Time-resolved extreme ultraviolet spectra of foil-excited 120-MeV Br ions have been recorded, using a spectrometer equipped with a position-sensitive detector. The spectra near λ=20–30 nm were measured at times after excitation of up to 6 ns. Prominent in the delayed spectra were the long-lived decays of the intercombination transitions in the Mg-like ${\mathrm{Br}}^{23+}$ and Al-like ${\mathrm{Br}}^{22+}$ ions. The present paper reports on lifetime measurements for intercombination transitions in highly charged bromine ions using such a position-sensitive detector.

We describe multiconfiguration linear-response (MCLR) approaches to the calculation of absolute photoionization cross sections. Three algorithms are described: (i) one in which the MCLR equations are used to derive the primitive spectrum of excitation energies and oscillator strengths; (ii) one in which the solutions of the MCLR equations are used to provide the even negative moments of the photoexcitation-photoionization spectrum; and (iii) one in which a pseudospectrum is obtained directly in the iterative procedure used to solve the MCLR equations. Either the primitive spectra (i), the moments (ii), or the pseudospectra (iii) are used as basic quantities in Stieltjes imaging to obtain the photoionization cross sections. Numerical demonstrations with large multiconfigurational self-consistent field reference spaces are given for the photoionization of HF, ${\mathrm{H}}_2$O, and Ne. Comparative calculations are performed in the random-phase approximation. Results are analyzed with respect to the fulfillment of gauge invariance, sum rules, basis-set completeness, and choice of correlating orbital spaces. Results for absolute photoionization cross sections from the MCLR algorithms agree very well with Stieltjes-imaging cross sections obtained from the semiempirically determined spectral moments of Meath and co-workers [Can. J. Phys. 55, 2080 (1977); 63, 1616 (1985)], and distinguish the raw experimental cross sections.

We investigate the problem of two-electron capture from heliumlike atomic systems by bare nuclei ${\mathit{Z}}_{\mathit{P}}$+(${\mathit{Z}}_{\mathit{T}}$;${\mathit{e}}_1$,${\mathit{e}}_2$${)}_{\mathit{i}}$→(${\mathit{Z}}_{\mathit{P}}$;${\mathit{e}}_1$,${\mathit{e}}_2$${)}_{\mathit{f}}$+${\mathit{Z}}_{\mathit{T}}$ at high incident energies, using the four-body formalism of the first- and second-order theories. Our goal is to establish the relative importance of the intermediate ionization continua of the two electrons in comparison with the usual direct path of the double electron transfer. For this purpose we presently introduce the boundary-corrected continuum-intermediate-state (BCIS) approximation, which preserves all the features of two-electron capture as a genuine four-body problem. The proposed second-order theory provides a fully adequate description of the fact that, in an intermediate stage of collision, both electrons move in the field of the two Coulomb centers. The previously devised boundary-corrected first Born (CB1) approximation can be obtained as a further simplification of the BCIS model if the invoked two-electron Coulomb waves are replaced by their long-range logarithmic phase factors defined in terms of the corresponding interaggregate separation R. The BCIS method is implemented on the symmetric resonant double charge exchange in collisions between α particles and He(1${\mathit{s}}^2$) at impact energies E≥900 keV. The obtained results for the differential and total cross sections are compared with the available experimental data and satisfactory agreement is recorded. As the incident energy increases, a dramatic improvement is obtained in going from the CB1 to the BCIS approximation, since the latter closely follows the measurement, whereas the former overestimates the observed total cross sections by two orders of magnitude.

Measurements and n-body classical-trajectory Monte Carlo calculations are presented for the dependence of recoil-ion emission on the azimuthal angle between a projectile and a recoil ion. From the measured momenta of the projectile and the recoil ion, the dependence of the electron emission on the azimuthal angle between projectile and electron is deduced. The recoil ions are found to be emitted to the opposite direction of the scattered projectile only for scattering angles larger than 1 mrad, while for smaller scattering angles, the momentum of the emitted electron results in a broad azimuthal angular distribution of the recoil ions. For a scattering angle of 0.55 mrad, the electrons are predominantly emitted opposite to the scattered projectile, yielding a direct proof of a binary encounter between projectile and electron.

