Energies of the ten (2l2${\mathit{l}}^{\prime }$) states of ions of the beryllium isoelectronic sequence are determined to second order in relativistic many-body perturbation theory. Both the second-order Coulomb interaction and the second-order Breit-Coulomb interaction are included. Corrections for the frequency-dependent Breit interaction are taken into account in lowest order only. The effect of the Lamb shift is also estimated and included. Comparison with experiment and other theoretical data is made. © 1996 The American Physical Society.

The decay times of ${\mathit{D}}_1$ emission lines of Rb and Cs atoms in superfluid helium were measured at 1.6 K as functions of helium pressure in a range from the saturated vapor pressure to about 25 atm. In the case of Cs, the measured decay time is almost independent of the helium pressure and is given by the lifetime of the 6${\mathit{P}}_{1/2}$ state of the free atom. However, we have found that the decay time of the Rb ${\mathit{D}}_1$ line decreases with increasing pressure. This suggests the coexistence of two processes leading from the excited P state to the ground state, one an ordinary ${\mathit{D}}_1$ emission transition and the other a process decaying via a lower energy state before reaching the ground state. The pressure-dependent quenching observed only for the Rb atom can be explained by the competition of the above two processes. A possible mechanism for quenching of the Rb 5P states is discussed in terms of the interaction potentials of the alkali-metal-atom–${\mathrm{He}}_{\mathit{N}}$ system, where two or six helium atoms are localized near the alkali core in a particular anisotropic configuration. The dynamics of processes leading from the excited P states of Na and Cs atoms are also discussed. © 1996 The American Physical Society.

We analyze the transition from a Zeeman to a Stark structure of a weakly split Rydberg n multiplet of the H atom in parallel magnetic and electric fields. The use of classical mechanics, topology, and group theory provides a detailed description of the modifications of dynamics due to the variation of the electric field. We focus on the point where the collapse of the Zeeman structure occurs, give the sequence of classical bifurcations responsible for the transition between different dynamic regimes, and compare it to the quantum energy-level structure. © 1996 The American Physical Society.

Ground-state energies of atoms and atomic chains in strong to superstrong magnetic fields (B≊2.4×${10}^6$ – 8.5×${10}^8$ T) are calculated within the single-particle scheme of a heuristic density-functional method. From these we find condensation energies and binding lengths. Using the local Dirac exchange functional, the corresponding equations are solved iteratively, expanding the wave functions with respect to Landau functions in the transverse directions and directly integrating along the longitudinal direction. Dropping the adiabatic approximation, we extrapolate our results with respect to the number of Landau channels taken into account. The values found by this extrapolation correspond to calculations without any restriction on the allowed wave functions. © 1996 The American Physical Society.

Scattering theory is extended to the description of processes during collision. Indeed, for large wave packets we may consider situations where time t, while large with respect to characteristic frequencies, is still smaller than the duration of the collision ${\mathit{t}}_{\mathit{c}}$. This leads to an asymptotic theory which is different from the usual S-matrix approach. There appear intermediate states with scattering cross sections which result from secular effects for t<${\mathit{t}}_{\mathit{c}}$, but differ from the values obtained for t≳${\mathit{t}}_{\mathit{c}}$ in the S-matrix theory. The consideration of such intermediate time scales is the normal procedure in many-body situations as realized, for example, in chemical reactions. Our method is closely connected to our work on the extension of quantum theory beyond the Hilbert space for large Poincaré systems. This theory applies to persistent scattering using delocalized density matrices (an example is plane waves). For this case, the S-matrix theory is not valid, while the results of our asymptotic approach remain valid for all times. Our theoretical predictions have been validated by numerical simulations. © 1996 The American Physical Society.

The effect of target e-e correlations on the angular distribution of fivefold differential cross section for the double ionization of helium by fast electrons has been studied. The helium ground-state wave functions due to Byron and Joachain [Phys. Rev. 146, 1 (1966)], Silverman, Platas, and Matsen [J. Chem. Phys. 32, 1402 (1960)], and Tweed and Langlois [J. Phys. B 20, 5213 (1987)] have been employed. The calculation has been done in the first-order Born approximation with the two ejected electrons being described by a product of two Coulomb waves. The correlations in the final state have been accounted for by suitable effective charges and the Gamow correlation factor. Kinematics corresponding to or close to the Bethe ridge and with small momentum transfer and low ejected electron energies has been chosen. When one (energetic of the two) of the electrons is ejected along the momentum transfer direction, the angular distribution of the other for angles in the range 180°±60° with respect to the first is found to bear clear signatures of e-e correlations in the ground-state wave function. © 1996 The American Physical Society.

