We demonstrate that electron momentum spectroscopy (EMS), based on the high-energy $\mathrm{}(e,2e)\mathrm{}$ reaction, can provide clean information on the electronic structure of the target that is not obscured by interchannel coupling in the ionization continuum. This is in sharp contrast to high-energy photoionization in which target states with different angular momentum, which are close in energy, are always mixed through interchannel coupling in the continuum. In atoms, in particular, the photoionization spectra of $\mathrm{nl}(l>\mathrm{}0\mathrm{})\mathrm{}$ subshells always have significant intensity due to admixture from the neighboring $\mathrm{ns}$ subshell. This mixing is shown to be negligible in electron momentum spectroscopy, a point we specifically illustrate for the $\mathrm{}(e,2e)\mathrm{}$ spectra of the argon $M$ shell.

Cross sections for electron-impact single and double detachment from $B^{-}$ have been measured from 0 to 200 eV. The single-detachment cross section peaks at 4–5 eV with a cross-section maximum of about ${10}^{-14}{\mathrm{cm}}^2$. A ${(2p}^3)$ ${}^{4}S$ state has recently been predicted to give rise to a resonance state in the $H^{2-}$ dianion [T. Sommerfeld et al., Phys. Rev. A 55 1903 (1997)]. We observe no resonances in the detachment cross section of $B^{-}$ and hence no sign of an equivalent shortlived $B^{2-}{(2p}^3)$ state. The ratio of the double- to single-detachment cross section reaches a constant value of 3% at energies above 50 eV. A simple model relates this number to a shake-off probability of about 90%. The ratio between double and single ionization of neutral atomic targets at high energy is also discussed, and the model relates this ratio to the shake-off probability in the sudden approximation.

$R$-matrix calculations have been carried out to investigate the effect of core-valence electron correlation on the scattering of electrons with Ca atoms at low energies. While the major feature of the $d$-wave shape resonance with a peak near 1.3 eV in the total cross section can be reproduced by including only the correlation among the valence electrons, we show that the existence of a low-energy peak near 0.15 eV, which has also been observed experimentally, can be obtained only if the correlations between the valence and core electrons are considered. This low-energy peak is shown not caused by a shape resonance, but rather due to the rapid decrease of the $s$- and $p$-wave phase shifts with collision energies. The present result also indirectly supports the previous conclusion that core-valence electron correlation is responsible for bringing the electron affinity of ${\mathrm{Ca}}^{-}$ from 72 to 15 meV, i.e., to a value which is much closer to the experiment.

The production and subsequent autoionization of the ${\mathrm{Ar}}^{+}{(}^1{D}_2{)6d}^{\prime }$ satellite state that is formed either by shake-up or recapture during the Auger decay of a $2p$ vacancy in Ar has been studied by photoelectron spectroscopy in the energy region from 243 to 256 eV. The creation of near zero energy electrons below and immediately above the $\mathrm{Ar}\hspace{0.5em}2p$ ionization threshold is discussed. Some ambiguous points in previous studies are clarified.

Collision-induced dissociation experiments have been conducted on ${\mathrm{CO}}_2^{+}$ ions using three types of ion sources, different collision energies, light and heavy targets, and three different apparatuses possessing different values of energy and angular resolution. Results of all these collision experiments establish that collision-induced dissociation dynamics is remarkably different from that obtained in photoionization experiments. Although the ${\mathrm{O}}^{+}\mathrm{}+\mathrm{}\mathrm{CO}$ channel is the energetically lowest dissociation pathway, dissociation into ${\mathrm{CO}}^{+}+\mathrm{O}$ is found to be the preferred channel. Our measurements of the ratio ${\mathrm{CO}}^{+}$ to ${\mathrm{O}}^{+}$ are substantially higher than that obtained in photoionization experiments. Determinations of kinetic energies released upon dissociation of ${\mathrm{CO}}_2^{+}$ are also made. Rationalization of our results is attempted in terms of curve crossings of low-lying excited states of ${\mathrm{CO}}_2^{+}$ ions.