Close-coupling techniques often break down when transitions to a continuum or quasicontinuum cannot be neglected. This work describes an extension of a recently proposed method [L. F. Errea, L. Méndez, and A. Riera, Chem. Phys. Lett. 164, 261 (1989)] to deal with such situations, when the coupling terms involve Coulomb potentials. A connection with the method of packet states is made, and a preliminary application of the method is presented.

The repulsive wall of the ground-state ${\mathrm{He}}_2$ interaction potential between 0.5 and 2 Å is determined by optimizing model potentials in order to fit the combined differential-cross-section data of Newman et al. [J. Geophys. Res. 90, 11 045 (1985)] and Nitz et al. [Phys. Rev. A 35, 4541 (1987)]. The results for two distinct forms of the model are consistent with one another and yield a potential which agrees with the semiempirical Hartree-Fock dispersion [HFD-B(He)] potential of Aziz et al. [Mol. Phys. 61, 1487 (1987)] near 1.6 Å but rises less steeply with decreasing internuclear separation.

The cross sections for electron-impact ionization of oxygen atoms in the (2${\mathit{p}}^4$${)}^3$P ground state to form ${\mathrm{O}}^{+}$ ions in the (2${\mathit{p}}^3$${)}^4$S, (2${\mathit{p}}^3$${)}^2$D, and (2${\mathit{p}}^3$${)}^2$P states have been calculated by the Born-Ochkur approximation for incident-electron energies from the threshold to 500 eV and by the method of exchange distorted waves (with Ochkur’s approximation) from the threshold to 200 eV. Comparisons are made with previous theoretical calculations and with experimental measurements. Our calculated cross sections agree well with the experimental values, the difference being about 15% at energies above 50 eV. The contributions from autoionization and from the inner-shell ionization (by removing the 2s electron) are also discussed. The cross-section calculation is extended to ionization of oxygen atoms in the [2${\mathit{p}}^3$${(}^4$S)${\mathit{n}}_0$${\mathit{l}}_0$${]}^3$${\mathit{L}}_0$ excited states by means of the Born-Ochkur approximation for ${\mathit{n}}_0$${\mathit{l}}_0$ up to (6,5) and the method of distorted waves for ${\mathit{n}}_0$${\mathit{l}}_0$=3s, 3p, and 3d.

The angular distributions of energetic electrons ejected at angles between 45° and 135° with respect to the incident-beam direction have been measured in coincidence with the charge states of the target recoil ions produced in multiply ionizing collisions for the 1-MeV/u ${\mathrm{O}}^{\mathit{q}+}$ (q=4,7)+Ar collision systems. These measurements have been made for ∼179-, ∼345-, and ∼505-eV electrons. Additionally, the energy distributions of electrons ejected into specific angular regions have been measured. Ar LMM satellite Auger electrons appear as a peak in the energy spectrum of electrons ejected at all large angles. The center of this peak is found at an electron energy of ∼179 eV. Electrons with ∼179 eV energy, ejected at large angles, are preferentially produced in coincidence with recoil ions of charge state 4+. Electrons with ∼345 eV energy and ∼505 eV energy ejected at large angles are preferentially produced in coincidence with recoil ions of charge state 3+. The angular distributions for these electrons are strongly peaked in the forward direction; essentially no electrons are observed at angles larger than 90°. These results are consistent with the dominant production mechanism for energetic electrons ejected at large angles being a binary-encounter process. Differential cross sections have been calculated from these angular distributions. They are on the order of ${10}^{\mathrm{-}21}$ ${\mathrm{cm}}^2$/(eV sr).

Cross sections for the photoionization of the argon 3s electron are presented from threshold up to 100 eV. In order to cover this whole energy range with comparable accuracy, we used two complementary methods: absolute measurements by photon-induced fluorescence spectroscopy (PIFS) and relative measurements by time-of-flight (TOF) photoelectron spectroscopy (PES). After calibration of the relative PES data by the most recent absorption results, the data of these independent measurements agree well with each other. The experiments were carried out with special emphasis to the threshold region (PIFS) and the region of the cross sectional minimum (TOF) where, in addition to the partial cross sections, the angular distributions of the photoelectrons were determined. The results are compared with published data and discussed with respect to matrix-element dependencies and correlation effects.

