The lifetime of the 2 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$ level in heliumlike krypton has been measured by an alternative method involving the use of a two-foil target. The experiment demonstrates the potential of this method which allows the standard beam-foil time-of-flight technique to be extended to shorter lifetimes. We measured a lifetime of 9.5±0.9 ps, which is in reasonable agreement with current theoretical calculations.

The problem of localization of atomic and molecular electron momentum is investigated. The atomic systems are studied in terms of the electron momentum localization function (EMLF), a momentum-space counterpart of the electron localization function introduced by Becke and Edgecombe [J. Chem. Phys. 92, 5397 (1992)]. The shell structure of atoms is better represented by the EMLF than the corresponding radial momentum density and Laplacian of the electron momentum density (EMD). A clear distinction is possible for monotonic and nonmonotonic decreasing atomic EMD’s from the EMLF plots. For molecules, the localization is studied via both the Laplacian and the EMLF and are found to be in agreement with each other. The critical structure of the EMD at p=0 is represented by the corresponding concentrations and depletions in the Laplacian contour plot.

We discuss theoretical foundations of the emerging area of quantum-optical studies of many-body systems: a quantum field theory of atoms interacting with photons. This theory is designed to describe quantum-statistical systems of cooled and trapped atoms undergoing spontaneous and stimulated emission processes. Formulated in the framework of the second quantization, this theory accounts fully for quantum-statistical properties of atoms as bosons or fermions. In particular, it applies to a description of the formation of the Bose-Einstein condensate in bosonic systems, as well as the spectroscopy and other quantum optical properties of the Bose-Einstein condensate. We derive the general second quantized Hamiltonian of this theory and discuss in detail various approximations.

A special type of two-center overlap integral is considered in which the integration does not extend over the whole space, but is constrained to the interior or exterior of a sphere about one of the centers of the orbitals. It is shown that exact, closed analytical expressions can be derived for this kind of two-center integral over Slater-type orbitals. These analytical expressions are of a very simple analytical structure and allow for a rapid integral evaluation. By applying the angular momentum recurrences which we proposed recently [W. Hierse and P. M. Oppeneer, J. Chem. Phys. 99, 1278 (1993)], the evaluation performance can be further accelerated. In the frequency occurring case of nearly equal scaling parameters the analytical integral expressions are totally stable, but in the (pathological) case of one vanishing scaling parameter the integral over the interior volume is not stable. Exact integration formulas for the latter limiting case are also given.

A scheme of generation of the generalized effective-core potential (generalized ECP) is described. Both valence and outermost core pseudo-orbitals are included in the generalized ECP construction scheme. Effective potentials have been generated for the atoms Xe, Pd, and Ag. The potentials and pseudo-orbitals are presented as linear combinations of Gaussians. Significant improvement (about 10 times) as compared with the standard ECP approach for reproduction of transition energies from all-electron calculations is demonstrated for transitions between the states with different configuration structures.

Spectra of rare-earth atoms praseodymium, Z=59, to ytterbium, Z=70, emitted from the high-temperature (1 keV) low-density (${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$) TEXT tokamak (at the Fusion Research Center, University of Texas, Austin) and high-density (${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$) laser plasmas have been recorded in the soft-x-ray range of 50–200 Å with an image intensifier detector and on photographic plates. The brightest n=4 to n=4 transitions of galliumlike ions have been identified and their emission patterns have been studied by comparison with ab initio atomic structure calculations and collisional radiative models under the respective plasma conditions. We have investigated the use of the ratios of the intensities of 4-4 transitions as indicators of plasma densities. This is possible owing to the doublet structure of the galliumlike ground state, which leads to a strong density dependence for ratios of transitions between low-lying levels. We have also used semiempirical ionization balance calculations to characterize the charge state distribution of the tokamak plasmas, in preparation for an investigation of the use of ratios of galliumlike to zinclike and copperlike emission features as indicators of whether the impurities are in coronal equilibrium or undergoing ionization.