The bound excited ${\mathrm{np}}^3{}^{2}D$ terms of ${\mathrm{Si}}^{\mathrm{-}},$ ${\mathrm{Ge}}^{\mathrm{-}},$ and ${\mathrm{Sn}}^{\mathrm{-}}$ have been investigated with a combination of single- and multiphoton tunable infrared laser experiments. The binding energies of the two $J=3/2\mathrm{}$ and $J=5/2\mathrm{}$ fine-structure levels were found to be 0.527 234(25) and 0.525 489(20) eV, respectively for ${\mathrm{Si}}^{\mathrm{-}},$ 0.401 44(10) and 0.377 27(6) eV for ${\mathrm{Ge}}^{\mathrm{-}},$ and 0.397 617(15) and 0.304 635(15) eV for ${\mathrm{Sn}}^{\mathrm{-}}.$ These results constitute improvements in accuracy over previous experimental term energies of up to four orders of magnitude and further provide experimental values for the ${}^{2}D$ fine-structure splittings: 14.08(20), 192.6(9), and $749.95\left(15\right)\mathrm{}{\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{Si}}^{\mathrm{-}},$ ${\mathrm{Ge}}^{\mathrm{-}},$ and ${\mathrm{Sn}}^{\mathrm{-}},$ respectively. In addition, the photodetachment thresholds of the ionic ${}^4{S}_{3/2}$ ground states have been reinvestigated. This resulted in improved electron affinities of 1.262 119(20), 1.389 521(20), 1.232 712(15), and 1.112 067(15) eV for ${\mathrm{C}}^{\mathrm{-}},$ ${\mathrm{Si}}^{\mathrm{-}},$ ${\mathrm{Ge}}^{\mathrm{-}},$ and ${\mathrm{Sn}}^{\mathrm{-}},$ respectively. Various attempts towards an observation of the very weakly bound ${\mathrm{C}}^{\mathrm{-}}{(}^{2}D)$ and ${\mathrm{Si}}^{\mathrm{-}}{(}^{2}P)$ terms remain unsuccessful.

We present theoretical integral and differential cross sections for transitions produced by electron collision with an excited ${\mathrm{H}}_2$ molecule ${(c}^3{\Pi }_u$ and $a^3{\Sigma }_g^{\mathrm{}(+)\mathrm{}})$ obtained with the Schwinger multichannel method in a four-state approximation. We also present the transition rates obtained with these cross sections. The results indicate the importance of these scattering processes in modeling ${\mathrm{H}}_2$ discharge.

A multichannel version of effective-range theory is developed, applicable to electron scattering by positive ions. The formalism allows for the inclusion of the effect of long-range inverse-power-law interactions arising from target distortion in the field of the projectile. A perturbation theory is described for the construction of asymptotic states accounting for the combined effect of Coulomb and polarization forces. The explicit extraction of threshold branch-point singularities leaves modified reaction-matrix elements that vary slowly with energy, thereby permitting the smooth extrapolation of scattering parameters through reaction thresholds. A systematic procedure is made available to determine the effect of long-range forces on quantum defects and resonance level shifts; it is applied here, as an illustration, in first order. The theory provides an extension, to a wider class of scattering systems and long-range interactions, of an analysis of threshold behavior in electron-ion reactions given some time ago [M. Gailitis, Sov. Phys. JETP 17, 1328 (1963)].

Excitation and charge transfer cross sections for collisions of protons with hydrogen are calculated by direct solution of the time-dependent Schrödinger equation on a three-dimensional Cartesian lattice. The $2s,$ $2p,$ $3s,$ $3p,$ and $3d$ excitation cross sections and the $1s,$ $2s,$ $2p,$ $3s,$ $3p,$ and $3d$ charge transfer cross sections from the $1s$ ground state at 10-, 40-, and 100-keV incident proton energy are found by projecting a time-evolved wave function onto the lattice target states of hydrogen. Excitation processes are calculated in the rest frame of the hydrogen atom, while capture processes are calculated in the rest frame of the proton. The computed excitation and charge transfer cross sections are in good agreement with recent experiments and other theoretical results based on coupled-channels methods.

Despite the importance of ${\mathrm{BCl}}_3$ in plasmas used for commercial etching processes, no calculations or measurements of elastic low-energy electron scattering from ${\mathrm{BCl}}_3$ have previously appeared in the literature. We therefore present calculations based on the complex Kohn method for elastic electron-${\mathrm{BCl}}_3$ scattering at incident electron energies below 8 eV. We find a near-zero-energy virtual state and a sharp temporary negative-ion resonance at 0.25 eV, which likely contributes to the large electron attachment cross sections observed in swarm studies.

