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Understanding how atoms interact with hot dense matter is essential for astrophysical and laboratory plasmas. Interactions in high-density plasmas broaden spectral lines, providing a rare window into interactions that govern, for example, radiation transport in stars. However, up to now, spectral line-shape theories employed at least one of three common approximations: second-order Taylor treatment of broadening operator, dipole-only interactions between atom and plasma, and classical treatment of perturbing electrons. In this Letter, we remove all three approximations simultaneously for the first time and test the importance for two applications: neutral hydrogen and highly ionized magnesium and oxygen. We found 15%-50% change in the spectral line widths, which are sufficient to impact applications including white-dwarf mass determination, stellar-opacity research, and laboratory plasma diagnostics.

We apply the molecular convergent close-coupling method to the calculation of linear polarization fractions for Fulcher-band fluorescence following electron-impact excitation of the ${\mathrm{H}}_{2}$ $d\phantom{\rule{0.16em}{0ex}}{}^{3}{\mathrm{\ensuremath{\Pi}}}_{u}$ state. The results exhibit the opposite threshold behavior compared to the only previous calculations [Meneses, Brescansin, Lee, Michelin, Machado, and Csanak, Phys. Rev. A 52, 404 (1995)], but are in agreement with the most recent measurements for the $Q(1),$ $R(1),$ and $Q(3)$ transitions [Maseberg, Bartschat, and Gay, Phys. Rev. Lett. 111, 253201 (2013)].

We use the adiabatic-nuclei molecular convergent close-coupling method to perform calculations of 0.01--1000 eV electrons scattering on the $c^{3}\mathrm{\ensuremath{\Pi}}_{u}$, $a^{3}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$, $B^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$, $C^{1}\mathrm{\ensuremath{\Pi}}_{u}$, and $EF^{1}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$ states of ${\mathrm{H}}_{2}$ in the $v=0$ vibrational level. Elastic, superelastic, ionization, and grand-total cross sections are presented, as well as cross sections for excitation of the $n=2$--3 singlet and triplet states of ${\mathrm{H}}_{2}$ (where $n$ is the atomic-limit principle quantum number). Comparison with available theoretical results is made. Good agreement is found with the recent $R$-matrix results [J. Phys. B 53, 245203 (2020)] for most of the exchange and dipole-forbidden transitions, but not for the dipole-allowed transitions. The sources of disagreement were found to be an unconverged partial-wave expansion and the utilization of the fixed-nuclei approximation (as opposed to adiabatic-nuclei) in the $R$-matrix calculations.

In this paper we propose a methodology to calculate the radiative properties of the diatomic molecular constituents of air, and utilize the present approach to model the radiative properties of nitrogen monoxide, NO. We also investigate the important physics involved in calculating accurate radiative quantities for air, such as the Planck and Rosseland mean opacities, and emission and absorption coefficients, as well as the couplings accounted for in rovibrational calculations. Complete active space self-consistent field multi-reference configuration interaction (CAS-MRCI) calculations were performed in order to model the NO X 2Π, a 4Π, b 4Σ−, 12Σ+, 22Σ+, 32Σ+, G 2Σ−, B′2Δ, (C, B) 22Π, (H′, L) 32Π, and 12Φ adiabatic states, and calculate the respective molecular data. The γ X 2Π − A 2Σ+, ɛ X 2Π − D 2Σ+, β′ X 2Π − B′2Δ, ‘11 000 Å’ A 2Σ+ − D 2Σ+, ‘infrared’ X 2Π − X 2Π and X 2Π − (C, B) 22Π (δ and β) band systems are investigated in monochromatic spectra calculations, as well as the Ogawa a 4Π − b 4Σ− band and several other band systems. Several conclusions are drawn, such as the importance of including the Ogawa band, which has not been included in previous air radiative models or comprehensive line-list calculations, as well as the importance of performing coupled rovibrational line-list calculations in order to accurately calculate the Rosseland means. We also found that the additional band systems modeled here contribute significantly to the total Planck and Rosseland means.

