Session T4: Electron-Atom and Electron-Molecule Collisions

Overset grid implementation of the complex Kohn variational method for electron-polyatomic molecule scattering

The complex Kohn variational method, which represents the continuum wave function in each channel using a combination of Gaussians and Bessel or Coulomb functions, has been successful in numerous applications to electron-polyatomic molecule scattering and molecular photoionization. The hybrid basis representation limits it to relatively low energies ($< 50$ eV) , requires an approximation to exchange matrix elements involving continuum functions, and hampers its coupling to modern electronic structure codes for the description of correlated target states. We describe a successful implementation of the method using completely adaptive overset grids to describe continuum functions, in which spherical subgrids are placed on every atomic center to complement a spherical master grid that describes the behavior at large distances. An accurate method for applying the free-particle Green’s function on the grid eliminates the need to operate explicitly with the kinetic energy, enabling a rapidly convergent Arnoldi algorithm for solving linear equations on the grid, and no approximations to exchange operators are made. Results for electron scattering from several polyatomic molecules will be presented.   

B-spine R-matrix with pseudostates calculations for electron-impact excitation and ionization of magnesium.

The B-spline R-matrix with Pseudo-States method [1,2] was employed to treat electron collisions with magnesium atoms. Predictions for elastic scattering, excitation, ionization, and ionization-excitation were obtained for all transitions between the lowest 25 states of Mg in the energy range from threshold to 100~eV. The accuracy of the results was checked by comparing with available experimental data and with results obtained in different approximations with increasing number of coupled states. The largest scattering model included 716 coupled states, most of which were pseudo-states that simulate the effect of the high-lying Rydberg continuum and, most importantly, the ionization continuum on the results for transitions between the discrete states of interest. Similar to our work on e-Be collisions~[3], this effect is particularly strong at ``intermediate'' incident energies of a few times the ionization threshold. The dataset generated from the largest model is estimated to be accurate to within a few percent for the cross sections of relevance for plasma modelling. [1] O.~Zatsarinny, Comp.\ Phys.\ Commun.~{\bf 174} (2006) 273. [2] O.~Zatsarinny and K.~Bartschat, J.~Phys.\ B~{\bf 46} (2013) 112001. [3] D.~V.~Fursa and I.~Bray, J.~Phys.\ B~{\bf 49} (2016) 235701.   

B-spline R-matrix calculations for electron-impact excitation of N$^{3+}$.

There are major discrepancies between recent ICFT (Intermediate Coupling Frame Transformation) [1] and DARC (Dirac Atomic R-matrix Code) calculations [2] regarding electron-impact excitation rates for transitions in several Be-like ions. To identify possible reasons for these discrepancies and to estimate the accuracy of the various results, we carried out independent B-Spline R-Matrix (BSR) calculations for electron-impact excitation of N$^{3+}$. Our close-coupling expansions contain the same 238 target states as the previous ICFT and DARC calculations, but with an improved representation of the target structure. We find close agreement among all calculations for the strong transitions between low-lying states, whereas serious discrepancies remain for the weak transitions and those involving high-lying excited states. The variations in the final results for the collision strengths are mainly due to differences in the structure description, specifically the inclusion of correlation effects, rather than the treatment of relativistic effects or problems with the validity of the three methods to describe the collision. [1] L.~Fern\'andez-Menchero {\it et al.}, Astron.\ Astroph.~{\bf 566} (2014) A104. [2] K.~M.~Aggarwal {\it et al.}, Mon.\ Not.\ R.~Astr.\ Soc.~{\bf 461} 3997.   

Electron excitation of Cesium and its application to plasma modeling

Electron impact excitation cross-sections and rate coefficients have been calculated using fully relativistic distorted wave theory for several fine-structure transitions from the ground as well as excited states of cesium. As an application, the calculated detailed cross-sections are used to construct a reliable collisional radiative (CR) model [1] to characterize the hydrogen-cesium plasma. These processes play dominant role in low pressure hydrogen-cesium plasma, which is relevant to the negative ion based neutral beam injectors for the ITER project. The calculated cross-sections and the extracted plasma parameters from the present model are compared with the available experimental and theoretical results [2]. [1] R.~K.~Gangwar, Dipti, R.~Srivastava and L.~Stafford, \textit{Plasma Sources Sci. Technol. }\textbf{25}, 035025 (2016). [2] Priti, Dipti, R K Gangwar and R Srivastava, \textit{J. Quant. Spectrosc. Radiat. Transf, } \textbf{187, } 426 (2017).   

