Time-resolved laser spectroscopy was used to measure radiative lifetimes of eight excited states of neutral sulfur in a laser-produced plasma. Excitation from the atomic ground state was performed with pulsed vacuum-ultraviolet (126–140 nm) radiation generated by resonant sum-difference four-wave mixing in krypton gas. The lifetimes of the ${3\mathrm{p}}^3$ ns${\mathrm{}}^3$ ${\mathrm{S}}^{\mathrm{o}}$ (n=5–7) and ${3\mathrm{p}}^3$ nd${\mathrm{}}^3$ ${\mathrm{D}}^{\mathrm{o}}$ (n=4,5) states were measured and are presented together with data available from literature. Relativistic Hartree-Fock calculations of radiative lifetimes were also performed for states where experimental data were available.

We have used third-order perturbation theory to calculate the effect of the Breit-Pauli relativistic interaction on the dipole polarizability of ${\mathrm{Li}}^{+}$ ions of order ${\mathrm{\alpha }}^2$ . Results are in good agreement with the value obtained by Johnson and Cheng [Phys. Rev. A 53, 1375 (1996)]. In addition, the retardation (Casimir) corrections to singly excited (${1\mathrm{s}}^2$ NL) states of neutral lithium have been computed. When both of these corrections are added to the previously calculated Rydberg state energies, 1σ agreement is found with experiment [N. E. Rothery et al., Phys. Rev. A 51, 2919 (1995)]. Addition of radiative (Lamb-shift) corrections produces essentially exact agreement.

Omitted end note.

Energy shifts of radiative corrections in atoms due to perturbing potentials are calculated. The potentials considered are a constant magnetic field, the magnetic field of a nucleus, and the Coulomb interaction from other electrons in many-electron atoms.

With a phase-modulated extreme ultraviolet pulsed laser source the frequency of the 1${\mathrm{}}^1$S-2${\mathrm{}}^1$P transition of helium at 58 nm has been measured. The phase modulation scheme enabled measurement and reduction of frequency chirp, usually limiting pulsed precision spectroscopy. From the measured transition frequency of 5 130 495 083(45) MHz, a fourfold improved value of the ground state Lamb shift of 41 224(45) MHz is deduced, in good agreement with a theoretical value of 41 233(35) MHz based on QED calculations up to order ${\mathrm{\alpha }}^5$${\mathrm{Z}}^6$. From these measurements, the well-known binding energy of the 2${\mathrm{}}^1$P state and the previously determined ${}_{}{}^4{}_{}{}^{}\mathrm{-}_{}^3{}_{}{}^{}$He isotope shift, accurate values for the ionization energies of the helium atom follow: 198 310.6672(15)${\mathrm{fcm}}^{\mathrm{-}1}$ for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ and 198 301.8808(15)${\mathrm{fcm}}^{\mathrm{-}1}$ for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$.

Formal separate expressions for the exact exchange and correlation potentials ${\mathrm{v}}_{\mathrm{x}}$ and ${\mathrm{v}}_{\mathrm{c}}$ are extracted from the formal line-integral expression of Holas and March for the whole exact exchange-correlation potential. Relations for the components of ${\mathrm{v}}_{\mathrm{c}}$ are extracted for each order in the electron-electron repulsion coupling constant, through use of the coupling-constant expansion of Görling and Levy for the external potential. The resultant expressions for ${\mathrm{v}}_{\mathrm{c}}$ and ${\mathrm{v}}_{\mathrm{x}}$ are separately path independent. The difference between ${\mathrm{v}}_{\mathrm{x}}$ and the Harbola-Sahni approximation to it, ${\mathrm{v}}_{\mathrm{x}}^{\mathrm{HS}}$, is identified as arising from a first-order contribution to the kinetic-energy density tensor. It is shown that this small correction to ${\mathrm{v}}_{\mathrm{x}}^{\mathrm{HS}}$, which we express in terms of perturbation theory, would be precisely zero if the Kohn-Sham determinant were identical to the Hartree-Fock determinant for the same density. In other words, ${\mathrm{v}}_{\mathrm{x}}^{\mathrm{HS}}$ would equal ${\mathrm{v}}_{\mathrm{x}}$ if the optimized effective potential determinant were the same as the Hartree-Fock determinant. This same property is shared by the Slater potential.

A set of 18 absolute cross sections for electron-impact-induced line radiation of Ne, Ar, and Kr in the spectral range between 46 and 100 nm was measured providing an accurate database for the use of an electron-impact excitation source as a source of calculable radiant flux. Unparalleled, low relative uncertainties, mostly below 4%, were achieved, mainly because the electron storage ring BESSY served as a primary standard source in the vuv to determine the responsivity of the spectrometer-detector system used for the cross-section measurements, and because a spinning rotor gauge was used as a secondary standard for the determination of the target gas density. A comparison of our cross-section data with published experimental and theoretical data is presented.