Many laboratory and industrial plasma applications require accurate modeling techniques to understand the interplay between microscopic and macroscopic processes. A prime example of this interplay is how particle and Monte Carlo (MC) simulation codes describe angular scattering of electrons following elastic scattering events. The forward peaked nature of high energy electron elastic scattering is relatively trivial to accurately describe in plasma simulations. However, for lower energy collisions, which produce near isotropic or backward peaked differential cross sections, there is not a strong consensus among the plasma modeling community on how to best describe these angular scattering trends. In this study, we propose a systematic method to approximate the aforementioned non-trivial angular scattering behavior with a formula that can be readily implemented in particle-in-cell (PIC) and/or MC plasma simulation codes. The present approach is specifically applied to fusion relevant atomic hydrogen and helium, as well as for molecular hydrogen, and results are also applicable to the atomic isotopes and homonuclear molecular isotopologues of these species. Comparisons between the present angular distribution function and benchmark scattering data were used to validate the proposed models. In addition, two-term Boltzmann calculations and PIC direct simulation MC simulations revealed that the proposed angular distribution function is accurate, agreeing very well with benchmark convergent close-coupling scattering calculations, and electron transport measurements. These studies confirmed that the present angular distribution function model can be utilized without the need of renormalization to the momentum transfer cross section (as opposed to using the elastic scattering integrated cross section), which has been suggested by several studies in order to correct for deficient angular scattering models, and to agree with transport measurements. Hence, the present anisotropic angular scattering model can be utilized to accurately model the momentum transfer as well as the electron trajectories of elastic collisions.

The low-energy electron-impact dissociation of molecular hydrogen has been a source of disagreement between various calculations and measurements for decades. Excitation of the ground state of ${\mathrm{H}}_{2}$ into the dissociative $b{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ state is now well understood, with the most recent measurements being in excellent agreement with the molecular convergent close-coupling (MCCC) calculations of both integral and differential cross sections [Zawadzki et al., Phys. Rev. A 98, 062704 (2018)]. However, in the absence of similar measurements for vibrationally excited or isotopically substituted ${\mathrm{H}}_{2}$, cross sections for dissociation of these species must be determined by theory alone. We have identified large discrepancies between MCCC calculations and the recommended $R$-matrix cross sections for dissociation of vibrationally excited ${\mathrm{H}}_{2}$, ${\mathrm{D}}_{2}$, ${\mathrm{T}}_{2}$, HD, HT, and DT [Trevisan et al., Plasma Phys. Contr. Fusion 44 1263 (2002); Plasma Phys. Contr. Fusion 44, 2217 (2002)], with disagreement in both the isotope effect and dependence on initial vibrational level. Here we investigate the source of the discrepancies, and discuss the consequences for plasma models, which have incorporated the previously recommended data.

Cosmic rays (CRs) at sub-TeV energies play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H$_{2}$. Their characterisation is important both for the interpretation of observations and for the development of theoretical models. The methods used so far for estimating the CR ionisation rate ($\zeta$) in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. We refine and extend the method proposed by Bialy (2020) to estimate $\zeta$ by observing rovibrational transitions of H$_{2}$ at near-infrared wavelengths, which are mainly excited by secondary CR electrons. Combining models of interstellar CR propagation and attenuation with the calculation of the expected secondary electron spectrum and updated H$_{2}$ excitation cross sections by electron collisions, we derive the intensity of the four H$_{2}$ rovibrational transitions observable in dense, cold gas: (1-0)O(2), (1-0)Q(2), (1-0)S(0), and (1-0)O(4). The proposed method allows the estimation of $\zeta$ for a given observed line intensity and H$_{2}$ column density. We are also able to deduce the shape of the low-energy CR proton spectrum impinging upon the molecular cloud. We present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar CR proton spectrum. We comment on the capability of the James Webb Space Telescope to detect these near-infrared H$_{2}$ lines, making it possible to derive for the first time spatial variation of $\zeta$ in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will be possible to test competing models of CR propagation and attenuation in the interstellar medium, as well as compare CR spectra in different Galactic regions., Comment: 14 pages, 11 figures, accepted by A&A

A Monte Carlo simulation of electron energy deposition in a gas of molecular hydrogen has been conducted with the aim of producing an ab initio estimate of energy deposition parameters such as the mean energy per ion pair. A set of cross sections obtained using the molecular convergent close-coupling method were used as input. At high incident electron energies the mean energy per ion pair was calculated to be 36.3 eV, agreeing with the recommended value of 36.5 ± 0.3 eV. This represents the first fully ab initio calculation of the mean energy per ion pair using a self consistent data set.

