An electron scattered by an atom or molecule experiences a generalized polarization potential due to bound-free correlation. A classical electrostatic model of this correlation potential is presented that is equivalent to a local-density response model recently used to compute van der Waals interaction constants. The frequency-dependent oscillator strength is parametrized as a function of local electron density for each volume element in an extended electronic system, atom, or molecule. A response surface is defined by a cutoff criterion that can be chosen for atoms to give the empirical static dipole polarizability. The classical model extends this theory to arbitrary geometry and to shorter-range interactions. A density-dependent dielectric constant is defined within the response surface. This model determines the polarization response and internal electric field due to an external test charge, as well as the generalized polarization potential acting on the test charge.

With reference to recent progress in preparing slow and intense Rb beams (rubidium atomic funnel and low-velocity intense source of atoms from a magneto-optical trap) we have calculated total and differential elastic scattering cross sections and scattering lengths for ground-state collisions of ${}^{85}$Rb as well as ${}^{87}$Rb. Our results are based on the state of the art $X^1{\Sigma }_g^{+}$ and $a{}^3{\Sigma }_u^{+}$ potentials. Pronounced resonances in the total scattering cross sections occur for collision velocities below 15 m/s. We propose scattering experiments with slow Rb beams in order to check the currently known singlet and triplet potential-energy curves.

A Feshbach resonance associated with the ${1s3s4s}^{4}S$ state of He${}^{-}$ has been observed in the He${(1s2s}^{3}S)$ + $e^{-}\left(\epsilon s\right)$ partial photodetachment cross section. The residual He${(1s2s}^{3}S)$ atoms were resonantly ionized and the resulting He${}^{+}$ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy $E_{\mathrm{r}}=2.959\mathrm{}255\left(7\right)\mathrm{}$ eV and a width $\Gamma =0.19\mathrm{}\left(3\right)\mathrm{}$ meV, in agreement with a recent calculation.

Ab initio autoionization widths for the Ne$\mathrm{}\left(3s\right)\mathrm{}$-Ar and Ne$\mathrm{}\left(3p\right)\mathrm{}$-Ar systems are calculated in a two-electron approximation. Using correctly normalized wave functions, the calculations result in absolute values for the autoionization widths. We use a modified coupled-channel code with ab initio potentials and ionization widths as input to calculate cross sections for ionization and intramultiplet mixing. The influence of ionization on the intramultiplet mixing process can thus be studied. Experimental results for intramultiplet mixing cross sections for the Ne$\mathrm{}\left(3p\right)\mathrm{}$-Ar system at a collision energy $〈E〉=1.7\mathrm{}$ eV are presented. These data and ionization cross sections obtained by Bussert et al. [Z. Phys. A 320, 105 (1985)] are in good agreement with coupled-channel calculations. For the Ne$\mathrm{}\left(3s\right)\mathrm{}$-Ar system the calculated ionization widths are in good agreement with empirical ionization widths derived in a multiproperty analysis of a wide range of scattering data. For this system we have to conclude that the ab initio results cannot explain the experimental cross section ratio ${Q(}^3{P}_0{)/Q(}^3{P}_2)$=1.3. Therefore, ab initio calculations of the real part of the potential are essential for a correct quantum-mechanical coupled-channel analysis.

The cross sections for capture, ionization, capture from pair production, and free pair production were measured for 0.96-GeV/nucleon U${}^{92+}$ and 0.405-, 0.96-, and 1.3-GeV/nucleon La${}^{57+}$ ions incident on Au, Ag, and Cu targets. The cross sections for capture from pair production, free pair production, ionization, and total capture (the sum of capture from pair production, radiative electron capture, and nonradiative capture) are analyzed as a function of collision energy, projectile, and target atomic numbers. We find that, when the collision energy is increased from 0.405 GeV/nucleon to 1.3 GeV/nucleon, the capture from pair production and the free pair production cross sections increase by almost a factor of 6, while the capture cross section decreases by two orders of magnitude. The ionization cross section is found to vary very weakly with the collision energy in the 1-GeV/nucleon energy range. We found a dependence of free pair production cross sections on the target and projectile atomic number to be close to $Z^2$, characteristic of an ionizationlike process. We also found a dependence of the capture from pair production cross sections on the target atomic number to be usually steeper than $Z_t^2$, and on the projectile atomic number, somewhat steeper than the $Z_p^5$, characteristic of a capturelike process. Theory and experiment are in some disagreement for capture from pair production, and free pair production, cross sections, but are in general agreement for the other capture processes and for ionization.

