Auger and radiative transition rates and fluorescence yields for the double-K-hole state have been calculated for five elements with 10≤Z≤36 using the multiconfiguration Dirac-Fock method. The Auger and x-ray energies are found to shift to higher energies by 68–269 and 100–388 eV, respectively, due to the presence of the K-shell spectator vacancy. For low-Z elements, the transition rates per K hole for the double-K-hole state are much larger than the corresponding rates for the single-K-hole state. As Z increases to higher values, the differences in transition rates diminish. K-shell fluorescence yields for the double-K-hole state compared to the results for the single-K-hole state are found to increase by 33% at Z=10 but by only 2.6% at Z=36.

We report studies of double-electron transfer in slow collisions (v≊0.2–0.4 a.u.) of the F-like (nine-electron) ions ${\mathrm{Si}}^{5+}$, ${\mathrm{Ar}}^{9+}$,${\mathrm{Sc}}^{12+}$, ${\mathrm{Ti}}^{13+}$, ${\mathrm{Fe}}^{17+}$, and ${\mathrm{Cu}}^{20+}$ with He atoms. The transfer populates doubly excited states of the Na sequence (eleven electrons) of these elements that we study via the subsequent electron emission using high-resolution Auger spectroscopic techniques. Comparison is made to spectra that are formed by single-electron capture by Ne-like (ten-electron) ions in the 2${\mathit{p}}^5$3s ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{0,2}$ metastable levels. Non-Coulomb (Breit interaction) Auger decay of the Na-like metastable level 2${\mathit{p}}^5$3s3p ${}_{}{}^4{}_{}{}^{}$${\mathit{D}}_{7/2}$ forms a prominent line in many of these spectra.

A charged projectile in dense matter generates a cascade of ionized electrons. Some of the ejected electrons are racing ahead of the projectile along the future primary track. Ionization induced by such electrons before the arrival of the projectile reduces the stopping power in the medium. This preionization effect increases with the third power of the projectile charge and approximately the second power of the density. The observed gas-solid differences in argon could be caused by preionization.

A theoretical investigation of charge transfer in ${\mathrm{He}}^{+}$+${\mathrm{H}}_2$ collisions in the ultralow-energy regime (meV) has been conducted. Molecular states have been obtained by using the multiconfiguration self-consistent-field method. The reaction rate constants have been calculated for dissociative and radiative charge-transfer processes. Agreement with the recent measurements of Schauer et al. [J. Chem. Phys. 91, 4593 (1989)] is satisfactory. The present study provides a theoretical rationale for the experimental findings by interpreting the detailed collision dynamics.

The argon photoabsorption cross section has been measured absolutely in the energy range around 3500 eV. Resonant K+L double photoexcitations and shake-up threshold jumps with an inner-shell 1${\mathit{s}}^1$2${\mathit{p}}^5$ double-hole configuration have been found. The observed energy positions agree quite well with those determined by relativistic self-consistent field calculations. Former determinations of the K+L shake-up threshold jump height have been confirmed.

Differential cross sections for rotational transitions from vibrationally excited sodium dimers (v=2 and v=31) by electron impact at 150 eV have been measured. The rotational inelasticity increases with the initial vibrational quantum number. Analysis of the data provides clear evidence for vibrational transitions as well. The cross section for vibrational transitions increases with the vibrational quantum number and is not small compared to the cross section for rotational transitions. The experimental data are well reproduced by a simple two-center model.

Time-dependent distortion of the initial electronic state in response to the motion of the projectile is employed in calculations of total and differential cross sections for the direct-Coulomb, K-shell-ionization process within the semiclassical approximation. Selected targets are chosen from Al through Ag for incident protons and helium and lithium ions in the velocity region that corresponds to energies of 0.175 through 5.0 MeV/u for a Cu target. The distortion is taken to include both energy and wave function (via polarization) of the initial-state K-shell electron. Discussion is presented with regard to the appearance of nonorthogonal basis states. The results of calculations for the total K-shell-ionization cross section in comparison with experimental data show good agreement for protons incident on targets from Ti through Cu. Some differences are noted for Al and Ag targets, and systematically larger discrepancies with projectile atomic number are obtained for He and Li projectiles. It is found that the introduction of temporally dependent distortion causes drastic changes in the behavior of the scattering amplitude at low velocities that have not been generally appreciated. Time-dependent binding gives rise to a large suppression of the cross section (over that induced by the well-known introduction of a constant increase in binding), while time-dependent polarization leads to a large enhancement. An explanation of this behavior is suggested. Possible causes of the differences between theoretical and experimental values of the cross section are considered. Some observations apropos of the introduction of temporally dependent quantities in approximate calculations are made.