Accurate calculation of spectral line broadening is important for many hot, dense plasma applications. However, calculated line widths have significantly underestimated measured widths for $\mathrm{\ensuremath{\Delta}}n=0$ lines of Li-like ions, which is known as the isolated-line problem. In this Letter, scrutinization of the line-width derivation reveals that the commonly used expression neglects a potentially important contribution from electron-capture. Line-width calculations including this process are performed with two independent codes, both of which removed the discrepancies at temperatures below 10 eV. The revised calculations also suggest the remaining discrepancy scales more strongly with electron temperature than the atomic number as was previously suggested.

Calculations of electron-impact excitation cross sections for molecular hydrogen have been performed using spherical- and spheroidal-coordinate formulations of the molecular convergent close-coupling method. We present a comparison and find good agreement between the results of these two techniques performed within the fixed-nuclei approximation for excitation from the ground X1Σg+ (v = 0) state of H2 to the B1Σu+ , C 1Πu, B′1Σu+ , D 1Πu, EF1Σg+ , b3Σu+ , c3Πu+ , a3Σu+ , e3Σu+ , h3Σg+ , and d 3Πu states. For the spheroidal-coordinate approach the adiabatic-nuclei method has been applied, allowing for a more reliable estimate of cross sections at near-threshold energies. Comparison of the adiabatic-nuclei cross sections with the corresponding fixed-nuclei cross sections is also presented.

We present benchmark integrated and differential cross-sections for electron collisions with H$_2$ using two different theoretical approaches, namely, the R-matrix and molecular convergent close-coupling (MCCC). This is similar to comparative studies conducted on electron-atom collisions for H, He and Mg. Electron impact excitation to the $b \ ^3\Sigma_u^+$, $a \ ^3\Sigma_g^+$, $B \ ^1\Sigma_u^+$, $c \ ^3\Pi_u$, $EF \ ^1\Sigma_g^+$, $C \ ^1\Pi_u$, $e \ ^3\Sigma_u^+$, $h \ ^3\Sigma_g^+$, $B' \ ^1\Sigma_u^+$ and $d \ ^3\Pi_u$ excited electronic states are considered. Calculations are presented in both the fixed nuclei and adiabatic nuclei approximations, where the latter is shown only for the $b \ ^3\Sigma_u^+$ state. Good agreement is found for all transitions presented. Where available, we compare with existing experimental and recommended data., Comment: 21 pages, 25 figures

We present a comprehensive set of vibrationally-resolved cross sections for electron-impact electronic excitation of the isotopologues of molecular hydrogen (D2, T2, HD, HT, and DT) initially in the ground electronic state. We apply the adiabatic-nuclei molecular convergent close-coupling (MCCC) method to calculate cross sections from threshold to 500 eV for excitation of all bound vibrational levels and dissociative excitation of the B 1 Σ u + , C 1 Π u , E F 1 Σ g + , B ′ 1 Σ u + , G K 1 Σ g + , I 1 Π g , J 1 Δ g , D 1 Π u , H 1 Σ g + , b 3 Σ u + , c 3 Π u , a 3 Σ g + , e 3 Σ u + , d 3 Π u , h 3 Σ g + , g 3 Σ g + , i 3 Π g , and j 3 Δ g electronic states from all bound vibrational levels of the ground electronic ( X 1 Σ g + ) state. Including the previously-published MCCC e-H2 cross sections (Scarlett et al., Atom. Data Nucl. Data Tables 137 (2021) 101361) the data set contains cross sections for over 60,000 electronic and vibrational transitions. The cross sections are presented in graphical form and provided as both numerical values and analytic fit functions in supplementary data files. The data can also be downloaded from the MCCC database at mccc-db.org .

Collisional radiative (CR) models for molecular hydrogen are of high relevance for performing qualitative and quantitative analysis of excited-state population densities measured in plasmas or predicting the dependence of plasma emission on parameter variations. Although the development of such models for H2 started decades ago, major uncertainties still exist regarding the most important set of input parameters, namely the cross sections for electron-impact excitation. The deviations between cross sections from different datasets are particularly pronounced in the energy region close to the threshold energy, strongly increasing the uncertainty of CR models applied to low-temperature plasmas. This paper presents experimental validation of a set of newly calculated non ro-vibrationally resolved electron-impact cross sections calculated for the triplet system of H2 using the molecular convergent close-coupling method in the adiabatic-nuclei formulation. These cross sections are implemented into a CR model based on the flexible solver Yacora. A first comparison of CR calculations with the different datasets to experimentally-determined population densities is performed at a planar ICP discharge for varying pressure (between 1 and 10 Pa) and RF power (between 700 and 1100 W). For the experimentally-accessible electron temperature and density range (2.5–10 eV and 1.8–3.3 × 1016 m−3, respectively), very good agreement between the model and experiment is obtained using the new data set, in contrast to previously used cross sections.