We report on an ab initio study of the single-photon ionization of ${\mathrm{Na}}_2$ from the electronic ground state (X${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$) and subsequent photodissociation of the resulting ion. The calculations are done as a function of internuclear distance (R=4.5–7.5 a.u.) and for a range of photoelectron energies 0.25–10 eV. The electronic continuum wave functions are determined using the so-called iterative Schwinger method, which is an ${\mathit{L}}^2$ method based on a single-center expansion of all relevant operators and wave functions. A Cooper minimum is found in the kπ-continuum’s p wave (l=1) around a photoelectron energy of approximately 4.5 eV. This does not, however, lead to significant observable implications for the total cross sections. The subsequent photodissociation of the ${\mathrm{Na}}_2^{+}$ ions is calculated using a numerical Numerov method, based on previously calculated data for the molecular potential-energy curves and dipole matrix elements. The spectrum resulting from photodissociation of ions created via photoionization of a thermal distribution of ${\mathrm{Na}}_2$ molecules is calculated and compared with the recent results of Kortyna et al. [Phys. Rev. A 50, 1399 (1994)]. Excellent agreement is found with the experimental values if the assumed initial temperature of the molecular beam is approximately 50 K, which is substantially colder than previously estimated. © 1996 The American Physical Society.

The C 1s partial photoionization cross section and photoelectron angular distribution of methane (${\mathrm{CH}}_4$) have been measured with high-energy resolution between threshold and 385 eV photon energy. From the analysis of the vibrational fine structure on the C 1${\mathit{s}}^{\mathrm{-}1}$ photoelectron line a vibrational energy of 396±2 meV and an equilibrium bond length of 1.039(±0.001) Å for the ${\mathrm{CH}}_4^{+}$ ion have been determined. The lifetime broadening was found to be 83(±10) meV. The weak feature in the photoabsorption cross section just above threshold does not influence the vibrational fine structure in a way typical for a shape resonance. We therefore suggest that it is due to doubly excited states of the type C (1s${)}^{\mathrm{-}1}$(Val${)}^{\mathrm{-}1}$(Ryd${)}_{\mathit{a}}^1$(Ryd${)}_{\mathit{b}}^1$, an assignment which is supported by recent Auger decay studies. Measurements of the shakeup structure revealed six satellite lines, one of which increases strongly in intensity at threshold, thus pointing to the existence of a conjugate shakeup process. © 1996 The American Physical Society.

We have investigated the threshold of photodetachment of ${\mathrm{Li}}^{\mathrm{-}}$ leading to the formation of the residual Li atom in the 2${\mathit{p}}^2$P state. The excited residual atom was selectively photoionized via an intermediate Rydberg state and the resulting ${\mathrm{Li}}^{+}$ ion was detected. A collinear laser-ion beam geometry enabled both high resolution and sensitivity to be attained. We have demonstrated the potential of this state-selective photodetachment spectroscopic method by improving the accuracy of Li electron affinity measurements an order of magnitude. From a fit to the Wigner law in the threshold region, we obtained a Li electron affinity of 0.618 049(20) eV. © 1996 The American Physical Society.

Stark mixing among excited degenerate states following capture or excitation by ion impact on hydrogenlike atoms is calculated at high velocities by using the perturbative formalism. For electron capture, the second-order Brinkman-Kramers approximation is found to describe the correct infinitesimal Stark rotation valid in the high-velocity regime. However, the second-order continuum distorted-wave approximation does not provide such a rotational form in that regime. It is concluded that the continuum distorted-wave amplitudes should not be Stark rotated even though a numerical coincidence is found between our formalism and the usual Stark rotation in the experimental range of interest. Finally for electron excitation, it is found that the perturbative formalism does not lead to a rotational structure. © 1996 The American Physical Society.

Differential cross sections and generalized oscillator strengths have been measured for two optically allowed transitions 5${\mathit{p}}^6$${(}^1$${\mathit{S}}_0$)→5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)5d(K=3/2,J=1) and 5${\mathit{p}}^6$${(}^1$${\mathit{S}}_0$)→5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)7s(K=3/2,J=1) and two optically forbidden transitions 5${\mathit{p}}^6$${(}^1$${\mathit{S}}_0$)→5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)5d(K=7/2,J=3) and 5${\mathit{p}}^6$${(}^1$${\mathit{S}}_0$)→5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)5d(K=5/2, J=3) in Xe by means of electron-energy-loss spectroscopy. These measurements are carried out for electron kinetic energies of 400 and 500 eV at small scattering angles (θ=1.4°–15.1°). Optical oscillator strengths have been determined by extrapolating the generalized oscillator strengths to zero momentum transfer, as (0.298±0.045) and (0.0738±0.011), for the 5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)5d(K=3/2,J=1) and 5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)7s(K=3/2,J=1) states, respectively. Integrated cross sections have also been determined for each impact energy. The errors are estimated to be less than 15%. © 1996 The American Physical Society.

