out in the present calculations. A set of B-spline orbitals is employed to represent the accurate continuum. The distinctive feature of the present calculations allows us to generate a more accurate description of the bound and continuum states than those employed before. Results. Total photoionisation cross-sections from the ground state of CH radicals and partial cross-sections corresponding to 1π, 3σ, and 2σ states of CH+ ions are presented for photon energies ranging from threshold to 80 eV. Extensive resonance structures, which are absent in previous studies, are observed for the first time near the ionisation threshold. The cross-section dataset obtained from the present calculations is expected to be sufficiently accurate and comprehensive for most current modelling applications involving the photon and CH radical scattering system.

Considering nonlocality in the optical potentials within eikonal models

Background: For its simplicity, the eikonal method is the tool of choice to analyze nuclear reactions at high energies ($E&gt;100$ MeV/nucleon), including knockout reactions. However, so far, the effective interactions used in this method are assumed to be fully local.Purpose: Given the recent studies on nonlocal optical potentials, in this work we assess whether nonlocality in the optical potentials is expected to impact reactions at high energies and then explore different avenues for extending the eikonal method to include nonlocal interactions.Method: We compare angular distributions obtained for nonlocal interactions (using the exact R-matrix approach for elastic scattering and the adiabatic distorted wave approximation for transfer) with those obtained using their local-equivalent interactions.Results: Our results show that transfer observables are significantly impacted by nonlocality in the high-energy regime. Because knockout reactions are dominated by stripping (transfer to inelastic channels), nonlocality is expected to have a large effect on knockout observables too. Three approaches are explored for extending the eikonal method to nonlocal interactions, including an iterative method and a perturbation theory.Conclusions: None of the derived extensions of the eikonal model provide a good description of elastic scattering. This paper suggests that nonlocality removes the formal simplicity associated with the eikonal model.

Multi-photon above threshold ionization of multi-electron atoms and molecules using the R-matrix approach

We formulate a computationally efficient time-independent method based on the multi-electron molecular R-matrix formalism. This method is used to calculate transition matrix elements for the multi-photon ionization of atoms and molecules under the influence of a perturbative field. The method relies on the partitioning of space which allows us to calculate the infinite-range free-free dipole integrals analytically in the outer region, beyond the range of the initial bound wave function. This approach is valid for an arbitrary order, that is, any number of photons absorbed both in the bound and the continuum part of the spectrum (below- and above-threshold ionization). We calculate generalized multi-photon cross sections and angular distributions of different systems (H, He, hbox {H}_{{2}}, hbox {CO}_{{2}}) and validate our approach by comparison with data from the literature.

F19(p,γ)20Ne and F19(p,α)O16 reaction rates and their effect on calcium production in Population III stars from hot CNO breakout

First generation, or Population III, stars have a different evolution than those of later generations owing to their initial primordial abundance composition. Most notably, the lack of carbon, oxygen, and nitrogen means that primordial massive stars must rely on the less efficient p−p chains, thereby requiring the star to contract to reach temperatures high enough to eventually trigger 3α reactions. Even small amounts of the C12(α,γ)O16 reactions begin feeding the CNO mass range and enable the CNO cycle to generate energy, but this occurs at higher temperature compared to later stellar generations. It is currently controversial if the observed enhanced abundances of Ca in the most metal-poor stars could be a result of the high temperature H-burning conditions in the first massive stars. The level of this enrichment depends on the hot breakout path from the CNO cycles via the F19(p,γ)Ne20 reaction. In this work, the rates of both the F19(p,γ)Ne20 and competing F19(p,α)O16 reactions are re-evaluated using the phenomenological R-matrix approach, simultaneously considering several F19(p,γ)Ne20, F19(p,α)O16, and F19(p,p)F19 data sets, to better characterize the rate uncertainties. It is found that the rate uncertainty for F19(p,γ)Ne20 reaction is considerably larger than previously reported. This is the result of undetermined interferences between observed resonances, a possible threshold state, possible subthreshold states, direct capture, and background levels. Additional experimental measurements are therefore needed to determine if F19(p,γ)Ne20 CNO breakout is responsible for Ca enrichment in metal-poor stars. Astrophysically, the breakout reaction revision makes it less likely that Ca observed in the most Fe-poor stars can originate in hot CNO breakout H-burning nucleosynthtesis, thereby casting doubt on the prevailing faint supernova scenario to explain the abundances observed in these stars.17 MoreReceived 5 January 2021Accepted 9 April 2021Corrected 1 June 2021DOI:https://doi.org/10.1103/PhysRevC.103.055815©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesFormation & evolution of stars & galaxiesH & He induced nuclear reactionsHydrostatic stellar nucleosynthesisLifetimes & widthsNuclear astrophysicsNuclear spin & parityNuclear structure & decaysRadiative captureResonance reactionsProperties20 ≤ A ≤ 38TechniquesNuclear data analysis & compilationNuclear PhysicsGravitation, Cosmology & Astrophysics