Using the concept of hidden adiabatic energy crossings in the complex plane of internuclear distance, the collision dynamics in the system ${\mathrm{He}}^{2+}$+H(n), n≤4 and ${\mathrm{H}}^{+}$+${\mathrm{He}}^{+}$(n), n≤6 is studied. About 160 molecular states, coupled by 220 isolated (hidden-crossing) radial transitions and by the rotational transitions at R≃0 between the m states of (Nl) manifolds up to the N=10 united-atom principal quantum number, have been included in the calculations of the evolution matrix of the system. The calculated cross sections for transitions between the discrete states (excitation and electron capture), and from any of the considered discrete states to the continuum (ionization), are compared with other theoretical and experimental results where these are available.

The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow ${\mathrm{Na}}^{+}$ ion impact at reduced velocity ṽ=0.1–0.9 has been measured. ${\mathrm{Na}}^{+}$ beam energies 7.3–684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities ṽ and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion–Rydberg-atom collisions at ṽ≤0.7. At very low energy (ṽ≤0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.

The angular distributions of dissociated deuterons for electron capture and ionization by bare oxygen ions with energies from 2 to 16 MeV in collision with molecular deuterium targets were measured. The dependence of the differential cross sections on the alignment of the molecular axis with respect to the beam axis was determined. The results show that in the transfer ionization and transfer excitation processes, the deuterium molecules are more likely to be aligned perpendicular to the incident beam than parallel to the beam. This feature can be qualitatively interpreted as resulting from the interference of capture amplitudes from the two atomic centers. In the double ionization and ionization excitation processes, little alignment dependence was observed.

Partial and total cross sections for electron capture in collisions of ${\mathrm{B}}^{4+}$ ions with H atoms from 100 meV/amu to 10 keV/amu have been rigorously determined by using the molecular-orbital expansion method modified by the inclusion of electron translation factors. Quantum-mechanical (3 channels) and semiclassical (12 channels) methods have been employed at collision energies lower than 30 eV/amu and higher than 15 eV/amu, respectively. The agreement of the present results with measurements above 1 keV/amu is reasonable. In our cross sections for the singlet manifold, rather large oscillatory structures are found below 10 eV/amu that are attributable to Stueckelberg-type oscillation. At collision energies below a few eV/amu, the ${\mathrm{B}}^{3+}$(1s3d) and ${\mathrm{B}}^{3+}$(1s3p) states are the dominant states for the triplet and singlet manifolds, respectively. However, the ${\mathrm{B}}^{3+}$(1s3p) state for the triplet and ${\mathrm{B}}^{3+}$(1s3d) state for the singlet take over above these energies. At energies above 1 keV/amu, three states, the ${\mathrm{B}}^{3+}$(1s3s), ${\mathrm{B}}^{3+}$(1s3p), and ${\mathrm{B}}^{3+}$(1s3d) states, contribute equally to the electron-capture process. A comparison of the present ${\mathrm{B}}^{4+}$ results with previous results for different projectiles with the same charge (i.e., ${\mathrm{Be}}^{4+}$, ${\mathrm{C}}^{4+}$, and ${\mathrm{N}}^{4+}$) reveals that although the cross sections of all these systems lie roughly within a certain range of magnitudes at intermediate collision energies (E=0.5–10 keV/amu), differences begin to emerge in magnitude and energy dependence below this energy. We examined the scalability of the cross section; some remarks on the scaling are given.

The impact-parameter representation is deduced from the formal scattering theory for electron capture by impact of bare ions on molecular targets. The role of the interactions between the projectile nucleus and the molecular nuclei is determined.

Doubly differential secondary-electron yields following the impact of 8-MeV/u ${\mathrm{U}}^{68+}$ and 3.5-MeV/u ${\mathrm{U}}^{38+}$ ions on 44-μg/${\mathrm{cm}}^2$ carbon-foil targets have been investigated experimentally. Electron emission yields were determined for ejection angles varied between 0° and 157° and electron energies ranging from 10 eV to 8 keV. The experimental data are compared to results from a theoretical model for production and transport of fast electrons in solids.

We present elastic through 7 ${}_{}{}^2{}_{}{}^{}$F differential, integrated, and total cross sections for electrons scattering from the ground state of potassium atoms. For the elastic channel, we also present spin asymmetries, and for the 4 ${}_{}{}^2{}_{}{}^{}$P channel we present spin asymmetries ${\mathit{L}}_{\mathrm{\perp }}$ for singlet, triplet, and spin-averaged scattering, optical excitation cross sections, and various photon polarization observables. These are calculated using the coupled-channel optical method. A total of 17 target states are explicitly coupled with the effect of the continuum included in the first six of these. The calculations have been performed at a range of energies from 1 to 100 eV. The results are generally in good agreement with available experiment. We find that the effect of the inclusion of the continuum is most evident in calculating spin asymmetries and singlet ${\mathit{L}}_{\mathrm{\perp }}$.