The photoionization of ${\mathrm{N}}_2$ leading to the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$, ${\mathit{A}}^2$${\mathrm{\Pi }}_{\mathit{u}}$, and ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ states of ${\mathrm{N}}_2^{+}$ has been studied using the random-phase approximation (RPA) and the multichannel frozen-core Hartree-Fock (MCFCHF) approximation. The RPA has been implemented as a coupled-channel method with a set of open and closed channels corresponding to excitation operators and with a set of closed channels corresponding to deexcitation operators. Thus in the case of the photoionization of ${\mathrm{N}}_2$ there are 14 coupled channels. Both the RPA and MCFCHF were solved using the Schwinger variational method with Padé-approximant corrections. Good agreement was found between the RPA results and the MCFCHF results when the dipole-mixed form of the cross section was used. In comparison with experiment, both of these methods show qualitative improvements over previous computational results. These calculations give the correct behavior of the cross section in the 1${\mathrm{\pi }}_{\mathit{u}}$→k${\mathrm{\pi }}_{\mathit{g}}$ channel without ad hoc corrections needed in earlier calculations. Also both of these methods exhibit autoionizing resonances lacking in earlier results. However, there are still a number of discrepancies between theory and experiment, including the position and shape of the autoionization resonances, and the behavior of the photoelectron asymmetry parameters in the (2${\mathrm{\sigma }}_{\mathit{u}}$${)}^{\mathrm{-}1}$ ionization channel, which is strongly perturbed by the shape resonance in the (3${\mathrm{\sigma }}_{\mathit{g}}$${)}^{\mathrm{-}1}$ ionization channel.

The first-order distorted-wave approximation (DWB1) has proved to be one of the most successful approaches for the theoretical calculation of electron-atom scattering. It is now evident that one must perform multistate close-coupling calculations or second-order perturbation calculations to improve significantly upon the DWB1, but these calculations require enormous computational efforts. Consequently, it is of interest to investigate the possibility of improving the standard DWB1. One such possibility that has not been investigated for electron-atom scattering is the use of realistic nonspherical distorting potentials. In this paper, the DWB1 formalism is presented for nonspherical distorting potentials, and the effects of such a potential are investigated for electron-impact excitation of the 2p state of hydrogen.

We derive level-to-configuration sum rules for dielectronic capture and for collisional excitation and ionization. These sum rules give the total transition rate from a detailed atomic level to an atomic configuration. For each process, we show that it is possible to factor out the dependence on continuum-electron wave functions. The remaining explicit level dependence of each rate is then obtained from the matrix element of an effective operator acting on the bound orbitals only. In a large class of cases, the effective operator reduces to a one-electron monopole whose matrix element is proportional to the statistical weight of the level. We show that even in these cases, nonstatistical level dependence enters through the dependence of radial integrals on continuum orbitals. For each process, explicit analytic expressions for the level-to-configuration sum rules are given for all possible cases. Together with the well-known J-file sum rule for radiative rates [E. U. Condon and G. H. Shortley, The Theory of Atomic Spectra (University Press, Cambridge, 1935)], the sum rules offer a systematic and efficient procedure for collapsing high-multiplicity configurations into ‘‘effective’’ levels for the purpose of modeling the population kinetics of ionized heavy atoms in plasma.