$L$-shell electron-impact ionization cross sections for highly charged uranium ions from fluorinelike ${\mathrm{U}}^{83+}$ through lithiumlike ${\mathrm{U}}^{89+}$ have been measured at 45-, 60-, and 75-keV electron energy. The cross sections were obtained from x-ray measurements of the equilibrium ionization balance in an electron beam ion trap. The measured cross sections agree with recent relativistic distorted wave calculations.

The Gaussian wave-packet phase-space representation is used to show that the expansion in powers of ħ of the quantum Liouville propagator leads, in the zeroth-order term, to results close to those obtained in the statistical quasiclassical method of Lee and Scully in the Weyl-Wigner picture. It is also verified that, propagating the Wigner distribution along the classical trajectories, the amount of error is less than that coming from propagating the Gaussian distribution along classical trajectories.

Intramultiplet mixing collisions have been investigated using polarized $\mathrm{Ne}\mathrm{}\left(3p\right)\mathrm{}$ atoms with a well-defined initial relative velocity, i.e., two initial-state vectors. By analyzing the final-state center-of-mass velocity with a (Doppler-detuned) probe laser with a well-defined polarization and detecting the laser-induced fluorescence we measure the differential cross section and the polarization of the final state, i.e., two final-state vectors. Measurements have been done at a collision energy $E=70\mathrm{}\mathrm{meV},$ for both the ${\alpha }_7$ and ${\alpha }_9$ (Paschen numbering) initial states. For the elastic ${\alpha }_7\to {\alpha }_7$ collisions the experimental results are strongly hindered by the undesired response on elastic scattering of the metastable $\mathrm{Ne}{(3s,}^3{P}_0)$ state. Inelastic collisions have been investigated for the ${\alpha }_9$ state, that forms a closed level system with the metastable $\mathrm{Ne}{(3s,}^3{P}_2)$ state. For the ${\alpha }_9\to {\alpha }_8$ transition experimental results are presented for both parallel and perpendicular orientations of the polarizations of the pump and probe laser, respectively, as well as for circular polarized $({\sigma }^{+},{\sigma }^{+})$ and $({\sigma }^{+},{\sigma }^{-})$ configurations. The experiments are compared to a full quantum-mechanical coupled channels calculation of the scattering process, using the model potentials of Hennecart and Masnou-Seeuws as input. The output of this calculation is combined with a density matrix description of the initial- and final-state polarization. Both shape of the differential cross sections and their relative scaling for different laser polarizarion configurations agree well with experiment. This gives strong support for the quality of the model potentials used.

A theoretical description of multiple ionization of atoms and molecules produced by an energetic ion impact is developed. It is based on the statistical energy-deposition model of Russek and Meli [Physica (Amsterdam) 46, 222 (1970)]. In this model the probability for the formation of a collision-induced final state with $n$ electrons in the continuum is obtained, assuming that its value is proportional to the volume of phase space available at that ionization state for a certain value of statistically distributed deposited energy. The model is extended in two respects. First, the deposited energy for each trajectory is considered as a fluctuating quantity with a certain distribution and the ionization probability is calculated as a weighted average over the distribution. Second, the mean value and straggling of the deposited energy are calculated within the Lindhard-Scharff local plasma approximation [Mat. Fys. Medd. K. Dan. Vidensk. Selsk. 27, No. 15 (1953)]. Sample calculations for collisions of protons and F${}^{4+}$ ions with neon atoms at an energy of 1 Mev/amu are presented and compared with calculations within the independent-electron approximation and with experimental data.