In this work we present a method developed to measure separately the radiative and nonradiative contributions to the cross section for collisional deexcitation of metastable hydrogenlike ions. We present the results of the first experiment applying this method, where deexcitation of metastable ${\mathrm{He}}^{+}\mathrm{}\left(2s\right)\mathrm{}$ ions in collisions with argon atoms is considered. First the total deexcitation cross section for 6.6 keV ${}^4{\mathrm{He}}^{+}\mathrm{}\left(2s\right)\mathrm{}$ ions is measured absolutely in a beam-gas attenuation experiment. Then the absolute cross section for nonradiative deexcitation is measured separately by coincident detection of projectiles keeping their initial charge state 1+ and recoil ions (formed only in nonradiative deexcitation events). Further, we discuss a semiclassical calculation of the radiative deexcitation cross section, in which the influence of competing electron-capture processes is taken into account in a semiempirical fashion. The result is in agreement with the measured radiative deexcitation cross section of $(5.4\mathrm{}\pm 1.3)\times {10}^{-16}\hspace{0.5em}{\mathrm{cm}}^2,$ which amounts to $(70\mathrm{}\pm 8)\%$ of the total deexcitation cross section.

It is argued that the cross section for ionization of a hydrogenlike ion penetrating a target at relativistic speed $v=\beta c$ saturates already at moderate values of $\gamma \equiv 1/\sqrt{1-{\beta }^2}.$ This is at variance with results published previously [R. Anholt and U. Becker, Phys. Rev. A 36, 4628 (1987)]. A simple model is developed and shown to yield cross sections in accordance with those of the only two experiments performed so far.

A method and apparatus have been developed for precise measurements of the ratios of total cross sections for electron-impact ionization and photoionization in rare gases based on the comparison of the total ion yields of the two ionization processes. Low uncertainties for the cross-section ratios are achieved above all by using a cryogenic electrical substitution radiometer as a primary detector standard in the soft-x-ray and vacuum ultraviolet spectral ranges in order to determine the impinging photon flux. On the basis of our measured cross-section ratios and well-known total photoionization cross sections we deduce absolute total electron-impact ionization cross sections of rare gases. As a result we present ratios of total electron-impact ionization cross sections to total photoionization cross sections and deduce total electron-impact ionization cross sections of neon in the energy range of electrons from 140 to 4000 eV and of photons from 100 to 1500 eV. Relative uncertainties as low as 1.3% for the cross-section ratios and 2.8% for the total electron-impact ionization cross sections have been achieved. A comparison of our cross-section data with published experimental and theoretical data is presented.

Single and double electron capture from He targets by ${\mathrm{Ar}}^{16+}$ ions have been studied at projectile velocities from 0.3 to 1.5 a.u. Cold-target recoil-ion momentum spectroscopy was used to record the energy gain and scattering angle simultaneously. For single capture, the reaction window is found to spread in width approximately as the square root of the projectile velocity and to shift slightly toward smaller energy-gain values as the velocity increases. The angular distributions center at the half Coulomb angle over most of the velocity range covered, but differ in shape from multichannel Landau-Zener model results. For double capture, transfer ionization dominates and feeds primarily $n$-symmetric states, where n is the principal quantum number. True double capture feeds mainly $n$-asymmetric states. The angular distributions for double capture lie outside the half Coulomb angle, indicating the importance of two-step processes in populating doubly excited states.

Electron-impact excitation cross sections and Maxwellian-averaged rate coefficients are calculated from the ground ${3d}^7{}^{4}F$ to the first ${3d}^7{}^{4}P$ and the second ${3d}^7{}^{2}G$ excited terms in ${\mathrm{Co}}^{2+}$ in close coupling with 8 and 27 terms; much larger transition arrays were first explored in a distorted-wave approximation. Both transitions are important elements of the emission spectrum of type Ia supernovae. We find that the two excitations are dominated by autoionizing resonances attached to higher-lying excited terms in the ${3d}^7,$ ${3d}^{6}4s,$ ${3d}^{6}4p,$ and ${3d}^{6}4d$ configurations. Those $\mathrm{LS}$ terms that are strongly excited from the ground ${3d}^7{}^{4}F,$ as found by distorted-wave calculations, are included in a large $R$-matrix calculation using asymptotics based on multichannel quantum defect theory.

