Time-dependent Close-coupling Research Articles

Electron-impact ionization of the tantalum atom

Electron-impact ionization cross sections for the ground configuration of the tantalum (Ta) atom are calculated using a combination of non-perturbative time-dependent close-coupling and perturbative distorted-wave methods. Direct ionization of the 6s and 5d subshells leading to single ionization are presented. The inclusion of a polarization potential in the 6s direct ionization leads to a small reduction in the cross section. Angular factors are included that allow the configuration-average direct ionization cross sections to be split into LS term resolution. The magnitude of the ionization cross section suggests that the ionization mean free path for neutral Ta will be similar to that for tungsten (W) plasma facing components in fusion experiments, leading to similarly large fractions of prompt redeposition.

Shake-Off Process in Non-Sequential Single-Photon Double Ionization of Closed-Shell Atomic Targets

Amusia and Kheifets in 1984 introduced a Green’s function formalism to describe the effect of many-electron correlation on the ionization spectra of atoms. Here, we exploit this formalism to model the shake-off (SO) process, leading to the non-sequential single-photon two-electron ionization (double photoionization—DPI) of closed-shell atomic targets. We separate the SO process from another knock-out (KO) mechanism of DPI and show the SO prevalence away from the DPI threshold. We use this kinematic regime to validate our model by making a comparison with more elaborate techniques, such as convergent and time-dependent close coupling. We also use our model to evaluate the attosecond time delay associated with the SO process. Typically, the SO is very fast, taking only a few attoseconds to complete. However, it can take much longer in the DPI of strongly correlated systems, such as the H− ion as well as the subvalent shells of the Ar and Xe atoms and Cl− ion.

Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact

We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen for an incident energy of approximately 76.45 eV, we obtain a deep minimum in the CB1 triply differential cross section (TDCS). However, the TDCC for 74.45 eV and the 3DW for 74.46 eV gave a dip in the TDCS. For positron-hydrogen ionization (breakup) we apply the CB1 approximation and a modified CB1 approximation. We obtain a deep minimum in the TDCS and a zero in the CB1 transition matrix element for an incident energy of 100 eV with a gun angle of 56.13 ° . Corresponding to a zero in the CB1 transition matrix element, there is a vortex in the velocity field associated with this element. For both electron- and positron-impact ionization of hydrogen the velocity field rotates in the same direction, which is anticlockwise. All calculations are performed for a doubly symmetric geometry; the electron-impact ionization is in-plane and the positron-impact ionization is out-of-plane.

Multicenter distorted-wave approach for electron-impact ionization of molecules

We have previously used the molecular three-body distorted-wave model to examine electron-impact single ionization of molecules. One of the possible weaknesses of this approach lies in the fact that the continuum electron wave functions do not depend on the orientation of the molecule. Here we introduce a model called the multicenter molecular three-body distorted-wave (MCM3DW) approach, for which the continuum electron wave functions depend on the orientation of the molecule at the time of ionization. The MCM3DW results are compared with experimental data taken from work by Dorn and colleagues [Ren et al., Phys. Rev. A 91, 032707 (2015); Phys. Rev. A 93, 062704 (2016); Phys. Rev. A 95, 022701 (2017); Phys. Rev. Lett. 109, 123202 (2012); Gong et al., Phys. Rev. A 98, 042710 (2018)] in which they measured triple differential cross sections for single ionization of molecular hydrogen while simultaneously determining the orientation of the ${{\mathrm{H}}_{2}}^{+}$ ion at the time of ionization. Comparisons are also made with previous theoretical calculations. It is found that orientation effects are important for low incident energy electrons. Very nice agreement with experiment and the time-dependent close coupling results is found for an incident electron energy of 26 eV. Orientation effects become relatively unimportant by the time the incident electron energy is 54 eV.

Proton impact excitation of the Na atom

A time-dependent close-coupling (TDCC) method is developed and used to calculate proton impact excitation cross sections for atoms. Cross sections for excitations of the Na atom are calculated for proton impact energies ranging from 1.0 to 100.0 keV. The TDCC cross sections are found to be in agreement for some energies with recommended cross sections from overall fits to theoretical and experimental atomic data.

Electron-impact single and double ionization of the Fe atom and atomic ions

Electron-impact single and double ionization cross sections for the Fe atom and atomic ions are calculated using a combination of perturbative distorted-wave (DW) and non-perturbative time-dependent close-coupling (TDCC) methods. The single ionization cross sections include contributions from outer subshell direct ionization and excitation-autoionization. The double ionization cross sections include contributions from direct double ionization, inner subshell direct ionization, and excitation-autoionization. The accuracy of the perturbative DW calculations for direct ionization of the outer subshells of Fe and Fe+ are checked by comparison with non-perturbative TDCC calculations. The cross sections for single and double ionization are then compared with crossed-beams measurements.