Photoionization of open-shell nitrogen confined in C60

Endofullerene photoionization of open-shell N@C60 is investigated using a coupled-channel, correlated R-matrix approach with additional modifications for a finite spherical-shell fullerene potential. Particular focus is on the initial-state dependence of the photoionization cross section. Comparisons show significant differences between the 1s 22s 22p 3(4S) initial state and the metastable 2D and 2P states, as well as strong confinement oscillations.

Photoionization cross section calculations of Ne-like Mo XXXIII

Within the framework of Breit Pauli R-matrix approach, a close-coupling calculation has been carried out for the process of photoionization of ground and excited states of Ne-like Mo XXXIII ion. The calculation includes 60 fine-structure levels and 15 parities for the target states. Level specific photoionization cross sections are computed for all levels at a fine energy mesh of 10−5 scaled Ryd, with 19500 energies for each Jπ = 0± - 6± symmetry, up to 2s22p43s (2S1/2) threshold at 212.33 Ryd above the first ionization threshold, 1s22s22p5 2Po3/2. In this article are presented cross sections for the photoionization of ground 1s22s22p6 1S0 and 1s22s22p5(2Po)3p 3P0, 1s22s22p5(2Po)3p 1S0, 1s22s2p63s 1S0, 1s22s22p53s(3Po2,1,0), 1s22s22p53s (1Po1) excited states. To explore the role of particle-hole, channels coupling and electron-electron correlation effects on the photoionization cross sections we have compared two calculation models, namely 8-target basic configurations and 5-target basic configurations, with and without 2s2p53l target states included into the model. Effects are highlighted in the photoionization cross section behavior of the excited of 2s2p63s of Mo XXXIII.We believe that this is the first such calculation for this ion to obtain the photoionization cross section and results are important for the modeling and diagnostics of laboratory and astrophysical plasmas.

A study of electron scattering from 1-1 C2H2F2 from 0.1 eV to 5 keV

Various electron- impact cross sections of 1-1-C2H2F2 molecule are reported in the energy range from 0.1 to 5000 eV using the cc-pVTZ basis set. The molecular wavefunctions of the target are obtained from the multi-center expansion of the Gaussian-type orbitals within a single determinant Hartree Fock self consistent field scheme. The elastic cross sections in the low energy range (below ionization threshold) are reported in different scattering models like static-exchange, static-exchange-polarization and close-coupling using the R-matrix approach. The resonances detected in these scattering models are comprehensively analysed. The computed shape resonances are in an excellent agreement with the experimental and theoretical values. In addition to elastic rate coefficients, we have also reported differential, momentum transfer and electronic excitation cross sections. The present elastic cross sections are in good accord with the other ab-initio calculations. However, the total cross sections obtained lie systematically above the experimental results. The Single Center Expansion (SCE) involving the use of local potentials is invoked to obtain differential, elastic and momentum transfer cross sections at energies beyond ionization threshold. The ionization cross sections are obtained using the Binary-Encounter-Bethe model. The total cross sections are obtained by incoherently summing the elastic and inelastic cross sections. The SCE based results and total cross sections are in good agreement with the experimental and theoretical results. The cross sections like elastic, momentum transfer and total obtained from two different approaches are found to match smoothly near ionization threshold thus helping in estimating the cross sections over a wide energy range.