The curvature, ρ’’(r), of the atomic electron density ρ(r) is studied using results from a bare-Coulomb-field (BCF) model, Hartree-Fock (HF), and configuration-interaction (CI) calculations. A region of nonconvexity in ρ(r), previously reported by Angulo, Dehesa, and Gálvez [Phys. Rev. A 42, 641 (1990)] for light atoms in a Hartree-Fock framework, is investigated for all atoms up to Z=92 and is found not to be an artifact of the basis set or the HF model. Numerical results for the BCF model show that the total electron density of an arbitrary number of closed shells is convex. However, for the same model with electrons filling orbitals according to Stoner’s restriction we find that nonconvexity of the density is a periodic property appearing around closed-shell ground-state hydrogenic configurations. Cusp conditions, reported earlier by Esquivel et al. [Phys. Rev. A 47, 936 (1993)] for the second derivative of the BCF density are verified for model atoms with s and p subshells. Using wave functions of near-HF accuracy we have found a region of nonconvexity in ρ(r) for atoms with Z=3–6, 16–32, and 45–92. Highly correlated densities of CI and Hylleraas-type quality for atoms of Li and Be isoelectronic sequences show that the nonconvex region of ρ(r) is largely unaffected by the inclusion of electron correlation. These results, coupled with those from the BCF model, lead us to suggest that it is the bare Coulomb field of the nucleus that is mainly responsible for the appearance of nonconvex regions in atoms. Furthermore, the degree of nonconvexity is shown to decrease as Z increases along the isoelectronic series. The contributions of different spin densities to the nonconvex electron densities is also studied. Finally, the behavior of the curvature of the electron density far from the nucleus is investigated. The ratio ρ’’(r)/ρ(r) is found to approach an asymptotic value from above or below, according to the magnitude of the ionization potential.

The low-frequency circularly polarized microwave ionization of highly excited states of the hydrogen atom is evaluated applying the adiabatic approximation in the classical limit. The ionization threshold is calculated by matching asymptotically the weak- and strong-field regimes, and the results are compared with numerical integrations of the equations of motion. Simulations for both linear and circular polarization are presented, corresponding to ensembles of initial states in experiments with linear polarized microwaves, at frequencies ω<0.6${\mathrm{\omega }}_{\mathit{K}}$, where ${\mathrm{\omega }}_{\mathit{K}}$=${\mathit{n}}^{\mathrm{-}3}$ is the initial Kepler frequency of a state with principal quantum number n. In contrast with the stochastic nature of the ionization for linearly polarized microwaves, the dynamic evolution in the case of circular polarization is quite regular and the threshold varies smoothly with frequency. As the static limit is approached, a scale-invariant switching-on procedure for the microwave field leads to a breakdown of the adiabatic approximation.

A numerical method has been developed to calculate the two-center integrals which can be used to evaluate the charge-transfer matrix elements arising in ion-atom collisions. The initial three-dimensional integrals are first reduced to one-dimensional integrals using a Feynman-integral technique. A special routine which uses continuation in the complex plane has been designed to evaluate the integrals in the region far away from the target to remove their rapid oscillation in this region. Then a Gauss-Laguerre integral method is used to compute this analytic continuation. The four-state charge-transfer matrix elements in proton-hydrogen scattering have been calculated to illustrate the method. The resultant coupled differential equations are solved by the Runge-Kutta method. Some cross sections for proton-hydrogen scattering have been evaluated and compared with available results.

The exact transmission coefficient of a one-dimensional potential barrier, V(x)=${\mathit{V}}_0$(1-[{1-exp(x/a)}/{1+c exp(x/a)}${]}^2$), of which the Morse and the Eckart barriers are special cases, has been obtained. Comparing the exact and WKB transmission coefficients, the limitation of the semiclassical method has been quantified. Another branch of this potential (c<0) has been shown useful in nucleus-nucleus fusion. Using the transmission coefficients, the exact energy eigenvalues of the inverted (${\mathit{V}}_0$→-${\mathit{V}}_0$) potential (oscillator) have been derived.

${\mathrm{H}}_3^{+}$ ions of velocity 4–7 a.u. have been passed through targets of several noble gases, He, Ne, Ar, and Xe, and the production of single H atoms, single ${\mathrm{H}}_2$ molecules, and pairs of H atoms has been studied. These are the main dissociation channels leading to the production of atomic and molecular neutral particles, other processes—formation of one ${\mathrm{H}}_3$ molecule, three H atoms, or one ${\mathrm{H}}_2$-H pair—being negligible as they require electron capture by these fast projectiles. The yields for the processes under study were measured as a function of target pressure, and the production cross sections extracted from the low-pressure region of the yield curves.

Dielectronic-recombination (DR) rate coefficients for the ground state of Ni-like ${\mathrm{Ta}}^{45+}$ have been calculated for the electron temperatures in the range 0.02<T<10 keV. The calculations were carried out in the isolated resonance approximation using the multiconfiguration Dirac-Fock model in intermediate coupling with configuration interaction. The total DR rate coefficients were obtained by including the contributions from the 3s, 3p, and 3d excitations. The rate coefficients are found to peak at very low temperature (∼80 eV) with a value of 3.58×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^3$/sec and reduce to 1.48×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^3$/sec at T=1 keV.