Analysis of multiple scattering contributions in electron-impact ionization of molecular hydrogen

We report a combined experimental and theoretical study on the low-energy (E0 = 31.5 eV) electron-impact ionization of molecular hydrogen (H2). Triple differential cross sections are measured for a range of fixed emission angles of one outgoing electron between and −130° covering the full 4π solid angle of the second electron. The energy sharing of the outgoing electrons varies from symmetric ( eV) to highly asymmetric (E1 = 1 eV and E2 = 15 eV). In addition to the binary and recoil lobes, a structure is observed perpendicular to the incoming beam direction which is due to multiple scattering of the projectile inside the molecular potential. The absolutely normalized experimental cross sections are compared with results from the time-dependent close-coupling (TDCC) calculations. Molecular alignment dependent TDCC results demonstrate that these structures are only present if the molecule axis is lying in the scattering plane.

Double photoionization of helium from the 1s2p excited state

Time-dependent close-coupling (TDCC) calculations are carried out for the double photoionization of Helium in the excited state. TDCC results are presented for the total and energy differential cross sections for the term. TDCC results are presented for the total and energy differential cross sections for the levels. Differences found between the level resolved double photoionization cross sections are due to varying degrees of continuum correlation found in the outgoing two electrons.

Propensity for distinguishing two free electrons with equal energies in electron-impact ionization of helium

We report a combined experimental and theoretical study on the electron-impact ionization of helium at ${E}_{0}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}70.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and equal energy sharing of the two outgoing electrons (${E}_{1}={E}_{2}=23\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$), where a double-peak or dip structure in the binary region of the triple differential cross section is observed. The experimental cross sections are compared with results from convergent close-coupling (CCC), $B$-spline R-matrix-with-pseudostates (BSR), and time-dependent close-coupling (TDCC) calculations, as well as predictions from the dynamic screening three-Coulomb (DS3C) theory. Excellent agreement is obtained between experiment and the nonperturbative CCC, BSR, and TDCC theories, and good agreement is also found for the DS3C model. The data are further analyzed regarding contributions in particular coupling schemes for the spins of either the two outgoing electrons or one of the outgoing electrons and the $1s$ electron remaining in the residual ion. While both coupling schemes can be used to explain the observed double-peak structure in the cross section, the second one allows for the isolation of the exchange contribution between the incident projectile and the target. For different observation angles of the two outgoing electrons, we interpret the results as a propensity for distinguishing these two electrons---one being more likely the incident projectile and the other one being more likely ejected from the target.

Ionisation of noble gas atoms and H2O by antiproton impact

SynopsisA time-dependent convergent close-coupling approach to antiproton collisions with noble gas atoms and H2O has been developed using a fully multielectron treatment of the targets. Integrated cross sections for single ionisation have been calculated in a wide range of impact energies from 5 keV up to 2 MeV. Obtained results are in good agreement with experiment.

Antiproton-impact ionization of Ne, Ar, Kr, Xe, andH2O

We calculate antiproton-impact total single ionization of Ne, Ar, Kr, Xe, and ${\mathrm{H}}_{2}\mathrm{O}$ using a time-dependent convergent close-coupling approach. The Ne, Ar, Kr, and Xe atom wave functions are described in a model of six $p$-shell electrons above a frozen Hartree-Fock core with only one-electron excitations from the outer $p$ shell allowed. For treating the water molecule we use a neonization method recently proposed by Montanari and Miraglia [J. Phys. B: At. Mol. Opt. Phys. 47, 015201 (2014)], which describes the ten-electron water molecule as a dressed Ne-like atom in a pseudospherical potential. In the present work the target states of noble gas atoms and water are obtained using a Laguerre basis expansion. For the noble gas atoms there is reasonably good agreement with the calculated single-ionization cross sections.

Generalized collisional radiative model for light elements: C: Data for the B isonuclear sequence

A first stage collision database is assembled which contains electron-impact excitation, ionization, and recombination rate coefficients for B, B+, B2+, B3+, and B4+. The first stage database is constructed using the R-matrix with pseudostates, time-dependent close-coupling, and perturbative distorted-wave methods. A second stage collision database is then assembled which contains generalized collisional-radiative ionization, recombination, and power loss rate coefficients as a function of both temperature and density. The second stage database is constructed by solution of the collisional-radiative equations in the quasi-static equilibrium approximation using the first stage database. Both collision database stages reside in electronic form at the IAEA Labeled Atomic Data Interface (ALADDIN) database and the Atomic Data Analysis Structure (ADAS) open database.

