The effects of relativity on resonance contributions to electron-impact ionization of ${\mathrm{Ar}}^{15+}$, ${\mathrm{Fe}}^{23+}$, ${\mathrm{Kr}}^{33+}$, and ${\mathrm{Xe}}^{51+}$ are investigated. The effects of relativity are found to increase the resonance strengths by 20% for ${\mathrm{Fe}}^{23+}$ and by as much as a factor of 3 for ${\mathrm{Xe}}^{51+}$. The magnetic quadrupole (M2) radiation for the 1s2s2${\mathit{p}}_{3/2}$ J=5/2 metastable state depends strongly on the atomic number, varying as ${\mathit{Z}}^8$. We find that including the M2 emission for the 1s2s2${\mathit{p}}_{3/2}$ J=5/2 state is essential for obtaining reliable cross sections for ions with Z≥26.

Compton scattering from bound electrons is studied within external field quantum electrodynamics and the independent-particle approximation (IPA), but without making use of any additional approximations, such as the impulse or incoherent-scattering factor approximations. Our calculations of the doubly differential cross section for scattering of unpolarized and polarized photons from bound atomic electrons as a function of scattered photon energy and angle are based on a numerical evaluation of the second-order S matrix in self-consistent screened atomic potentials. Such calculations permit the simultaneous discussion of all regions of the Compton spectrum for scattering from any atomic subshell. We present a systematic theoretical investigation of this process for atomic inner subshells at energies where binding effects in these subshells are important. We also discuss the efficient evaluation of the total atom scattering cross section in these cases. For individual subshells, we discuss the applicability of widely used approximate methods with regard to the spectral features they describe. For the K shell, we compare with earlier attempts at calculations within the relativistic S-matrix framework. We also discuss the cross section singly differential in scattered photon angle, emphasizing the contribution of terms neglected when making the incoherent-scattering factor approximation, as well as implications for total cross sections and for attenuation coefficients. Finally we discuss several recent experiments, including the efforts to find the infrared rise for soft photons.

The multiconfiguration Hartree-Fock method for continuum wave functions has been used to calculate the scattering length and phase shifts over extremely low energies ranging from 0 to 1 eV very accurately for electron-helium scattering. The scattering length is calculated very accurately with wave functions computed exactly at zero energy, resulting in an upper bound of 1.1784. The electron correlation and polarization of the target by the scattering electron, which are very important in these calculations, have been taken into account in an accurate ab initio manner through the configuration-interaction procedure by optimizing both bound and continuum orbitals simultaneously at each kinetic energy of the scattered electron. Detailed results for scattering length, differential, total, and momentum-transfer cross sections obtained from the phase shifts are presented. The present scattering length is found to be in excellent agreement with the experimental result of Andrick and Bitsch [J. Phys. B 8, 402 (1975)] and the theoretical result of O’Malley, Burke, and Berrington [J. Phys. B 12, 953 (1979)]. There is excellent agreement between the present total cross sections and the corresponding experimental measurements of Buckman and Lohmann [J. Phys. B 19, 2547 (1986)]. The present momentum-transfer cross sections also show remarkable agreement with the experimental results of Crompton, Elford, and Robertson [Aust. J. Phys. 23, 667 (1970)].

We have determined the distribution of n states initially populated in a double- or multielectron-capture collision of ${\mathrm{Kr}}^{\mathit{q}+}$ (q=17,18) with Kr at low energy (≊5 keV/u). Radiative decay curves of Rydberg transitions of the projectile ions were obtained by translating the target gas jet along the projectile beam. As a result for ${\mathrm{Kr}}^{18+}$ projectiles, we found that an initial population of ${\mathrm{Kr}}^{16+}$ equally distributed from n=14 to 19 allows the observed Kr xvii 13-14 radiative decay curve to be reproduced relatively well. Furthermore, coincidence time-of-flight spectroscopy of photons emitted by recoil ions of given charge states was used to measure the radiative decay of recoil-ion transitions. From the Kr vii 7i-8k decay curve we deduced an initial population distribution of the ${\mathrm{Kr}}^{6+}$ recoil ion centered around n=20. These results lead to a better understanding of the mechanisms of double-electron capture responsible for populating Rydberg states, especially those in asymmetric configurations.