For very slow collisions, the ionization cross section depends on the projectile kinematics in the combined field of the nucleus and atomic electrons, on the time evolution of the electron wave function, and on the recoil of the nucleus. First-order semiclassical cross sections in the united-atom limit were calculated for the Thomas-Fermi and Hartree-Fock screening potentials. The time evolution of the wave function was considered by the model of a perturbed harmonic oscillator and by a simplified atomic model involving only a few lowest states. Theoretical K-shell ionization cross sections were compared with the mean experimental data calculated for groups of atoms with neighboring atomic numbers. It was pointed out that the averaging process, also employing theoretical data, depends essentially on the projectile kinematics in the screened atomic field. Taking this into account, good agreement between mean experimental and theoretical cross sections was obtained. © 1996 The American Physical Society.

An approximation procedure is introduced for the evaluation of the parameters that determine the positions, widths, and asymmetric shapes of resonance profiles associated with autoionizing states. The essential feature of the method is the availability of a minimum principle to aid in the optimization of trial functions. Such functions, suitably antisymmetrized, are used in the variational calculation of the nonresonant background contribution to the continuum wave function describing a resonant electron-ion scattering process. The rigor of the minimum principle can be maintained, even for atomic systems consisting of more than two electrons, if sufficiently accurate trial bound-state wave functions are employed in the description of ionic target states and those discrete states of the full system that are embedded in the continuum. The analysis of the general problem is preceded, for illustrative purposes, by a study of a solvable model of a resonant two-channel scattering system; the utility of the minimum principle is demonstrated by comparison of exact and variationally determined s-wave cross sections. The applicability of the formalism to the calculation of resonant photoionization and bound-state Compton scattering amplitudes is discussed briefly. © 1996 The American Physical Society.

We report results from a reformulation of the relativistic bremsstrahlung code of Tseng and Pratt. This permits calculation of the triply differential cross section ${\mathit{d}}^3$σ of bremsstrahlung (electron-photon coincidence measurements) in electron scattering on neutral atoms and ions. The cross section ${\mathit{d}}^3$σ is viewed as a more sensitive test of the theory, and predictions are needed for comparison with the more systematic and accurate experiments which are now being undertaken. The reformulation represents an extension of the previous code (which only calculated the doubly differential cross section), again utilizing partial-wave and multipole expansions in a screened potential within the independent particle approximation, but differently organizing their summation. The best previous predictions for the triply differential cross sections are due to Elwert and Haug, under assumptions less restrictive than Born approximation yet valid for high-Z elements only at very high incident electron energy. While we confirm differences from the Elwert-Haug results, we do not see a systematic improvement in the agreement with the limited previous experimental data, and further experiments are awaited with interest. © 1996 The American Physical Society.

Electron capture in ${\mathrm{O}}^{2+}$${(}^3$P${,}^1$D)+He collisions is studied theoretically by using a semiclassical molecular representation with six molecular channels at collision energies above 50 eV and by using a fully quantum-mechanical molecular representation with three Π channels below these energies. The ab initio potential curves and nonadiabatic coupling matrix elements for the ${\mathrm{HeO}}^{2+}$ system are obtained from multireference single- and double-excitation configuration-interaction calculations employing a relatively large basis set. The present total cross sections for electron capture by the ground-state ${\mathrm{O}}^{2+}$ ions are found to be in reasonable accord with those calculated by Gargaud, Bacchus-Montabonel, and McCarroll [J. Chem. Phys. 99, 4495 (1993)] below 30 eV/u but are slightly larger above this energy. Partial cross sections for the l distribution are also slightly different. Cross sections for electron capture by the metastable ${\mathrm{O}}^{2+}$ ions decrease much more sharply than those for the ground-state ion as the energy is lowered, reaching a difference between them approximately as large as one to two orders of magnitude below 100 eV. The present rate coefficient for the reaction is approximately ${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$/s above 10 000 K, suggesting that the small rate coefficient of about ${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^3$/s at 20 000 K observed by Kwang and Fang [Phys. Rev. Lett. 71, 4127 (1993)] for electron capture by the ground-state ion might be caused, in part, by a mixture of ground and metastable ions in their experiment. © 1996 The American Physical Society.

