The polarization of Lyman-α radiation, produced by electron-impact excitation of atomic hydrogen, has been measured over the extended energy range from near threshold to 1800 eV. Measurements were obtained in a crossed-beam experiment using a silica-reflection linear polarization analyzer in tandem with a vacuum-ultraviolet monochromator to isolate the emitted line radiation. Comparison with various theoretical calculations shows that the present experimental results are in good agreement with theory over the entire range of electron-impact energies and, in particular, are in excellent agreement with theoretical convergent-close-coupling (CCC) calculations performed in the present work. Our polarization data are significantly different from the previous experimental measurements of Ott, Kauppila, and Fite [Phys. Rev. A 1, 1089 (1970)].

Absolute elastic differential cross sections for electron collision with carbon dioxide $\left({\mathrm{CO}}_2\right)$ at impact energies from 1.5 to 100 eV and scattering angles from 15° to 130° have been measured. Also, a calculation has been made that uses two different types of close-coupling approaches and covers all scattering angles in the same energy region. The measurements are in excellent agreement with observations by other authors. They also agree with the present calculation for all energies above 10 eV. The agreement becomes less satisfactory as the energy decreases below 8 eV, particularly at scattering angles below 60°, where the measurements show a conspicuous shoulder around this angle in the energy between 5 and 7 eV. Integral and momentum-transfer cross sections have been estimated from extrapolations to 0° and 180°. Extensive comparisons with recent other theories are included.

We complete a previous description of classical ionization in ${\mathrm{He}}^{2+}+\mathrm{H}$ collisions, extending the energy range of the illustrations and using as initial conditions either a microcanonical distribution or a hydrogenic one that produces better cross sections near threshold. We call those ionizing electrons that are bound in a frame where the nuclear motion is suppressed molecular-type electrons. We illustrate the evolution of these electrons in phase space, and show that it may properly be called saddle-point ionization at low nuclear velocities, while they give rise at high energies to the standard direct ionization process.

We study long-range diagonal adiabatic corrections to the interaction potentials of the ground state of homonuclear alkali-metal dimers. We show that in the long-range limit the diagonal adiabatic matrix elements may be expressed as a series of inverse powers of the internuclear distance. Each of the nonzero coefficients of this expansion is a mass-dependent correction term to the dispersion coefficients. They are computed by using a complex integral representation, that transforms the molecular problem into an atomic one. Numerical results are presented.

Cadmium $\mathrm{}(e,2e)\mathrm{}$ energy spectra have been measured which show the effects of electron impact exchange processes at large scattering angles. As the scattering angle is increased from 2° to 15° the relative peak intensities of the ${}^3{P}_1$ and ${}^1{P}_1$ ${4d}^9{5s}^{2}5p$ autoionizing resonances change by a factor of 3. A semiempirical model is presented which seeks to parametrize all the data in terms of a single constant.

Differential cross sections (DCS’s) for excitation of the $2{}^{1}S$ and $3{}^{1}S$ states in helium have been measured at small scattering angles from 0° to 15° for the electron-impact energies from 100 to 500 eV. Measurements were performed with a high angular resolution, better than 1°, and with an accuracy of the angle position of 0.2°. Distinct forward peaking features have been observed at minute scattering angles lower than about 3° for impact energies higher than 200 eV. Experimental DCS’s as functions of the scattering angle are compared with theoretical calculations based on various kinds of approximation method. Effective generalized oscillator strengths (GOS’s) for the $2{}^{1}S$ and $3{}^{1}S$ excitations were deduced from the DCS’s for impact energies 100–500 eV as a function of the squared momentum transfer $K^2.$ A systematic discrepancy between the measured effective GOS’s and the theoretical GOS calculated by the first Born approximation has been displayed at the low limit of $K^2.$ The effective GOS’s appear to approach the theoretical GOS very slowly as the impact energy increases, however, the GOS for low values of $K^2$ still does not agree with the theoretical GOS up to the impact energy 800 eV for the $2{}^{1}S$ excitation.

We perform a gauge transformation on the time-dependent Dirac equation describing the evolution of an electron in a heavy-ion collision to remove the explicit dependence on the long-range part of the interaction. We solve, in an ultrarelativistic limit, the gauge-transformed Dirac equation using light-front variables and a light-fronts representation, obtaining nonperturbative results for the free pair-creation amplitudes in the collider frame. Our result reproduces the result of second-order perturbation theory in the small charge limit while nonperturbative effects arise for realistic charges of the ions.