The production of free electron-positron pairs in relativistic heavy-ion collisions is investigated within first-order time-dependent perturbation theory. An analytic expression for the differential pair-production cross section is obtained by employing Furry-Sommerfeld-Maue wave functions for the description of continuum states in the external field of the target nucleus. The angular distributions of electrons and positrons and cross sections are calculated and compared with previous results.

A charge-transfer experiment has been done in which ${\mathrm{Ar}}^{7+}$ was incident on sodium in the ground state and first excited state. Absolute capture cross sections were determined for projectile velocities in the range of 0.1–1.0 a.u. The experimental findings are compared with the results of semiclassical calculations.

We present measurements of absolute excitation differential cross sections for electron scattering by ground-state sodium in the 3P manifold at 2.3, 2.4, 2.5, 2.6, 3.0, 3.3, and 3.7 eV in the angular range 1° to 60°. No calibration or normalization procedures are involved. Comparisons with computational results of the close-coupling approximation and experimentally obtained Δ${\mathit{M}}_{\mathit{s}}$- and Δ${\mathit{M}}_{\mathit{L}}$-changing cross sections are made.

Relative triple-differential cross sections for ionization of Xe 4d and 5p orbitals have been measured in coplanar asymmetric kinematics. The scattered-electron energy is held at 1000 eV while the ejected-electron energy is changed so as to select energy-loss values ΔE in or out of the region characterized by the giant resonance in the 4d→ɛf channel. The ionization mechanism as well as the interference between the direct and resonant processes have been investigated by varying the momentum transfer in a collision at each ejected-electron energy. Strong interchannel coupling between the 5p and 4d shells in the region of the giant resonance has been observed in (e,2e) experiments. Distorted-wave-Born-approximation calculations have been compared with the experimental triple-differential cross section and used to guide the discussion.

Fast (0.35–1.2-MeV) ${\mathrm{He}}^{\mathrm{-}}$ ions produced by double electron capture have been observed in ${\mathrm{He}}^{+}$-He collisions. The thickness of the gas-jet target (4×${10}^{13}$–${10}^{15}$ atoms/${\mathrm{cm}}^2$) has been calculated without any separate calibration. Single-collision conditions have allowed a direct measurement of the total cross section of the double electron capture. These data can be related to the development of theoretical studies on double electron capture.

In the plane-wave Born approximation (PWBA), scaling relations for K-shell generalized oscillator strengths and energy-loss cross sections of atoms are given. The total K-shell ionization cross sections, obtained at high impact energy, are used to obtain the Bethe collisional parameter ${\mathit{c}}_{\mathit{K}}$ for atoms ranging from carbon to gold. These values of ${\mathit{c}}_{\mathit{K}}$ are, in general, significantly different from those obtained previously with the help of Fano plots at relatively low impact energies. Furthermore, using Hippler’s modification of the PWBA [Phys. Lett. A 144, 81 (1990)], total K-shell ionization cross sections of various atoms by electron and positron impact are calculated over a wide energy range. The electron-impact ionization cross sections for light atoms are in satisfactory agreement with the experimental data.

Using a crossed-beam technique, we have measured differential vibrational-excitation cross sections of the electronic ground state of molecular oxygen by electron impact. The angular range covered was from 12° to 156°, while the incident energy range of the electrons was from 5 to 15 eV. Integrated cross sections and vibrational-state energies were also obtained. The cross sections exhibit a maximum near 10 eV incident energy.