In this work we study the double-differential cross sections (DDCSs) of ejected electrons in single-ionization collisions of protons with helium atoms. The final state of the emitted electron is modeled by the correlated wave function ${\Phi }_2,$ a confluent hypergeometric function of two variables. We introduce a series representation of the ${\Phi }_2$ wave function in terms of two-body Coulomb-like states, corresponding to the electron-projectile and electron-target relative motions. We consider undistorted and eikonal initial states that give rise to the Born ${\Phi }_2$ and eikonal initial-state ${\Phi }_2$ approximations, respectively. In both cases we obtain analytic series representations of the transition matrices, which exhibit a strong numerical convergence. We obtain DDCSs in ion-helium collisions in the intermediate- to high-energy regime with the Born ${\Phi }_2$ approximation and show that it qualitatively agrees with the available experimental data.

Individual contributions to radiative stabilization in ${\mathrm{Ne}}^{10+}+\mathrm{He}$ collisions were studied at projectile energies from 1 to 150 keV. In these collisions doubly excited states ${3lnl}^{\prime }$ and ${4lnl}^{\prime }\hspace{0.5em}(n>~4)$ can be produced by uncorrelated two-electron transitions or by dielectronic processes caused by the electron-electron interaction. Compared to our previous study of stabilization, we extended the projectile-energy range down to 1 keV and the present analysis is based entirely on experimental data. Radiative stabilization was evaluated by combining our recent results for Auger decay cross sections with available radiative and nonradiative branching ratios. The total amount of radiative stabilization was found to be nearly constant $(\approx 0.35)$ within the studied energy range. However, the individual contributions to stabilization were found to change significantly. At 1 keV the major contribution to stabilization is shown to be due to the decay of the nonequivalent electron configurations ${3lnl}^{\prime }\hspace{0.5em}(n>~6)$ produced during the collision by the dielectronic process of autoexcitation.

The Moyal formalism utilizes the Wigner transform and associated Weyl calculus to define a phase-space representation of quantum mechanics. In this context, the Weyl symbol image of the Heisenberg evolution operator admits a generic semiclassical expansion that is based on classical transport and related $O\left({ħ}^2\right)$ quantum corrections. For two atom systems with a mutual pair interaction described by a spherically symmetric potential, the predictive power and convergence properties of this semiclassical expansion are investigated via numerical calculation. The rotational invariance and tensor structure present are used to simplify the semiclassical dynamics to the point where numerical computation in the six-dimensional phase space is feasible. For a variety of initial Gaussian wave functions and a selection of different observables, the $O\left({ħ}^0\right)$ and $O\left({ħ}^2\right)$ approximations for time dependent expectation values are determined. The interactions used are the Lennard-Jones potentials, which model helium, neon, and argon. The numerical results obtained provide a first demonstration of the practicality and usefulness of Moyal quantum mechanics in the analysis of realistic atomic systems.

Energy loss of slow ($v\approx 0.06\hspace{0.5em}\mathrm{a}.\mathrm{u}.\mathrm{}$), highly charged ${\mathrm{Ar}}^{9+}$ ions scattered from a Au(111) single-crystal surface has been investigated for different incident angles ($\psi =5\mathrm{}°$ and $37.5°$) and a fixed scattering angle of $75°.$ The energy position of the elastic peak agrees well with the value expected for the binary elastic scattering. At small incidence angle $\left(5\mathrm{}°\right)$ an additional inelastic energy loss was identified in spite of the large scattering angle. This additional energy loss is found to compare well with a combination of a modified Firsov model with processes of electron capture, recapture, Auger transitions, and image charge acceleration during the ion-surface interactions.

The wake potential induced by heavy ions in a free-electron gas is used as the interacting potential in the binary collisional formalism (BCF). Detailed results are presented for the stopping power, energy-loss distributions, mean free path, and electron angular distributions. The results were compared with those obtained with the Yukawa central potential and the dielectric formalism on equal footing. A link between the dielectric and binary collision formalisms was established. The BCF was found to be very sensitive with respect to the dielectric function used in the wake potential. Dressed projectiles are also examined, and their distributions compared. The binary formalism is found to be well behaved. At high impact velocities, as expected, it provides only the collisional term, missing the excitation of collective mode.