The complex Kohn variational method is used to study low-energy electron collision processes involving ${\mathrm{CH}}_3\mathrm{Cl}.$ The elastic differential and momentum transfer cross sections are studied from low electron energy (0.5 eV) through the resonance region (3.5 eV) to 10 eV. The effects of target correlation and polarization are studied. A comparison is made between the results of this study and experimental results available for this system.

Individual $L$-shell x-ray production cross sections were measured with ${}^{12}$C ions in the energy range 0.22– 0.83 MeV u${}^{-1}$ on Cd, In, Sb, Te, Ba, Tb, and Yb targets. From these measurements the $L$-subshell ionization cross sections were extracted and compared with the predictions of the direct ionization theories, i.e., with perturbed stationary state theory with energy loss, Coulomb deflection and relativistic corrections, the semiclassical approximation (SCA) in the united-atom limit, and SCA coupled-channel calculations involving a few lowest lying states. Better overall agreement between experiment and theory was found for the SCA theory. Inclusion of the SCA coupled-channel calculations improved the agreement for the $L_2$-subshell in the very adiabatic region, but theory fails to describe $L_1$-ionization cross sections, especially in the 0.4 $<{\xi }_{L_1}<$ 0.7 region. The degree of multiple ionization of outer shells caused by C ion bombardment was estimated from the x-ray line energy shifts and by comparing the x-ray intensity ratios for carbon ions and protons.

With a relativistic partial-wave method we calculated numerically the photon-energy-angle distributions of electron bremsstrahlung in the field of atoms with atomic number $Z=1\mathrm{}$ and 79 for kinetic energies of incident electrons $T_1=2\mathrm{}$ to 5 MeV. We have found that in this intermediate-electron-energy region, the Born approximation works fine for low-$Z$ elements, as expected. Our partial-wave results indicate that the shape of photon-energy-angle distributions is almost independent of atomic-electron screening. Our partial-wave results also indicate that both the high-energy approximation and the Born approximation predictions are quite good for the shape of photon-energy-angle distributions for small photon angles. We compared our partial-wave results with experiments.

A Galilean invariant theory of atomic collisions is constructed for straight-line trajectories of nuclei with arbitrary impact parameters. The theory is based on Sturmian expansions in Fourier space, and provides an exact description of excitation and ionization processes for a wide range of collision velocities. Advantages of Sturmian sets over conventional eigenstates are emphasized. Detailed spectra of electrons ejected in head-on ion-atom collisions are calculated.

A technique is described for detecting light fragment ions from dissociation of molecular ions. The crossed-beams method is adapted from a previous approach by implementation of ion optics, dispersion, and detection methods that are particularly focused on detection of the light ions from the breakup of heteronuclear molecular ions. Absolute cross sections as a function of interaction energy are reported for the process $e+{\mathrm{CD}}^{+}\to e+{\mathrm{D}}^{+}+\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s},$ where the ${\mathrm{CD}}^{+}$ targets are characteristic of emission from a discharge ion source.

Many-body perturbation theory for general model spaces (quasidegenerate theory) has been applied to compute excitation energies for doubly excited autoionizing states in N${}_2$. Special attention is given to the energy range 20–30 eV. A few autoionizing single excited states with excitation energies around 18 eV are also studied. The many-body method is in addition used to compute electric dipole transition moments (oscillator strengths) from the ground state to the doubly excited states. Autoionization rates (lifetimes) are also calculated, as well as partial rates for autoionization to the three lowest electronic states in N${}_2^{+}$. The computed results show encouraging agreement with resonances observed in branching ratios for vibrationally resolved photoionization cross sections.