We present a comprehensive set of vibrationally-resolved cross sections for electron-impact electronic excitation of molecular hydrogen suitable for implementation in collisional-radiative models. The adiabatic-nuclei molecular convergent close-coupling method is used to calculate cross sections for excitation of all bound vibrational levels and dissociative excitation of the B 1 Σ u + , C 1 Π u , E F 1 Σ g + , B ′ 1 Σ u + , G K 1 Σ g + , I 1 Π g , J 1 Δ g , D 1 Π u , H 1 Σ g + , b 3 Σ u + , c 3 Π u , a 3 Σ g + , e 3 Σ u + , d 3 Π u , h 3 Σ g + , g 3 Σ g + , i 3 Π g , and j 3 Δ g electronic states from all v i = 0 –14 bound vibrational levels of the ground electronic ( X 1 Σ g + ) state. The data set consists of cross sections from threshold to 500 eV for over 5000 transitions, representing all possible electronic and vibrational transitions between the X 1 Σ g + state and the n = 2 –3 singlet and triplet states (where n refers to the united-atoms-limit principle quantum number). The cross sections are presented in graphical form and provided as both numerical values and analytic fit functions in supplementary data files. The data can also be downloaded from the MCCC database at http://mccc-db.org .

The feasibility of producing the molecular antihydrogen anion ${{\overline{\mathrm{H}}}_{2}}^{\ensuremath{-}}$ in the laboratory is investigated. Utilizing reaction rates calculated here involving the interaction of laser excited-state antihydrogen atoms held in magnetic minimum traps, key processes are identified that could lead to anion production, as well as competing effects leading to anti-atom loss. These are discussed in the context of present-day and near-future experimental capabilities.

A minority relativistic electron component can arise in both laboratory and naturally occurring plasmas. In the presence of high-atomic-number ion species, the ion charge state distribution at a low bulk electron temperature can be dominated by relativistic electrons, even though their density is orders of magnitude lower. This is due to the relativistic enhancement of the collisional excitation and ionization cross sections. The resulting charge state effect can dramatically impact the radiative power loss rate and the related Bethe stopping power of relativistic electrons in a dilute plasma.

We report cross sections for electron-impact dissociative excitation of the $B{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, $C{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$, $D{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$, ${B}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, and $E,F{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ singlet states of molecular hydrogen from all ${v}_{i}=0--14$ vibrational levels of the ground $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ state. Calculations are performed using the adiabatic-nuclei convergent close-coupling method formulated in prolate spheroidal coordinates from threshold to 500 eV. Agreement with previous calculations varies with transition and impact energy, ranging from excellent to poor. Agreement with available experiment is generally good.

The electron impact $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}\ensuremath{\rightarrow}b{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ transition in molecular hydrogen is one of the most important dissociation pathways to forming atomic hydrogen atoms, and is of great importance in modeling astrophysical and industrial plasmas where molecular hydrogen is a substantial constituent. Recently, it has been found that the convergent close-coupling (CCC) cross sections of Zammit et al. [Phys. Rev. A 95, 022708 (2017)] are up to a factor of 2 smaller than the currently recommended data. We have determined normalized differential cross sections for excitation of this transition from our experimental ratios of the inelastic to elastic scattering of electrons by molecular hydrogen using a transmission-free time-of-flight electron spectrometer, and find excellent agreement with the CCC calculations. Since there is already excellent agreement for the absolute elastic differential cross sections, we establish benchmark differential and integrated cross sections for the $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}\ensuremath{\rightarrow}b{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ transition, with theory and experiment being essentially in complete agreement.

We apply the adiabatic nuclei convergent close-coupling method to electron-impact dissociative excitation of ${\mathrm{H}}_{2}$ in the low-energy regime. Differential and integrated cross sections are presented for excitation of the $b^{3}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ state, the primary pathway to dissociation of ${\mathrm{H}}_{2}$ at low energies. Agreement with experiment is satisfactory. Results are also presented for the isotopologues ${\mathrm{D}}_{2}$, ${\mathrm{T}}_{2}$, HD, HT, and DT, which show a pronounced isotope effect near threshold in both the differential and integrated cross sections.