The huge ${\mathrm{He}}^{\mathrm{-}}$ ${}_{}{}^4{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ shape resonance was reexamined in photodetachment using a titanium:sapphire laser tuned over the energy range 9740–10 300 ${\mathrm{cm}}^{\mathrm{-}1}$. This range covered well below the He (2 ${}_{}{}^3{}_{}{}^{}$P) threshold and included the entire resonance region. The uncertainties in the earlier data [Peterson et al., Phys. Rev. Lett. 55, 692 (1985)] were greatly reduced. The new (old) experimental value for the electron affinity of He (2 ${}_{}{}^3{}_{}{}^{}$S) is 77.67±0.12 meV (77.6±1.0 meV), in fair agreement with the most accurate theoretical value, 77.51±0.04 meV, of Bunge and Bunge [Phys. Rev. A 30, 2179 (1984)]. This resonance lies 10.80±0.07 meV above the He 2 ${}_{}{}^3{}_{}{}^{}$P threshold, and has a decay width of Γ=7.16±0.07 meV. The ratio of the cross section at resonance to the continuum at the threshold is ${\mathrm{\sigma }}_{\mathit{R}}$/${\mathrm{\sigma }}_0$=89±5. This study provides by far the most detailed experimental view of an electronic shape resonance.

We have measured the intensities of extreme ultraviolet (EUV) lines of the Ar VIII spectrum produced in collisions of ${\mathrm{Ar}}^{8+}$ ions ($v=0.13-0.40\mathrm{}$ a.u.) with atomic cesium and determined the relative populations of the orbital-angular-momentum subshells for an electron captured at 16- and 160-keV projectile energies. At 16 keV, the partial cross sections for capturing into low-$l$ levels are larger than that for high-$l$ levels, while, at 160 keV, only high-$l$ levels are significantly populated. The high-energy $l$ distribution is in good agreement with Monte Carlo calculations of Pascale et al. Phys. Rev. A 42 5305 1990. The experimental results at 16 keV are partly in qualitative agreement with a close coupling study of Harel and Jouin J. Phys. B 21 859 1988.

Differential cross sections and angular correlation parameters are calculated for electron-impact excitation of the 3 ${}_{}{}^2{}_{}{}^{}$P state in hydrogen from the ground $1^2$S state using a distorted-wave approximation. The calculations are compared with the recently reported experimental and close-coupling theoretical results of J. F. Williams, A. T. Stelbovics, and I. Bray [J. Phys. B 26, 4599 (1993)]. Present results show reasonably good agreement with both of their theoretical and experimental results.

The orientation created in the charge-exchange process ${\mathrm{B}}^{3+}$+He→${\mathrm{B}}^{2+}$(2p)+${\mathrm{He}}^{+}$ is considered, emphasizing the problem of the formation of the angular distribution in the final state. The interfering contributions to the orientation parameter are generated by the trajectories corresponding to the same scattering angle but differing by the impact parameters. Such trajectories exist only when the short-range repulsion on the initial diabatic potential curve is taken into account. Thus, the orientation effects probe the core of the atom-atom interaction in the initial state.

We have calculated the angular distribution functions for Auger electrons emitted from Be-like Mg, Fe, and Mo following 1s-2p excitation by electron impact. The calculations were carried out in intermediate coupling with configuration interaction using the relativistic distorted-wave and multiconfiguration Dirac-Fock methods. We found strong angular asymmetry for many transitions. We also found that relativistic effects can completely change the characteristics of the angular distribution for transitions which have many contributing partial waves, even for ions as light as Fe.

The intercombination transitions between l-resolved Rydberg levels due to collisions with fast electrons are studied within the Ochkur approximation for principal quantum numbers n≤20, where l is the angular momentum. The use of the Heisenberg correspondence principle for radial integrals enables one to obtain analytic expressions for any term in the multipole expansion of the cross section for collisions with an arbitrary n and l change. For transitions involving angular momenta that are small compared to the principal quantum numbers, the semiclassical and the exact results are found to be fairly close, within the accuracy consistent with the Born approximation. As a result, the semiclassical approach furnishes a ready estimate for cross sections, and its validity range proves broad enough to cover the region of large momenta transferred to the atom. The numerical results also confirm that dipole transitions have no dominance over the collisions with other Δl values, in accord with the general model of intercombination scattering.

Experimental studies have been made of superelastic and inelastic collision processes involved in the state-selective single-electron capture reaction ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$+He(1 ${}_{}{}^1{}_{}{}^{}$S)${\to }^3$${\mathrm{He}}^{+}$(n)+${\mathrm{He}}^{+}$(n’), where n and n’ are the final principal quantum-number states of the collision products. Total cross sections have been measured at projectile energies of 15, 30, and 45 keV/u, by examining the energy loss-gain of the fast ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ product ions, for the superelastic n=n’=1 channel, and for the sum of the inelastic n=2, n’=1 and n=1, n’=2 states. Measurements are also presented for Ne and Ar targets.