The thermally averaged total electron spin-flip cross sections (${\mathrm{\sigma }}_{\mathrm{SF}}$) for Mu-Cs have been measured in a 2 atm ${\mathrm{N}}_2$ moderator at 543, 566, and 643 K using the muon-spin-rotation technique. Within an overall experimental error of ±15%, the measured cross sections can be taken as temperature independent, ${\mathrm{\sigma }}_{\mathrm{SF}}$=39.7±6.0×${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$, though there may be a trend to decreased values at the lower temperatures. This average value is considerably lower than either the early calculated results of Dalgarno and Rudge [Proc. R. Soc. London Ser. A 286, 519 (1965)] or the more recent ones of Cole and Olson [Phys. Rev. A 31, 2137 (1985)] for the corresponding H-Cs collisions, indicating a dramatic isotope effect.

We have applied a recently proposed ${\mathit{L}}^2$ method to evaluate total and partial photodetachment cross sections of ${\mathrm{H}}^{\mathrm{-}}$ up to the N=3 threshold. The method was specially designed to evaluate continuum states when interchannel coupling is very strong, so that the ${\mathrm{H}}^{\mathrm{-}}$ ion is an ideal system to test its validity. The calculated cross sections exhibit two Feshbach resonances below N=2, the well-known shape resonance just above that threshold, and five Feshbach resonances converging to N=3. The structure of these resonances is analyzed in detail, as well as the behavior of the partial cross sections in these regions. We also provide a complete set of Fano and Starace parameters that are useful to interpret the total and partial photodetachment spectra in the resonance regions. Finally, our results are compared with the available experimental data as well as with other theoretical calculations.

The optical-excitation-function method has been used in a crossed-atom and electron-beam arrangement to measure the electron-impact cross section of the copper 4 ${}_{}{}^2{}_{}{}^{}$P→4 ${}_{}{}^2{}_{}{}^{}$S resonance lines (324.8, 327.4 nm) from threshold (3.8 eV) to 8 eV. Relative experimental cross-section data are normalized at an energy of 1000 eV with respect to first Born theory that includes the 4 ${}_{}{}^2{}_{}{}^{}$S→4 ${}_{}{}^2{}_{}{}^{}$P resonance transition with an oscillator strength of 0.652 and cascading from the (3${\mathit{d}}^{10}$nd${)}^2$D states with n=4,...,10. The measured Cu 4 ${}_{}{}^2{}_{}{}^{}$S→4 ${}_{}{}^2{}_{}{}^{}$P cross section is compared with recent theoretical calculations in close-coupling approximation. Very good agreement is found with the ten-state close-coupling theory of Scheibner and Hazi (private communication).

We present a fully-${\mathit{L}}^2$-integrable calculation of the partial photoionization cross sections of He between the N=3 and 4 thresholds. The energy positions, total and partial autoionization widths, and Fano and Starace parameters for the 4lnl’ doubly excited states lying in this region are also provided. We have found that only one series of resonances with large autoionization widths is observed in the total-cross-section spectrum. On the other hand, several resonance series are exhibited in the partial N=3 cross sections. Our results also show that the 2p and 3p cross sections dominate the N=2 and 3 ones, respectively. Good agreement with the existing experimental data is found.

Studies of destruction of very-high-n (100<n<400) Rydberg atoms in collisions with the polar targets HF and ${\mathrm{NH}}_3$ are reported. Analysis of the data using the essentially-free-electron model suggests that, for ultralow electron energies (∼80 μeV–1.4 meV), the cross section σ(ɛ) for rotationally inelastic scattering of electrons by a polar target varies approximately as 1/ɛ, where ɛ is the electron energy.