We calculate the kinetic-energy release distributions of fragments produced for electron-impact dissociation of the vibrationally excited molecular hydrogen ion ${{\mathrm{H}}_{2}}^{+}$ and its isotopologues ${{\mathrm{D}}_{2}}^{+}$ and ${{\mathrm{T}}_{2}}^{+}$. Here we apply the adiabatic-nuclei convergent close-coupling method and compare results with several different methods, including the $\ensuremath{\delta}$ approximation. Results are presented for a number of dissociative excitation transitions and dissociative ionization as a function of the initial vibrational state of the molecule. We confirm that the square root approximation is a good approximation for the adiabatic-nuclei kinetic-energy release cross sections of ${{\mathrm{H}}_{2}}^{+}$. Agreement with experiment, where available, is good.

Positron scattering from the vibrational ground state of the hydrogen molecular ion has been investigated using the single-center convergent close-coupling (CCC) method. Cross sections for dissociative excitation, dissociative ionization, total inelastic scattering, and proton production have been calculated at incident positron energies from 10 to 500 eV. For this energy range the mass stopping power and mean excitation energy have also been calculated for both the positron and electron cases. Since there are no experiments or other calculations for positron scattering on the hydrogen molecular ion, the obtained results have been compared with corresponding CCC results for positron collisions with He+ and electron collisions with H2+. Comparative analysis allows to estimate the quality of the approximations used in the present calculations.

The single-center adiabatic-nuclei convergent close-coupling method is used to investigate positron collisions with molecular hydrogen (${\mathrm{H}}_{2}$) in the ground and first vibrationally excited states. Cross sections are presented over the energy range from 1 to 1000 eV for elastic scattering, vibrational excitation, total ionization, and the grand total cross section. The present adiabatic-nuclei positron-${\mathrm{H}}_{2}$ scattering length is calculated as $A=\ensuremath{-}2.70{a}_{0}$ for the ground state and $A=\ensuremath{-}3.16{a}_{0}$ for the first vibrationally excited state. The present elastic differential cross sections are also used to ``correct'' the low-energy grand total cross-section measurements of the Trento group [A. Zecca et al., Phys. Rev. A 80, 032702 (2009)] for the forward-angle-scattering effect. In general, the comparison with experiment is good. By performing convergence studies, we estimate that our ${R}_{m}=1.448{a}_{0}$ fixed-nuclei results are converged to within $\ifmmode\pm\else\textpm\fi{}5%$ for the major scattering integrated cross sections.

We report the electron impact integrated and differential cross sections for excitation to the $b{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}, a{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}, c{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Pi}}}_{u}, B{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}, E,F{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}, C{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}, e{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}, h{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}, d{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Pi}}}_{u}, {B}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}, D{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}, {B}^{\ensuremath{'}}{}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$, and ${D}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$ states of molecular hydrogen in the energy range from 10 to 300 eV. Total scattering and total ionization cross sections are also presented. The calculations have been performed by using the convergent close-coupling method within the fixed-nuclei approximation. Detailed convergence studies have been performed with respect to the size of the close-coupling expansion and a set of recommended cross sections has been produced. Significant differences with previous calculations are found. Agreement with experiment is mixed, ranging from excellent to poor depending on the transition and incident energies.

The convergent close-coupling method has been used to solve the electron-hydrogen molecule scattering problem in the fixed-nuclei approximation. Excellent agreement with experiment is found for the grand total, elastic, electronic-excitation, and total ionization cross sections from the very low to the very high energies. This shows that for the electronic degrees of freedom the method provides a complete treatment of electron scattering on molecules as it does for atoms.

The immense progress in the field of electron–atom scattering, where pure Coulomb potentials are involved, has been translated to collision processes in Debye plasmas. The Debye–Huckel potential is used to describe plasma Coulomb screening effects depending on the temperature and density of the plasma. We provide benchmark results of electron–helium scattering for momentum transfer cross sections, discrete excitation and ionization over a broad range of incident energies and Debye radii.

We present calculations of vibrationally resolved cross sections for excitation of the , , , , and electronic states of molecular hydrogen. Here we apply the adiabatic nuclei convergent close-coupling method formulated in two-center prolate spheroidal coordinates. We find significant disagreement with previous calculations, where available.