We study the time evolution of wave packets on coupled energy levels including the irreversible effect of spontaneous emission. As a test case we choose a simple two-state level-crossing problem of the Landau-Zener type. The numerical treatment of this situation is demanding, and we explore two approaches based on different views of the quantum-mechanical dynamics. First, we solve the time evolution of the wave packet using the traditional master equation giving directly the ensemble averages. Second, we use the recently developed Monte Carlo simulation method, which gives the individual members of an ensemble of quantum states undergoing spontaneous quantum jumps. The results are compared and found to agree when large enough ensembles are used. We discuss the computational requirements for the two methods and we find both quite demanding but qualitatively different.

Absolute cross sections for electron-impact single ionization of ${\mathrm{Si}}^{+}$ and ${\mathrm{Si}}^{2+}$ have been measured using crossed beams of ions and electrons and calculated using a configuration-average distorted-wave method. Corrections have been made for metastable components and small fractions of nitrogen impurities in the incident ion beams. Excitation-autoionization measurably enhances the cross sections of both ${\mathrm{Si}}^{+}$ and ${\mathrm{Si}}^{2+}$. Ionization rate coefficients and fitting parameters are presented for the experimental data.

The effects of collisional resonances and radiative cascades on the linear polarization of He-like iron (Fe xxv lines from 1s2l to 1${\mathit{s}}^2$ levels have been investigated. Detailed calculations have been carried out for the 1s3l3l’ resonance contributions to electron-impact excitation rates from the 1${\mathit{s}}^2$ ground level to the individual magnetic sublevels of 1s2l configurations. Excitation collision strengths from 1${\mathit{s}}^2$ to 1s3l sublevels as well as radiative cascade transitions from 1s3l to 1s2l’ sublevels have also been computed. The autoionization transition-matrix elements (for the resonance effects) and the collison strengths (for the radiative cascades) have been computed in the distorted-wave approximation using intermediate coupling with fine-structure mixing multiconfiguration bound wave functions. The results indicate that the collisional resonance contributions, when averaged over a small energy range just covering them, have a somewhat significant depolarizing effect on the (1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$, electric dipole) w line, the (1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$, magnetic quadrupole) x line, and the (1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$, fine-structure electric dipole) y line. However, the averaged polarization degree over some single resonances can reach high values.

Now for the (1${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2s ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$, relativistic magnetic dipole) z line the individual resonances are found to induce a polarization degree less than 15% in absolute value but after averaging over all 1s3l3l’ resonances z remains practically unpolarized. Concerning radiative cascades, it is shown that they can create a relative small degree of polarization on the z line, the highest value being close to 14%. For w, x, and y lines, the cascades have a weak depolarizing effect, except for x at large incident energy. It is expected that the contributions from 1snl cascades as well as 1s3lnl’ resonances, with n≥4, would not greatly change the results already obtained.

Autoionizing resonances in the compound system of an electron and a helium ion are observed in kinematically complete ionization experiments for electrons on helium atoms. We calculate the differential cross section for comparison with these experiments in an equivalent local form of the distorted-wave impulse approximation. Resonant scattering amplitudes are calculated by a six-state momentum-space coupled-channel method.

A photoionization cross-section calculation of atomic tungsten is performed in the formalism of many-body perturbation theory for photon energies ranging from the ionization threshold of tungsten to 150 eV. Nonrelativistic orbitals are used in the basis set and relativistic corrections are included. We consider excitations from the 4f, 5s, 5p, 5d, and 6s subshells. The effects of the strong 5${\mathit{p}}^6$5${\mathit{d}}^4$→5${\mathit{p}}^5$5${\mathit{d}}^5$ and 4${\mathit{f}}^{14}$5${\mathit{d}}^4$→4${\mathit{f}}^{13}$5${\mathit{d}}^5$ transitions are included as resonant contributions to the 5d partial cross section. Our results indicate that the 5d partial cross section dominates the total cross section below 100 eV.

The propensity rules for populating magnetic substates in electron-capture processes from excited target atoms are examined theoretically using the close-coupling expansion method with two-center atomic orbitals. It is shown that if the quantization axis is chosen to be perpendicular to the scattering plane, the dominant magnetic substate populated for each final nl state is m=-l, irrespective of the magnetic quantum number of the initial state. Electron-capture processes from 2s and oriented 2p states of hydrogen atoms in collisions with ${\mathrm{He}}^{2+}$ and ${\mathrm{Li}}^{3+}$ are examined.