Effective-range theory, for both single-channel and multichannel scattering by systems interacting with short-range forces at low energies, provides a simple representation of the energy dependence of scattering matrix elements near reaction thresholds in conformity with the Wigner threshold law. A modification of effective-range theory for single-channel electron-atom scattering, required to account for long-range polarization forces, was developed some years ago. A multichannel extension of that theory is presented here, allowing for a superposition of asymptotic power-law potentials, with asymptotic wave functions expressed as sums of Bessel functions. Threshold branch-point singularities are extracted explicitly, leaving modified reaction-matrix elements that vary smoothly with energy. For target states that are not spherically symmetric the asymptotic wave functions account for inverse-cube law interactions that couple the various channels. The theory provides an unambiguous prescription for analytic continuation of scattering parameters from just above to just below a reaction threshold, a feature that may be useful, for example, in studying resonant cross sections based on above-threshold calculations. The present work represents an extension, to a wider class of scattering systems and long-range interactions, of an earlier analysis of threshold behavior [M. Gailitis and R. Damburg, Proc. Phys. Soc. London 82, 192 (1963)] applicable to electron-hydrogen scattering in the field of the inverse-square potential arising from the linear Stark effect.

We reexamine the Casimir and Lifshitz theories of intermolecular forces at nonzero temperature. For dilute media and atomic interactions, the limits of validity of the London–van der Waals potential between atoms at finite temperature are established by a detailed asymptotic analysis. In the retarded limit, the Casimir-Polder interaction potential is shown to be rigorously correct only in the limit of zero temperature. At any nonzero temperature a different analytic form obtains and is derived. We then consider Casimir forces between perfectly conducting plates. Existing results for the case of intervening vacuum are recovered by a different method. Moreover, we show that the Mellin transform technique and theory of generalized ζ functions allows a detailed asymptotic treatment of a system of perfectly conducting plates with an intervening electron plasma, useful in the modeling of forces between metal plates, where the finite metallic skin depth is an important consideration.

We use the density-matrix formalism to parametrize a spin-polarized $\mathrm{}(e,2e)\mathrm{}$ experiment where a $p$ electron is ejected from a closed-shell system and the fine structure of the ion is resolved experimentally. This formulation allows a definition from general principles of the recently observed spin up-down asymmetry [X. Guo and co-workers, Phys. Rev. Lett. 76, 1228 (1996); J. Phys. B 30, 4097 (1997)] due to the fine-structure effect [S. Jones et al., Phys. Rev. Lett. 72, 2554 (1994); G. F. Hanne, in Abstracts of Contributed Papers, XVIIth International Conference on the Physics of Electronic and Atomic Collisions, edited by W. R. MacGillivray, I. E. McCarthy, and M. C. Standage (Hilger, Bristol, 1992), p. 199] and the complete definition of potentially observable quantities such as the polarization of the fast- and slow-emitted-electron beams and the orientation of the ion beam after the collision. These parameters are defined for the general case where the incoming-electron beam has an arbitrary polarization $\mathbf{P}{=(P}_x{,P}_y{,P}_z)$ and calculated in an asymmetric kinematic for the case where the initial-electron beam is, respectively, unpolarized and polarized perpendicular to the scattering plane.

Using a merged-beam technique, the absolute, total electron-capture cross section has been measured for collisions of B${}^{4+}$ ions with hydrogen (deuterium) atoms at collision energies between 60 and 1200 eV/amu. The data are compared to two semiclassical coupled-channel molecular-orbital calculations, which differ by 30% with respect to each other over the entire energy range, and to two previous measurements at higher energies that differ by up to a factor of 2.

The differential cross section of the valence one-photon ionization of ${\mathrm{N}}_2$ is computed in the random phase approximation using large $L^2$ basis sets of Gaussian-type orbitals (GTO’s) and a $K$-matrix-based technique. Several special polynomial spherical GTO’s are included in the basis set used to adequately represent the orbitals lying in the electronic continuum. Ionization channels originating from the $3{\sigma }_g,$ $1{\pi }_u,$ $2{\sigma }_u,$ and $2{\sigma }_g$ occupied orbitals are considered, both in separated and interacting channel approaches.

The cross sections for double photoionization of helium and the ratios of double to single ionization have been measured from the double-ionization threshold to 820 eV. The results are in very good agreement with several recent calculations.

Based on the multiple-scattering $X\alpha$ method, the photoabsorption spectra near the $1s$ threshold of ${\mathrm{CH}}_4,$ ${\mathrm{NH}}_3,$ ${\mathrm{H}}_2\mathrm{O}$, and HF with ten electrons, the same as the Ne atom, are calculated. The calculated near-threshold spectra are in good agreement with the experimental spectra. With the calculated results we can provide clear theoretical assignments for the recent highly resolved experimental spectra. In order to elucidate the attractive effect of the localized inner-shell core hole, we have adopted the ground state self-consistent field potential, the transition state model potential, and the $Z+1\mathrm{}$ model potential, respectively. The results of the $Z+1\mathrm{}$ model are in better agreement with the experimental measurements. Along the isoelectronic molecules, the antibonding orbitals are also the initial orbitals of the corresponding Rydberg series and can be regarded as “precursor orbitals” (i.e., mixed valence and Rydberg orbitals). It is interesting that the sums of the oscillator strengths of such precursor orbitals are approximately proportional to the numbers of the antibonding orbitals.