A two-center close-coupling method that includes positronium (Ps) formation channels has been developed for positron scattering on molecular hydrogen. Calculations are performed within the fixed-nuclei approximation. Results for the grand total, Ps-formation, and direct ionization cross sections are presented. In general, good agreement with experiment is seen in all the calculated cross sections.

We have extended the $ab$ $initio$ convergent close-coupling method to electron scattering from the vibrationally excited molecular hydrogen ion and its isotopologues. Calculations have been performed within the adiabatic-nuclei approximation. Results are presented for dissociative excitation and ionization as a function of the initial vibrational state of the molecules. Comparison with experiment is excellent across the energy range from near threshold to 1 keV.

To apply spectroscopy as a diagnostic tool for dense plasmas, a theoretical approach to pressure broadening is indispensable. Here, a quantum-statistical theory is used to calculate spectral line shapes of few-electron atoms. Ionic perturbers are treated quasistatically as well as dynamically via a frequency fluctuation model. Electronic perturbers are treated in the impact approximation. Strong electron-emitter collisions are consistently taken into account with an effective two-particle T-matrix approach. Convergent close-coupling calculations give scattering amplitudes including Debye screening for neutral emitters. For charged emitters, the effect of plasma screening is estimated. The electron densities considered reach up to n(e) = 10(27) m(-3). Temperatures are between T = 10(4) and 10(5) K. The results are compared with a dynamically screened Born approximation for Lyman lines of H and H-like Li as well as for the He 3889 Å line. For the last, a comprehensive comparison to simulations and experiments is given. For the H Lyman-α line, the width and shift are drastically reduced by the Debye screening. In the T-matrix approach, the line shape is notably changed due to the dependence on the magnetic quantum number of the emitter, whereas the difference between spin-scattering channels is negligible.

We have extended the convergent close-coupling method to investigate electron scattering from the vibrationally excited molecular hydrogen ion H${{}_{2}}^{+}$, within the Born-Oppenheimer approximation. Results are presented for proton-production and dissociative-ionization cross sections. The comparison with experiment is excellent across the energy range from near threshold to 1 keV.

The $ab$ $initio$ convergent-close-coupling method has been extended to positron-molecule collisions within the adiabatic (fixed-nuclei) approximation. Application to molecular hydrogen at energies from 0.1 to 1000 eV has yielded convergent total ionization and grand total cross sections over most of the energy range. We find that very large calculations are required for convergence, even in the case of low-energy elastic scattering, due to the effects of positronium formation. In general, the comparison with experiment is good.

Electron-helium scattering in weakly coupled hot-dense (Debye) plasma has been investigated using the convergent close-coupling method. The Yukawa-type Debye-Hueckel potential has been used to describe plasma Coulomb screening effects. Benchmark results are presented for momentum transfer cross sections, excitation, ionization, and total cross sections for scattering from the ground and metastable states of helium. Calculations cover the entire energy range up to 1000 eV for the no screening case and various Debye lengths (5-100 a{sub 0}). We find that as the screening interaction increases, the excitation and total cross sections decrease, while the total ionization cross sections increase.

Electron-hydrogen scattering in weakly coupled hot-dense plasmas has been investigated using the convergent-close-coupling method. The Yukawa-type Debye-Hueckel potential has been used to describe the plasma screening effects. The target structure, excitation dynamics, and ionization process change dramatically as the screening is increased. Excitation cross sections for the 1s{yields}2s,2p,3s,3p,3d and 2s{yields}2p,3s,3p,3d transitions and total and total ionization cross sections for the scattering from the 1s and 2s states are presented. Calculations cover the energy range from thresholds to high energies (250 eV) for various Debye lengths. We find that as the screening increases, the excitation and total cross sections decrease, while the total ionization cross sections increase.

The electronic self‐energy ∑e is an important input in a quantum‐statistical theory for spectral line profile calculations. It describes the influence of plasma electrons on bound state properties. In dense plasmas, the effect of strong, i.e. close, electron‐emitter collisions can be considered by three‐particle T‐matrix diagrams. These digrams are approximated with the help of an effective two‐particle T‐matrix, which is obtained from convergent close‐coupling calculations with Debye screening. A comparison with other theories is carried out for the 2p level of hydrogen at kBT = 1 eV and ne = 2⋅1023 m−3, and results are given for ne = 1⋅1025 m−3.