Femtosecond pump-probe techniques are applied to the measurement of above-threshold ionization (ATI) electrons to observe the influence of vibrational wave-packet dynamics on molecular above threshold ionization in real time. When ionized by a single laser pulse structural changes in the ATI electron spectra map the changes of the vibrational wave-packet evolution in the resonant intermediate electronic states as a function of laser intensity. Molecular vibrational motion in the coherently coupled electronic states dominates the electron energy distribution rather than the energy splitting induced by Rabi oscillations.

A resonance state with three equivalent electrons and one proton has been identified. Large-scale calculations have been performed using the complex-rotation technique, and the results indicate that the investigated ${}_{}{}^4{}_{}{}^{}\mathrm{S}_{}^{\mathrm{o}}{}_{}{}^{}$ state of ${\mathrm{H}}^{2\mathrm{-}}$ is strongly correlated. The findings are rationalized by comparing with bound doubly charged anions, isoelectronic states of the Li atom, and calculations in the infinite-dimension limit. In addition, the emerging picture is sustained by an analysis of the complex-rotated wave function.

We present absolute term- and level-selective cross sections for specific single- and double-electron capture processes in slow (16 keV) ${\mathrm{C}}^{4+}$-Ne collisions. The results are deduced through a combined analysis of earlier translational energy-gain data and the present photon-emission cross sections in the region 40–240 nm. We find that double-electron capture is dominated by processes in which two electrons appear to change orbitals in a single interaction of two diabatic quasimolecular potential curves—a type of process which rarely has been isolated before. Furthermore, we observe a strong preference for population of 2s2p over ${2\mathrm{p}}^2$ configurations in these one-step two-electron capture reactions. This is a remarkable configuration selectivity, since reaction channels leading to 2s2p and ${2\mathrm{p}}^2$ configurations cross with the incident channel at almost the same internuclear distances.

In this work the autoionization of the configuration Ca ${3\mathrm{p}}^5$3d4s4p of free calcium atoms has been investigated by the use of angle-resolved photoelectron spectroscopy. In a first step, atoms in the ground-state Ca ${3\mathrm{p}}^6$${4\mathrm{s}}^{21}$${\mathrm{S}}_0$ were laser excited and aligned to the first optically excited-state Ca ${3\mathrm{p}}^6$${4\mathrm{s}4\mathrm{p}}^1$${\mathrm{P}}_1$. From this intermediate state, with the help of synchrotron radiation (31.5–34.5 eV), the closed 3p subshell was opened, leading to the population of Ca ${3\mathrm{p}}^5$3d4s4p autoionizing states. The autoionization decay of these states to both the ground state and shakeup states of singly charged ions was investigated using two types of photoelectron spectrometers. An angle-integrating spectrometer allowed us to tentatively assign the dominating angular momenta and to measure the relative partial cross sections. Furthermore, by combination with an angle-resolving spectrometer the angle dependence of the outgoing photoelectrons in the region of 32.7–33.3 eV was completely characterized for the three main diagram lines. Completeness was achieved in the sense that for action of linearly polarized light sources maximal information was extracted.

The rotational excitation of the title linear polyatomic target, treated as a rigid rotor is computed using a fully ab initio interaction potential recently employed to obtain integral elastic cross sections [Gianturco and Stoecklin, J. Phys. B 29, 3933 (1996)] and to unravel several resonances in the low-energy region. The present study looks at the rotationally inelastic processes which can occur in the energy range across the long-lived shape resonance at 3.9 eV and finds that the resonant process strongly enhances the overall efficiency of the rotational excitation by the electron projectile, as shown by the computed values of the average rotational energy transfers. Angular distributions are also evaluated at different collision energies and compared with earlier calculations.

Medium modification of scattering properties in interacting Bose systems are considered by solving the Bethe-Salpeter equation. An equation of state for the normal phase (generalized Beth-Uhlenbeck formula) is given using the in-medium phase shifts to include two-particle correlations. Conclusions are drawn for systems of bosonic atoms with repulsive interaction such as sodium ${(}^{23}$Na) and rubidium ${(}^{87}$Rb). It is shown that the negative slope of the in-medium phase shift at zero momentum and the absolute value of the in-medium scattering phase shift for low scattering energies increase with density.

