Large-scale $R$-matrix-with-pseudostates calculations for electron collisions with argon atoms, using a recently developed parallel version of our B-spline $R$-matrix code, are reported. The calculations were carried out in the semirelativistic $\mathit{jK}$-coupling scheme. They are intended to provide converged (with respect to the number of coupled states) results for electron-impact excitation of individual target states with dominant configurations $3p^{5}4s$, $3p^{5}4p$, $3p^{5}3d$, and $3p^{5}5s$ for incident electron energies from threshold to 300 eV. The close-coupling expansion includes 500 target states, with the lowest 78 states representing the bound spectrum and the remaining 422 the ionization continuum. The results reveal dramatic reductions of the predicted excitation cross sections at intermediate energies due to a strong influence of coupling to the target continuum and the higher-lying Rydberg states. Comparison with available experimental data for excitation raises questions about the absolute normalization in the measurements.

The $B$-spline $R$-matrix-with-pseudostates (BSR) method is employed to treat electron collisions with nitrogen atoms. Predictions for elastic scattering, excitation, and ionization are presented for all transitions between the lowest 21 states of nitrogen in the energy range from threshold to 120 eV. The structure description has been further improved compared to a previous BSR calculation by Tayal and Zatsarinny [ J. Phys. B 38, 3631 (2005)]. This change in the structure model, together with the inclusion of a large number of pseudostates in the close-coupling expansion, has a major influence on the theoretical predictions, especially at intermediate energies, where many of the excitation cross sections are reduced significantly. Ionization cross sections for the ground and metastable initial states are also provided. Finally, we carry out an accurate ab initio treatment of the prominent shape resonance just above the elastic threshold.

The question of stability of a given quantum system made up of charged particles is of fundamental interest in atomic, molecular, and nuclear physics. In this work, the stability for the negatively charged positronium (Ps)-like ions or the three-body system ( $Z{e}^{+},e^{-},e^{-}$ ) with Yukawa potentials is studied using correlated exponential wavefunctions based on the Ritz variational method. We obtained the critical screening parameter ${\mu }_C$ as a function of the continuously varied nuclear charge $Z$ , the critical nuclear charge $Z_C$ as a function of the screening parameter $\mu$ , and the ionization energies in terms of the screening parameter $\mu$ and $Z$ . The critical nuclear charge for the bare Coulomb system ( $Z{e}^{+},e^{-},e^{-}$ ) obtained using 700-term correlated exponential wavefunctions is in accord with the reported results. The ionization energy, ${\mu }_C$ , and $Z_C$ for the Yukawa system ( $Z{e}^{+},e^{-},e^{-}$ ) exhibit interesting behaviors. The present study describes the possible nonexistence of Borromean binding as well as Efimov states. The possible existence of quasi-bound resonances states for the negatively charged screened Ps-like ions is briefly discussed.

We present the absolute triple differential cross section (TDCS) for single ionization of $\mathrm{Ne}\left(2p\right)$ at an impact energy of 599.6 eV and $\mathrm{Ar}\left(3p\right)$ at 195 eV. The role of the postcollision interaction (PCI) is studied using a high-order distorted-wave Born approximation model with a continuum distorted-waves expansion. Both the second- and third-order effects are considered in the present calculations, and the third-order distorted wave Born approximation model is reported in the $(e,2e)$ reaction. The calculated results show satisfactory agreement with experimental data. The magnitude of the absolute TDCS is enhanced by a factor 2–3 when the strength factor $\gamma$ of the PCI amplitude is summarized just from 0 to 2. This proves that the PCI plays an important role in the absolute TDCS of the $(e,2e)$ reaction in the intermediate-energy region.

The Dirac B-spline R-matrix (DBSR) method is employed to treat low-energy electron collisions with thallium atoms. Special emphasis is placed on spin polarization phenomena that are investigated through calculations of the differential cross-section and the spin asymmetry function. Overall, good agreement between the present calculations and the available experimental measurements is found. The contributions of electron exchange to the spin asymmetry cannot be ignored at low impact energies, while the spin–orbit interaction plays an increasingly significant role as the impact energy rises.