The multichannel multiconfiguration Dirac-Fock method has been used to study the final-continuum-state configuration interaction (FCSCI) and relaxation effects in Xe ${\mathit{N}}_{4,5}$OO Auger-electron spectrum. FCSCI has been found to cause a drastic intensity redistribution and an 11% reduction of the total transition rate. The relaxation effect has been calculated in the full many-electron picture by using separately optimized initial- and final-state orbitals. The relaxation effect varies from an 8% increase to a 16% decrease of transition rates. The dominant contribution to the relaxation is found to come from the contraction and expansion of orbitals of participating electrons and from the conjugate Auger amplitudes.

Molecular-alignment effects in the transfer excitation of ${\mathrm{H}}_2$ by high-velocity heavy ions are studied using a two-step mechanism with amplitudes evaluated from first-order perturbation theory. Two-electron transfer excitation is treated as a result of two independent collision processes (excitation and electron transfer). Cross sections for each one-electron subprocess as well as the combined two-electron process are calculated as functions of the molecular-alignment angle. Within the independent-electron approximation, the dynamic roles of electron excitation and transfer in conjunction with molecular alignment are explored. While both excitation and transfer cross sections may strongly depend on molecular alignment, it is electron transfer that is largely responsible for the molecular-alignment dependence in the transfer excitation process. Interpretation of some experimental observations based on this model will also be discussed.

We present a formulation for low-energy positron-molecule collisions based on the Schwinger multichannel method (SMC) which was previously adapted for electron-molecule scattering. We have preserved very important features of the SMC such as its ability to deal with electronically inelastic scattering and its applicability to molecular targets of arbitrary geometry. Our implementation also allows for the inclusion of a substantial number of closed electronic channels to represent polarization effects, which are very important at low impact energies. As an application, to illustrate the form with which polarization is represented in this formulation, we have calculated elastic integral and differential cross sections for ${\mathit{e}}^{+\mathrm{-}}$${\mathrm{H}}_2$ and ${\mathit{e}}^{+\mathrm{-}}$${\mathrm{CH}}_4$ collisions. We included all possible single-particle excitations from the target and found substantial disagreement with respect to model potential calculations and attenuated-beam experimental data. We discuss possible reasons for these discrepancies.

The x-ray emission following the impact of highly charged ${\mathrm{Xe}}^{\mathit{q}+}$ (q=44–48) ions of 7-keV/q energy on a Cu surface has been measured. Theoretically we have derived an analytic formula which allows us to calculate the 2l-nl’, 3l-nl’, and 4l-nl’ (n≤5), L, M, and N x-ray transition energies averaged over spin and angular-momentum quantum numbers (L and S) as a function of the electron occupation numbers (${\mathit{k}}_{\mathit{i}}$) for the states Q=1${\mathit{s}}_1^{\mathit{k}}$2${\mathit{s}}_2^{\mathit{k}}$2${\mathit{p}}_3^{\mathit{k}}$3${\mathit{s}}_4^{\mathit{k}}$3${\mathit{p}}_5^{\mathit{k}}$3${\mathit{d}}_6^{\mathit{k}}$4s ${}_7{}^{\mathit{k}}{}_{}{}^{}\mathit{p}_8^{\mathit{k}}{}_{}{}^{}$4${\mathit{d}}_9^{\mathit{k}}$4${\mathit{f}}_{10}^{\mathit{k}}$5${\mathit{s}}_{11}^{\mathit{k}}$5${\mathit{p}}_{12}^{\mathit{k}}$. In accordance with our theoretical predictions the observed L x-ray structures shift toward higher energies and increase in intensity relative to the M x rays with increasing charge state. We have also observed an increase in high-energy satellite-line intensities with the increasing number of 2p vacancies. In calculating the radiative-transition probabilities we have found that the dominating electric-dipole transitions for L and M x rays are of the kind 2p-3d and 3d-4f, respectively. A comparison between measured peak positions and calculated transition energies reveals that direct feeding by approximately 13 electrons can explain the main features of the observed x-ray emission spectra.