Two-electron capture with an emission of a single photon (TESP) in collisions of highly charged ions with light atoms is considered. Such a process is actually a time-reversed double photoionization but occurring at specific kinematics. In the lowest order in the interelectron interaction, the TESP probability is determined by two diagrams which are evaluated analytically by means of the Coulomb Green's function. The calculated ratio of the radiative double-electron capture and single recombination cross sections is in fair agreement with the data obtained in the recent experimental study of this phenomenon.

Integral cross sections for electron-impact excitation from the helium 2 ${}_{}{}^3{}_{}{}^{}$ S and 2${\mathrm{}}^1$ S metastable excited states, to all energetically higher S, P, and D excited states, are reported from threshold to 200 eV. These cross sections have been obtained using the distorted-wave approximation, and are compared to other experimental and theoretical results. Special attention is paid to obtaining an adequate representation for the wave functions of the n${\mathrm{}}^1$ S excited states (n=2, 3, and 4). The total integral electronic excitation cross sections from each of these metastable states, summed over n for fixed L of the final state, and then summed over L, are also reported. These results are combined with other cross-section data to obtain the total inelastic integral cross section for inelastic electron scattering from each of these two helium metastable states.

Triple differential cross sections (TDCS's) have been calculated for ionization of ${\mathrm{He}}^{+}$ by electron impact in the coplanar asymmetric geometry for three different incident energies 250, 500, and 1000 eV, and fixed value of the ejected energy 5 eV and scattering angles (4° and 10°), and in the coplanar symmetric geometry for incident energies of 250–2000 eV as well. The final-state wave function used here considers all three two-body interactions and satisfies the proper asymptotic Coulomb boundary condition. The initial channel wave function involves a Coulomb wave due to the long-range Coulomb attraction between the incident electron and the screened ionic nucleus. Parts of the calculated TDCS's are compared with the existing theoretical results.

In the context of the independent-electron model, we evaluate the transition amplitude for simultaneous transfer and excitation to occur in ion-atom collisions. For the capture transition amplitude, we use the continuum-distorted-wave approximation, and for the excitation amplitude, we use the first Born approximation. The results are compared directly with the continuum-distorted-wave approximation in a four-body approach for the ${\mathrm{S}}^{15+}$ +H system.

A complete treatment of electron-ion recombination of e+Fe III→Fe II employing a unified method is presented. The treatment incorporates both the radiative and dielectronic recombinations in a self-consistent manner. Total recombination rate coefficients are obtained from photoionization cross sections, and from collision strengths for dielectronic recombination calculated using the precise theory of Bell and Seaton [J. Phys. B 18, 1589 (1985)]. Large-scale computations for both of these quantities are carried out in the close coupling approximation using the R-matrix method with an eigenfunction expansion that includes 83 states of Fe III dominated by the ground ${3\mathrm{d}}^6$, and the excited ${3\mathrm{d}}^5$4s and ${3\mathrm{d}}^5$4p configurations. Both the total and state-specific recombination rate coefficients are obtained. Comparison of the present results with the previous ones shows considerable difference for most of the temperature regions. The present results provide accurate and self-consistent recombination rates, in the temperature range of practical applications (T<${10}^5$ K), for ionization balance in photoionization models employing the detailed photoionization cross sections from the Opacity Project.

Spectra of K x rays from Ar ions (incident energy 15 MeV/u) traveling in thick solid targets of Be, C, NaF, and KCl were measured with a curved crystal spectrometer for the purpose of investigating the dependence of the H-like Ar 2p-state population fraction on projectile energy and target atomic number. Information pertaining to the energy dependence of the 2p fraction was obtained from the Doppler-shifted average energies and peak shapes associated with the Lyman-α lines. Model calculations that took into consideration the fraction of ions with one 2p electron and x-ray absorption as a function of depth in the target were performed. Generally good agreement between the experimental results and the predictions of the model were observed for all of the targets except NaF.

For N charged particles of equal masses moving in the field of a heavy residual charge, an approximate analytical solution of the many-body time-independent Schrödinger equation is derived at a total energy above the complete fragmentation threshold. All continuum particles are treated on equal footing. The proposed correlated wave function represents, to leading order, an exact solution of the many-body Schrödinger equation in the asymptotic region defined by large interparticle separations. Thus, in this asymptotic region the N-body Coulomb modifications to the plane-wave motion of free particles are rigorously estimated. It is shown that the Kato cusp conditions are satisfied by the derived wave function at all two-body coalescence points. An expression of the normalization of this wave function is also given. To render possible the calculations of scattering amplitudes for transitions leading to a four-body scattering state, an effective-charge method is suggested in which the correlations between the continuum particles are completely subsumed into effective interactions with the residual charge. Analytical expressions for these effective interactions are derived and discussed for physical situations.