Theoretical method to study electron-impact rotational excitation of neutral molecules

A general theoretical approach to study rotational excitation in collisions of an electron with a neutral molecule is developed [1]. Scattering matrices for the process are generated using the UK R-matrix method [2, 3] and then used to compute rotational excitation cross sections for an energy range from 0 to a few eV for different combinations of initial and final rotational states of the target molecule. The approach is applied to obtain cross sections for rotational excitation of HCCH (acetylene). References: 1. M. Khamesian, \textit{Theoretical study of negative molecular ions relevant to the interstellar and laboratory plasma,} Ph. D. thesis, University of Central Florida, 2016. 2. J. Tennyson, Phys. Rep. \textbf{491}, 29 (2010). 3. J. Tennyson, D. B. Brown, J. J. Munro, I. Rozum, H. N. Varambhia, and N. Vinci. Quantemol-N: an expert system for performing electron molecule collision calculations using the R-matrix method. J. Phys. Conf. Series, 86:012001, 2007.   

Dissociative recombination of HCl+: direct an indirect mechanisms

We present the study of the Dissociative Recombination (DR) of HCl$^+$ treating both the direct and indirect dissociation mechanisms. The relevant electronic states are calculated from ab initio principles by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the ion and Rydberg states. Direct and indirect DR cross sessions were calculated independently and added incoherently. The former was obtained by solving the time-dependent Schrodinger equation and propagating the wave packets along the resonant states, while the latter was computed using a theoretical model. In this model, an upper bound for the indirect process is obtained using a vibrational frame transformation of the elements of the scattering matrix at energies just above the ionization threshold. Vibrational excitations of the ionic core from the ground vibrational state to the first three excited states are considered and autoionization is neglected. The calculated cross section is compared to measurements.   

The conversion of resonances to bound states in the presence of a Coulomb potential and the computation of autoionization lifetimes from quantum defects

The conversion of resonant metastable states to bound states with changing potential strength in the presence of a Coulomb potential proceeds by a mechanism fundamentally different from the same process in the case of short-range potentials. This phenomenon, which can accompany changes in molecular geometry, is central to the physics of the process of dissociative recombination of electrons with molecular cations. We verify computationally that there is no direct connection between a resonance pole of the $S$-matrix and the bound state poles for several model problems. We present a detailed analysis of the analytic structure of the scattering matrix in which the resonance pole remains distinct in the complex plane while a new state appears in the bound state spectrum. Nonetheless, as might be expected from quantum-defect theory, there is a close analytic relation between the resonant behavior of scattering at positive energies and the energies of the bound states. This connection allows the width of a resonance at low energies to be calculated directly from the behavior of the quantum defects with changing potential strength or molecular geometry.   

Intramolecular electron transfer in transient ammonia anion resonances

We report a combined experimental and theoretical study of dissociative electron attachment (DEA) dynamics of ammonia. Fragment momentum imaging experiments performed at MPIK Heidelberg and LBNL Berkeley found that DEA involving two electronic Feshbach resonances produce both H- and NH2- from ammonia. Two-body dissociation producing H- occurs via direct dissociation on either of two resonant anion states, with the lower and higher energy resonances leading to ground state and electronically excited NH2*, respectively. Using ab initio electronic structure theory we found that dissociation to H and NH2- involves a virtual anion state that asymptotically approaches the lower of these two dissociation limits, with nonadiabatic coupling due to electron transfer at considerable N-H distances. Through complex Kohn electron scattering calculations we examine the electron attachment probabilities in the molecular frame and compare these with measured fragment angular distributions to analyze and draw conclusions on the transient anion dynamics for each dissociation channel.   

Spin entanglement in elastic electron scattering from lithium atoms.

In two recent papers [1,2], the possibility of continuously varying the degree of entanglement between an elastically scattered electron and the valence electron of an alkali target was discussed. In order to estimate how well such a scheme may work in practice, we present results for elastic electron scattering from lithium in the energy regime of 1$-$5~eV and the full range of scattering angles~$0^\circ - 180^\circ$. The most promising regime for Bell-correlations in this particular collision system are energies between about 1.5~eV and 3.0~eV, in an angular range around $110^\circ \pm 10^\circ$. In addition to the relative exchange asymmetry parameter, we present the differential cross section that is important when estimating the count rate and hence the feasibility of experiments using this system. [1] K.~Blum and B.~Lohmann, Phys.\ Rev.\ Lett.~{\bf 116} (2016) 033201. [2] B.~Lohmann, K.~Blum, and B.~Langer, Phys.\ Rev.\ A~{\bf 94} (2016) 032331.   

Positron Annihilation in the Undergraduate Laboratory

While there are a variety of undergraduate laboratory experiments in the literature, they tend to focus on specific positron experiments and use specialized equipment that limit their flexibility. Here we present a positron spectroscopy experimental apparatus designed for the undergraduate lab. Rather than specialized pulse processing the apparatus utilizes a PC oscilloscope as its primary data acquisition utility with pulse processing happening in software instead of hardware. This allows the apparatus to explore a variety of physical phenomena with the positron annihilation including material science, 2 and 3 gamma annihilation properties, polarimetry via Compton scattering, QED tests, and local hidden variable theories. The supporting software is flexible and allows students to pursue these experiments through exploration rather than simply supporting data acquisition.