Angular distributions of fragment ions (${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$) produced in dissociative double photoionization of carbon monoxide have been studied in the photon energy region 39–100 eV using a photoion-photoion coincidence technique and a source of synchrotron radiation. The asymmetry parameter β ranges from 0.69 at 39 eV to -0.19 at 100 eV. The observations of anisotropy in the dissociative double photoionization from the valence orbitals of CO are discussed in relation to the symmetry considerations involved in the transition.

Radiative relaxation times of rovibrational transitions for the electronic ground state of ${\mathrm{HD}}^{+}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_{}^{18}{}_{}{}^{}$${\mathrm{O}}^{+}$, and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{}^{13}{}_{}{}^{}$${\mathrm{C}}^{+}$ are calculated. These lifetimes are compared with the storage time in a heavy-ion storage ring where molecular ions can be stored, in order to provide vibrationally and rotationally cold ions. The relevance of the results to the measurement of dissociative recombination of cold molecular ions is discussed.

Electron-energy spectra for ${\mathrm{H}}^{\mathrm{-}}$ doubly excited states resulting from collisions of ${\mathrm{H}}^{\mathrm{-}}$ with He are rigorously calculated within a semiclassical molecular representation by including couplings between doubly excited states and continuum states and their interference with direct-detachment processes. An energy sampling procedure, based on the Gauss quadratures, is used to discretize continuum states. The present theoretical result clarifies mechanisms of excitation to doubly excited states, quantitatively reproduces the experimental spectra first observed by Risley and Geballe in 1974, separates the contributions from each of three doubly excited states, and identifies the cause of the interference between channels arising from double-electron excitation and direct detachment with simultaneous excitation.

Angular and momentum distributions have been measured for electron-positron pairs created in peripheral collisions of 6.4-TeV bare sulfur ions with thin targets of Al, Pd, and Au. Singly and doubly differential cross sections are presented for 1–17 MeV/c electrons and positrons detected independently and in coincidence as pairs. Integrated yields for pair production are found to vary as the square of the target nuclear charge. Relative angular and momentum differential cross sections are effectively target independent. Various physical parameters are deduced from the coincident electron and positron data, including probability distributions for the pair transverse momentum, the pair total energy, and the positron fraction of the pair energy. Where possible, results of these measurements are compared with theory.

We measured relative cross sections for true double capture (TDC) and autoionizing double capture (ADC) in the collisions of slow bare ${\mathit{Z}}^{\mathit{Z}+}$ and hydrogenlike ${\mathit{Z}}^{\left(\mathit{Z}\mathrm{-}1\right)+}$ ions (Z=7–13) on rare gases. We found that the probabilities for radiative stabilization ${\mathit{P}}_{\mathrm{rad}}$=${\mathrm{\sigma }}_{\mathrm{TDC}}$/(${\mathrm{\sigma }}_{\mathrm{TDC}}$+${\mathrm{\sigma }}_{\mathrm{ADC}}$) are much stronger when the binding energies of the projectile captured electrons are comparable to those of the limits of the Rydberg (3,n), (4,n), and (5,n) series. The observed strong effect of the projectile core on the probabilities for radiative stabilization of both electrons shows that the relaxation processes of the Rydberg electron play an important role.

The direct numerical approach using a finite-difference propagation algorithm that we developed for electron-hydrogen scattering for total angular momentum L=0 [Y.D. Wang and J. Callaway, Phys. Rev. A 48, 2058 (1993)] is generalized to arbitrary L. This algorithm can treat the full Coulomb interaction of the two electrons. Since solutions are propagated outward to the asymptotic region, the propagation scheme is effective and suitable for extracting scattering quantities such as the S-matrix and K-matrix. The present algorithm is also attractive for its handling of large data arrays, which is a key limit to the direct numerical solution of partial differential equations in general. We applied this method to calculate phase shifts and resonances below the n=2 threshold, elastic and inelastic scattering cross sections at energies between the n=2 and n=3 thresholds. Effects of long-range channel coupling were investigated. Detailed comparisons with other calculations are given.

The vibrational excitation to v=1 level of the ground electronic state X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ of nitrogen was measured in a positive column during the 40-μs pulsed electric discharge, and in the afterglow. The discharge was operating at 6 Torr pressure and at current densities in the range 0.7–6.4 A/${\mathrm{cm}}^2$. Temporal evolution of vibrational excitation was measured using coherent anti-Stokes Raman scattering (CARS). Vibrational temperatures were found to range from 700 to 4000 K. The results are in fair agreement with model calculations based on energy transfer to vibrational excitation. Vibrational temperatures of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ state, obtained from direct CARS measurements, were up to a factor of 3 below those deduced from the second positive system emission.