Differential cross section for the double photoionization of Mg

The time-dependent close-coupling (TDCC) method is used to calculate the energy and angle triple differential cross section for the double photoionization of Mg() at a photon energy of 55.49 eV to compare with recent converged close-coupling (CCC) calculations and in support of recent experiments at ELETTRA. Comparisons are made between the TDCC, CCC, and experimental results for equal energy sharing at scattering angles of , , and , and for unequal energy sharing at and . Comparisons are made between the TDCC and CCC results for equal and unequal energy sharing at . In addition, TDCC, CCC, and experimental results are compared for complex scattering amplitudes and the TDCC method is used to calculate single energy differential cross sections at photon energies of 30, 35, 40, 45, and 55.49 eV. Although there are scattering angles in the triple differential cross sections at 55.49 eV at which the TDCC, CCC, and experimental results are quite different, the overall agreement between the theories and experiment is reasonable for such small cross sections.

Electron-impact ionization of the N atom

Electron-impact ionization cross sections for the ground and excited states of the N atom are calculated using the non-perturbative R-matrix with pseudo-states and time-dependent close-coupling (TDCC) methods, as well as the perturbative distorted-wave method. The TDCC and distorted-wave results for the () excited configurations are much larger than for the ground configuration. In all cases the TDCC results are substantially lower than the distorted-wave results. The ionization cross section results will lead to a better understanding of moderately dense astrophysical and laboratory plasmas containing nitrogen.

Theoretical and experimental(e,2e)study of electron-impact ionization of laser-aligned Mg atoms

We have performed calculations of the fully differential cross sections for electron-impact ionization of magnesium atoms. Three theoretical approximations, the time-dependent close coupling, the three-body distorted wave, and the distorted wave Born approximation, are compared with experiment in this article. Results will be shown for ionization of the $3s$ ground state of Mg for both asymmetric and symmetric coplanar geometries. Results will also be shown for ionization of the $3p$ state which has been excited by a linearly polarized laser which produces a charge cloud aligned perpendicular to the laser beam direction and parallel to the linear polarization. Theoretical and experimental results will be compared for several different alignment angles, both in the scattering plane as well as in the plane perpendicular to the incident beam direction.

Close-coupling approach to antiproton-impact breakup of molecular hydrogen

An ab initio time-dependent convergent close-coupling approach to describing antiproton collisions with molecular hydrogen or the hydrogen molecular ion has been developed. The approach accounts for all possible orientations of the molecular target. Averaging over the molecular orientations is performed fully analytically. For the molecular hydrogen target calculated orientation-averaged total cross sections for single ionization and proton production are compared with several experiments over the energy range of 1–2000 keV. Results for single ionization are in good agreement with experiment, except for the region around the experimental maximum. For proton production reasonable agreement with experiment is observed above 40 keV.

Photoionization-Excitation and Double Photoionization of He@C60

The R-matrix with pseudostates (RMPS) and time-dependent close-coupling (TDCC) methods are used to calculate photoionization-excitation and double photoionization cross sections of He and He@C60.

Target Structure-Induced Suppression of the Ionization Cross Section for Low-Energy Antiproton-Molecular Hydrogen Collisions: Theoretical Confirmation

Theoretical confirmation of the experimentally observed phenomenon [Knudsen et al., Phys. Rev. Lett. 105, 213201 (2010)] of target structure-induced suppression of the ionization cross section for low-energy antiproton-molecular hydrogen collisions is given. To this end a novel time-dependent convergent close-coupling approach to the scattering problem that accounts for all possible orientations of the molecular target, has been developed. The approach is applied to study single ionization of molecular hydrogen on the wide energy range from 1 keV to 2 MeV with a particular emphasis on low energies. Results for the orientation-averaged total single ionization cross section are compared with available experimental data and good agreement is found at low (<20 keV) and high (>90 keV) energies. A minor discrepancy is found within a small energy gap near the maximum of the cross section.

Ejection of Quasi-Free-Electron Pairs from the Helium-Atom Ground State by Single-Photon Absorption

We investigate the single-photon double ionization of helium at photon energies of 440 and 800eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia etal. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.

An R-matrix with pseudo-state approach to the single photon double ionization and excitation of the He-like Li+ ion

The success of the R-matrix with pseudo-state (RMPS) method to model single photoionization processes, benchmarked against dedicated synchrotron light source measurements, is exploited and extended to investigate, single photon double ionization cross-sections for the He-like, Li+ ion. We investigate these processes from both the ground state and the excited 1s2s 1, 3S metastable levels of this He-like system. Comparisons of the results from the RMPS method are made with other state-of-the-art theoretical approaches such as time-dependent close-coupling (TDCC), B-splines and the convergent close-coupling (CCC). Excellent agreement with various other theoretical approaches are achieved but differences occur which are highlighted and discussed. For the ground state the peak of the cross-section is ∼2 kilo-barns (Kb), that for the 1s2s 1S state is ∼6 and ∼1 Kb for the corresponding 1s2s 3S state. All the cross-sections for single photon double ionization are extremely small, being in the region of 2–10 Kb, or less, rendering their experimental determination extremely challenging.