Collision-induced dissociation experiments on ground-state ${\mathrm{S}}_2^{+}$ and ${\mathrm{CS}}^{+}$ have been carried out using a crossed-beam ion translational energy spectrometer with a very high angular resolution (≤${10}^{\mathrm{-}4}$ rad). The distributions of kinetic energies released upon dissociations ${\mathrm{S}}_2^{+}$→${\mathrm{S}}^{+}$+S, ${\mathrm{CS}}^{+}$→${\mathrm{S}}^{+}$+C, and ${\mathrm{CS}}^{+}$→${\mathrm{C}}^{+}$+S in 3-keV collisions with ${\mathrm{N}}_2$ have been measured; dissociation channels have been identified with the aid of computer simulations of trajectories through the apparatus of ions possessing different center-of-mass kinetic-energy release (${\mathit{K}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$) components. In the case of ${\mathrm{S}}^{+}$ formation from ${\mathrm{S}}_2^{+}$, dissociation occurs via excitation to repulsive ${\mathrm{S}}_2^{+\mathrm{*}}$ electronic states, giving rise to ${\mathit{K}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ components of 1.5 and 2.0 eV; lower ${\mathit{K}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ values (0 and 0.3 eV) are obtained upon dissociation of a bound excited state of ${\mathrm{S}}_2^{+}$ by a curve-crossing mechanism. Dissociation of ${\mathrm{CS}}^{+}$ occurs via rotational resonances, giving rise to narrow, symmetrical structure in the ${\mathrm{S}}^{+}$ translational energy spectrum; direct dissociation also occurs which yields relatively high ${\mathit{K}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ values due to the breaking of the strong C≡S triple bond. The ${\mathrm{CS}}^{+}$→${\mathrm{C}}^{+}$+S channel is considerably weaker.

The angle-dependent cross section for perturbed overlapping autoionization lines excited in ion-atom collisions is derived analytically. This electron angular distribution shows strong enhancement in the forward direction, and includes interference between overlapping lines that profoundly affect the cross section in the forward direction. The relative cross sections of autoionization lines excited in ${\mathrm{He}}^{+}$+He collisions at 5, 10, and 15 keV are calculated and found to be in good agreement with experiment.

Cross sections for double photoionization of He are calculated using the lowest-order many-body perturbation theory. There are three amplitudes contributing in the present calculation. They represent the three mechanisms for double ionization, namely, two-step-1, shake off, and ground-state correlation. It is explicitly shown that the cross section for each of these mechanisms depends strongly on the adopted form of the dipole interaction as indicated by Dalgarno and Sadeghpour [Phys. Rev. A 46, 3591 (1992)]. Our final results obtained by the sum of three amplitudes do not depend on the choice of the dipole formula at photon energies above 1 keV. The ratios of the cross sections for double ionization to single ionization are in excellent agreement with recent experimental results at energies 2–12 keV.

The angular anisotropy ${\mathrm{\alpha }}_2$ and spin polarization ${\xi }_2$ parameters have been calculated for Ar ${\mathit{L}}_3$MM, Kr ${\mathit{M}}_{4,5}$NN, Xe ${\mathit{N}}_{4,5}$OO, and Xe ${\mathit{M}}_{4,5}$NN Auger transitions using the multichannel multiconfiguration Dirac-Fock method (MMCDF). The results of our MMCDF calculations are in good general agreement with experiment. We have also determined the separate contributions of the exchange, relaxation, and channel interaction to the angular anisotropy of the Auger process. We have shown that especially for Auger lines having at least two strong partial amplitudes corresponding to different values of the orbital angular momentum both the exchange and channel interaction can change the value of the ${\mathrm{\alpha }}_2$ parameter significantly and sometimes even change its sign. Because of the additional eigenchannel phase shifts the ${\mathrm{\alpha }}_2$ parameters are more sensitive to channel interaction than to the exchange. In contrast the relaxation has a small effect on the angular distribution of Auger electrons. A comparative study showed that the ${\mathrm{\alpha }}_2$ parameters are usually less sensitive to correlation than the branching ratios or even the total Auger rates. This was traced back to universal scaling of the modulus of transition amplitudes and to the existence of one dominating ionization channel in most transitions. The ${\xi }_2$ parameters were small for all strong Auger lines and exceedingly sensitive to all correlation effects.