Ionization probabilities for highly excited hydrogen atoms interacting with a metal surface are calculated within the time-dependent close-coupling approach. In order to simulate current experiments, the effect of a uniform external electric field is taken into account. Adiabatic resonance states determined in the fixed-atom approximation are used as basis states in the time-dependent calculations. The velocity-dependent dynamic couplings acting between the resonance states are found to influence the total ion production for low-velocity Rydberg atoms.

Auger $\mathrm{LMM}$ spectra and preliminary model simulations of Ar${}^{9+}$ and metastable Ar${}^{8+}$ ions interacting with a clean monocrystalline $n$-doped Si(100) surface are presented. By varying the experimental parameters, several spectroscopic features have been observed providing valuable information for the development of an adequate interaction model. On our apparatus the ion beam energy can be lowered to almost mere image charge attraction. High data acquisition rates could still be maintained yielding an unprecedented statistical quality of the Auger spectra.

The emission of electrons from metal clusters for fast excitation processes is studied, modeling Coulomb excitation through a bypassing ion. The time scales of the process and the trends with excitation energy are estimated within a simple model and computed in detail using a time-dependent density-functional formalism. The numerical study also allows one to estimate nonlinear effects. These are surprisingly small up to rather high excitation energies in the multiplasmon regime.

We use the results of a Lagrangian formulation of the dynamics of ions in a Penning trap to calculate the motional frequencies of the ions as a function of the trapping parameters. We then add realistic perturbations to the ideal trapping fields in the formalism and deduce the effects of these perturbations on the ion motion. For an ideal trap there exist values of the applied trapping fields that result in a degeneracy in the ion oscillation frequencies associated with different types of motion. We show that this motional frequency degeneracy is lifted by the inclusion of the effect of a tilt of the trap axis with respect to the axis of the applied magnetic field, leading to an “avoided crossing” between the oscillation frequencies. We calculate typical ion orbits for trap parameters that give oscillation frequencies near the avoided crossing between the axial and modified cyclotron frequencies. We generalize the analysis to include the motion of ions in a combined (Penning-Paul) trap and perform an experiment to test the predictions of the theory for a degeneracy between the modified cyclotron frequency and the axial frequency for ${\mathrm{Mg}}^{+}$ ions held in a tilted combined trap. The oscillation frequencies are measured for a range of tilt angles using a photon-photon correlation technique. There is good agreement between the experimental results and the theoretical predictions. The method we describe may prove to be a useful means by which trap imperfections can be identified and subsequently removed.

We propose an atomic funnel consisting of a dark hollow, divergent laser beam and an evanescent light in a short micron-sized hollow optical fiber. The cold atoms extracted from a magneto-optical trap experience effective Sisyphus cooling in the dark hollow beam. We calculate the potential barriers for atomic guiding in the evanescent light and the dark hollow beam, and analyze the atomic losses during the guiding and funneling process. Our results show that the equilibrium three-dimensional root-mean-square momentum of about $4.5ħk$ (temperature $\sim 2.5\mu \mathrm{K}$) and the funneling efficiency of about 95% could be obtained. A low-velocity, intense coherent atomic beam can be generated in this funnel scheme and its potential applications in atom optics are briefly discussed.

Optical orientation of reagents is one approach to achieving selectivity in a chemical reaction and has been used successfully to shift the point of equilibrium in bulk reaction mixtures of alkali-metal atoms and dimers. In addition to the continuous control of the point of equilibrium in the dimerization of sodium atoms, the control of the ortho-para nuclear spin distribution is shown here to also be a function of the degree of atomic spin orientation over a broad range. The direction of the spin conversion is para to ortho. For a mutlicomponent vapor, the optical orientation of ${}^6\mathrm{Li}$ in a mixture of lithium-6 and lithium-7 is shown to produce a continuous and selective control of the isotopic dimer concentrations. ${}^6{\mathrm{Li}}_2$ (para) is selectively destroyed in the presence of the other dimer species. In this way the composition of a multicomponent metal vapor system at chemical equilibrium can be manipulated. The constraint produced by the optical orientation of one of the reagents offers a technique whereby the equilibrium yield of a multicomponent bulk chemical reaction can be laser controlled in a selective and continuous fashion.