The reaction processes in a ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$→A ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) flash lamp (600-Torr, 10-cm-long, pulsed capillary discharge) are analyzed to estimate the spontaneous emission rate and a device gain. The observed dominance of ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$→A ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) radiation over all other radiative transitions, together with predicted rates for these radiative transitions, are used to characterize the plasma. New reactions are proposed between He(2 ${}_{}{}^3{}_{}{}^{}\mathrm{P}$) and the ground-state molecular ions that produce ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$) by rapid two-body momentum exchanges and explain the absence of He(2 ${}_{}{}^3{}_{}{}^{}$P) and ground-state molecular ions in the discharge. A first approximation that all ion-electron recombinations result in ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$) provides a conservative gain estimate of 0.4% per meter for this device. ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$→B ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) absorption is identified as a key loss process that will limit the tuning range of a potential laser. An investigation of excimer laser discharges reveals the presence of the ${\mathrm{He}}_2^{+}$(C ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$) molecule at pressures of 0.1–10 atm.

Total cross sections for double photoionization of He by high-energy photon impact are evaluated by use of the correlated double-continuum wave function. Calculated ratios of the double- to single-ionization cross sections are compared with the very recent observations by Levin and co-workers [Phys. Rev. Lett. 67, 968 (1991); Phys. Rev. A 47, R16 (1993)]. The role of the final-state correlation is also examined.

We present theoretical photoionization cross sections from the 1sns ${}_{}{}^{1,3}{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ bound excited states of the helium atom based on a configuration-interaction procedure for the continuum, using a finite ${\mathit{L}}^2$-basis set constructed from a nearly complete set of B-spline-based one-particle hydrogenic orbitals. In addition to the nonresonant spectra near the first ionization threshold, resonant structures associated with selected sp,2${\mathit{n}}^{\pm }$ and 2pnd ${}_{}{}^{1,3}{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ autoionization series below the ${\mathrm{He}}^{+}$ N=2 threshold are examined in detail. The widths and resonance energies of all ${}_{}{}^{1,3}{}_{}{}^{}$P autoionization series are also reported and compared with other available theoretical and experimental results.

Within the framework of standard electroweak gauge theory, parity-violation effects on the Auger emission from atomic ions are examined. Highly charged ions are prime candidates due to their simple atomic structure and large electroweak charge. To be specific, the Auger decay of the 2${\mathit{s}}^2$ J=0 level in ${\mathrm{U}}^{90+}$ is analyzed. In QED, the polarization-asymmetry parameter is identically zero. It is found that neutral-current mixing of the 2${\mathit{s}}^2$ J=0 and 2s2p J=0 levels yields a polarization asymmetry of ${10}^{\mathrm{-}7}$.

The cross section for dielectronic recombination of ${\mathrm{C}}^{4+}$ ions in the 1${\mathit{s}}^2$ ground state has been measured with an energy resolution of ≊2 eV at the heavy-ion storage ring TSR (at Heidelberg) up to the 1s2s${(}^{1,3}$S) and 1s2p${(}^{1,3}$P) excitation thresholds just below 310 eV. Resonance energies and absolute cross sections are found to be well reproduced by theoretical calculations except in a region extending ≊5 eV below the almost degenerate excitation thresholds 1s2s${(}^1$S) and 1s2p${(}^3$P). These discrepancies imply that theory overestimates the cross section, energy-integrated over all dielectronic resonances, by ≊10%.

Electron correlation in the 4d hole state of Ba has been studied by comparing the experimental 4${\mathit{d}}^{\mathrm{-}1}$→6${\mathit{s}}^{\mathrm{-}2}$ Auger electron and 4d photoelectron spectrum with results based on the Bethe-Born approximation and relativistic multiconfiguration Dirac-Fock calculations. The 4d-hole-state correlation is found to result in a reversal of the 4${\mathit{d}}_{5/2}$:4${\mathit{d}}_{3/2}$ branching ratio in going from the photoelectron spectrum to the Auger spectrum in accordance with experimental results.

Double-electron excitation processes to (2l,3l’) and (3l,3l’) manifolds of He atoms by multiply-charged-ion impact have been theoretically investigated using a close-coupling method at the (1–10)-MeV/u energy regime. Hyperspherical wave functions are adopted to take full account of strongly correlated motion of two atomic electrons in He. For the excitation processes to the (2l,3l’) manifold, the cross sections to the ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ states show the most rapid increase as projectile charge ${\mathit{Z}}_{\mathit{p}}$ increases, while that to the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ states indicate the slowest. For the 3l3l’ excitation processes, the cross sections to the ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ and ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ states increase at almost the same rate. It is found that the isomorphism of the charge-density plots of the channel wave functions between the initial and final states governs collision dynamics in direct double-electron excitation processes when incident multiply charged ions strongly interact with the target He atoms.

A noniterative eigenchannel R-matrix approach, combined with the quantum-defect method, is formulated to describe the predissociation of ${\mathrm{H}}_2$ in the Rydberg state 3pπD ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{+}{}_{}{}^{}$ by the low-lying 3pσB’ ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ and 2pπC ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{+}{}_{}{}^{}$ states belonging to the same Rydberg channel. The Coriolis coupling leading to the dissociation has been accounted for by a rovibrational frame transformation. The predissociation line positions and widths for the v=3 to v=11 levels have been calculated. Very good agreement between experiment and theory is obtained.