Total elastic and transport cross sections for the (pμ${)}_{\mathit{n}}^{\mathrm{*}}$+p, (dμ${)}_{\mathit{n}}^{\mathrm{*}}$+d, and (tμ${)}_{\mathit{n}}^{\mathrm{*}}$+t systems (n is the principal quantum number) are calculated in the energy range 0.04≤E≤10 eV using the classical and quasiclassical approach. Electron screening is taken into account. The obtained elastic cross sections are large, especially for small E. The experimental methods of measuring these cross sections are discussed. © 1996 The American Physical Society.

Double electron transfer processes for the system ${\mathrm{Fe}}^{17+}$+He have been studied utilizing high-resolution Auger electron spectroscopy. A thorough analysis of the experimental conditions (single collision condition and metastable fraction content at the incident ion beam and their influence) has been performed, leading to the following conclusions. The double capture leads to the preferential population of 2${\mathit{p}}^5$5l8${\mathit{l}}^{\prime }$ configurations sharing their decay between Auger processes and radiative cascade transitions. The observed sodiumlike ‘‘quartet’’ and ‘‘doublet’’ states result from cascade contributions from higher-lying levels. Tentative mechanisms for the production of the quartet states are suggested using a spin-flip model. © 1996 The American Physical Society.

Circular dichroism in the photoelectron angular distribution (CDAD) of the 7${\mathit{P}}_{\mathit{J}}$ and 8${\mathit{P}}_{\mathit{J}}$ states of cesium have been measured using plane-polarized light to excite and align the atoms and alternately right- and left-circularly polarized light to ionize them. The experiments are compared with the results of theory, calculated using the Hartree-Fock wave functions with many electron correlations included in the random-phase approximation with exchange. A comparison of theory and experiment allows information on the alignment of the resonant state to be extracted from the experimental data. No CDAD effect is observed for the ${\mathit{nP}}_{1/2}$ states that cannot be aligned, whereas CDAD is observed for the 7${\mathit{P}}_{3/2}$ and 8${\mathit{P}}_{3/2}$ states. The CDAD signal is significantly larger for the 8${\mathit{P}}_{3/2}$ state than for the 7${\mathit{P}}_{3/2}$ state, indicating a greater retention of the optical alignment of the 8${\mathit{P}}_{3/2}$ state within the time sampled by the ionizing laser pulse. The differences in the retention of alignment for the two states are attributed to effects of hyperfine depolarization, caused by coupling of the electronic angular momentum to the nuclear spin. © 1996 The American Physical Society.

Charge-state dependence for K-shell x-ray production cross sections in ${}_{13}{}^{}{}_{}{}^{}\mathrm{Al}$ bombarded by 2–12-MeV ${}_6{}^{}{}_{}{}^{}\mathrm{C}$ ions with charge states from 2+ to 6+ was measured using a Si(Li) detector. A thin Al target was used to ensure single collision conditions. Contributions of the electron capture as well as direct ionization to the inner-shell ionization were determined by an analysis of the charge-state dependence of the target x-ray production. The measurements are compared with the prediction of the ECPSSR theory using a single-hole fluorescence yield. The ECPSSR theory is based on the perturbed stationary state (PSS) formalism and relativistic effects (R) for the target electrons, and energy loss (E) and Coulomb deflection (C) of the projectile. In general, this theory gives reasonable agreement with the data for carbon ions without K vacancies while it overpredicts the data for carbon ions with K vacancies. The significant underprediction of the data at the lowest energy is likely associated with the molecular-orbital effect that is not accounted for in the ECPSSR theory. © 1996 The American Physical Society.

Different contributions to radiative stabilization in ${\mathrm{Ne}}^{10+}$+He collisions at projectile energies of 10 and 150 keV are studied. For both energies, radiative stabilization is found to be equal to ≊0.3 when referred to the total double capture. In ${\mathrm{Ne}}^{10+}$+He collisions doubly excited states 3${\mathit{lnl}}^{\prime }$ and 4${\mathit{lnl}}^{\prime }$ (n≥4) are either produced by uncorrelated two-electron transitions or by dielectronic mechanisms due to electron-electron interaction. A strong contribution (≊0.15) to the stabilization follows from the decay of near-equivalent electrons 3${\mathit{lnl}}^{\prime }$ and 4${\mathit{lnl}}^{\prime }$(n=4,5). Another major contribution (0.10–0.25) to stabilization is due to the decay of configurations 3${\mathit{lnl}}^{\prime }$ (n≥6) of nonequivalent electrons produced by the dielectronic process of autoexcitation. A small contribution is found to be due to the configurations 3${\mathit{lnl}}^{\prime }$ (n≳9) created by dielectronic phenomena in the postcollisional and asymptotic regions (≊0.04). © 1996 The American Physical Society.