Cross sections have been determined for electron transfer and ionization in collisions between ${\mathrm{H}}^{+},$ ${\mathrm{He}}^{2+},$ ${\mathrm{Li}}^{3+},$ ${\mathrm{Be}}^{4+},$ ${\mathrm{B}}^{5+},$ ${\mathrm{C}}^{6+},$ ${\mathrm{N}}^{7+}$ ions and target ${\mathrm{C}}^{5+}\mathrm{}\left(1s\right)\mathrm{}$ ions for projectile energies 125–1000 keV/amu using a coupled-Sturmian-pseudostate approach [Phys. Rev. A 35, 3799 (1987)]. A comparison is made with results using simpler approaches, and rules for scaling with the projectile nuclear charge are considered. Except for ${\mathrm{H}}^{+},$ the results for electron transfer to the ground state are estimated to be converged to 1%; for ionization, the convergence is probably to 10–20 %.

The collisional quenching of excited $\mathrm{Na}\mathrm{}\left(4p\right)\mathrm{}$ by He and ${\mathrm{H}}_2$ has been studied using the semiclassical, impact-parameter, close-coupling method based on a molecular expansion augmented by the plane-wave translation factor. Interactions between the colliding partners are approximated using suitable pseudopotentials. At low velocities $0.0005<~v<~0.001\mathrm{a}.\mathrm{u}.\mathrm{},$ interference oscillations which depend on the spatial alignment of the electron charge distribution before the collision are present in the cross sections for ${\mathrm{H}}_2$ quenching perturber. The cross sections due to ${\mathrm{H}}_2$ are larger than those of He atoms throughout the energy region. At $T=470\mathrm{}\mathrm{K}$ the present quenching rate coefficients show good agreement with the experimental data of Kleiber et al. [J. Chem. Phys. 98, 1101 (1993)].

The energy-loss straggling and energy width of states of slow ions interacting with a homogeneous electron gas are evaluated within a quadratic response theory and the random-phase approximation. These results are compared with corresponding results determined from a fully nonlinear scattering theory approach. The quadratic response theory is shown to be a good approximation for high electron densities and small ion charges.

Energy losses of ${\mathrm{H}}^{+}$ fragments repelled parallel and antiparallel to the beam direction at dissociation are measured for a glancing-angle incidence of 0.15–0.6-MeV/amu ${\mathrm{H}}_2^{+}$ ions on a clean (001) surface of SnTe. The sum of the energy losses of the specularly reflected fragments is larger than twice that of protons of the same velocity reflected from the surface. For the interpretation of the vicinage effect on the stopping power of the surface, the energy losses of the pairs of fragments resulting from the dissociation of ${\mathrm{H}}_2^{+}$ are calculated at the glancing-angle scattering. The contribution of single-electron excitation to the energy loss of the pair is calculated with the harmonic-oscillator model [J. Basbas and R. H. Ritchie, Phys. Rev. A 25, 1943 (1982)] and that of collective excitation by the superposition of surface wakes of two protons. A dependence of the vicinage effect on the distance from the surface is found from the calculated stopping powers, where the pronounced effect becomes weak as the distance decreases. The calculated energy losses of the reflected pairs of protons agree fairly with those measured. The agreement of the calculated energy losses with those measured supports the calculated dependence of the vicinage effects.

Measurements of antiproton stopping powers around the stopping-power maximum are presented for targets of Al, Si, Ti, Cu, Ag, Ta, Pt, and Au. The Low Energy Antiproton Ring antiproton beam of 5.9 MeV is degraded to 50–700 keV, and the energy loss is found by measuring the antiproton velocity before and after the target. Target thicknesses have been determined accurately by weighing and Rutherford backscattering techniques. The antiproton stopping powers are found to be reduced by around 35% for both light and heavy elements near the electronic stopping-power maximum as compared to the equivalent proton stopping power. The antiproton stopping powers and the Barkas effect; that is, the difference in stopping power between protons and antiprotons is compared to theoretical estimates, based on a harmonic-oscillator model and an electron-gas model, and good agreement is obtained.