Angular-distribution asymmetry parameters for photoelectrons produced by two-photon ionization of the Ar atom and two-photon detachment of the ${\mathrm{F}}^{\mathrm{-}}$ ion are calculated for photon energies below the thresholds for single-photon ionization and single-photon detachment, respectively. Effects of electron correlations are included by perturbative methods. Good agreement is obtained between our results and those of a recent experimental angular-distribution measurement of two-photon detachment of the ${\mathrm{F}}^{\mathrm{-}}$ ion at λ=532 nm [C. Blondel, M. Crance, C. Delsart, and A. Giraud (unpublished)].

Assuming the approximation of pair independence, the number of pairs produced by a classical, imperturbable electromagnetic source is shown to be described by a Poisson distribution. The perturbational calculation of pairs by colliding heavy-ion beams is treated by straightforward summation of the diagrammatically defined series. The extended calculation, in which all orders of interaction between colliding ions and the electron-positron system are included, is also demonstrated to result in a Poisson-number distribution. Applications to current heavy-ion colliders are made.

With the advent of laser pulses of ultrashort duration, the possibility of exploring the details of atomic collision phenomena has become a reality. However, the estimate of the duration of pulse that is expected to produce new effects is made difficult as it crucially depends on the details of the interatomic potential, the collision time, as well as the characteristics of the wave packets describing the atoms. Following the effective potential approach developed by Basu and Sengupta (unpublished) to study the wave-packet propagation in a potential field, we have evaluated the trajectories and the pulse duration necessary for different wave packets. The method has been applied to the investigation of two atomic systems, one of which has also been studied by others. The results obtained for the Na-Ar system are found to compare favorably with those calculated by the split operator method. Also, the present method has the advantage of being direct and simple, and the error may easily be estimated. A comparison of the two methods is also given in the Appendix. Finally, the present study also predicts for the Ne-Ar system that the expected new effects may be observed with the available experimental facility.

Metastable molecular hydrogen in the ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{\mathrm{-}}{}_{}{}^{}$(v=5, N=1) state is excited by absorption of a photon to the ${\mathit{i}}^3$${\mathrm{\Pi }}_{\mathit{g}}$(v’=5, N’=1, N’=2) quasibound levels. Dissociation of the excited molecules occurs by tunneling through the barrier in the electronic potential of the ${\mathit{i}}^3$${\mathrm{\Pi }}_{\mathit{g}}$ state. The quasibound levels appear as partially overlapping resonances in the photodissociation cross section. The cross section is determined as function of both photon energy and dissociation angle. The coherence of the excitation process is observed experimentally as a change of the photofragment angular distribution as a function of the photon energy. Ab initio calculations qualitatively reproduce the findings. Possible causes of quantative differences are discussed.

A recent technique, based on differential subshell ionization by tuned synchrotron radiation, has been applied to measurements of L-subshell Coster-Kronig yields of Ni, Cu, and Mo from ${\mathit{L}}_2$ and ${\mathit{L}}_3$ Auger spectra. Results for Ni are ${\mathit{f}}_{23}$=0.6+0.2, ${\mathit{f}}_{12}$=0.4+0.2, and ${\mathit{f}}_{13}$=0.5+0.2; for Cu, ${\mathit{f}}_{23}$=0.8+0.1, ${\mathit{f}}_{12}$=0.44+0.06, and ${\mathit{f}}_{13}$=0.3+0.2; for Mo, ${\mathit{f}}_{23}$=0.15+0.02, ${\mathit{f}}_{12}$=0.15+0.02, and ${\mathit{f}}_{13}$=0.61+0.06. Measured transition probabilities are compared with previously available information. The results are generally consistent with the body of theoretical and experimental data. For Ni and Cu, the present measurements of ${\mathit{f}}_{23}$ confirm that the ${\mathit{L}}_2$-${\mathit{L}}_3$${\mathit{M}}_{4,5}$ Coster-Kronig channel is accessible in the metals, even though it is energetically cut off in free atoms. Further improvements in synchrotron-radiation sources are likely to make it possible to throw light on several critical questions by means of the present method.

An alternative approach is presented to the problem of computing atomic and molecular photoionization cross sections by use of basis sets. The present method is based on a simple integral equation that relates the real polarizability on the imaginary frequency axis to the photoionization cross section. Hence the computational work consists in obtaining polarizabilities for imaginary frequencies. Many-body perturbation theory is used for that purpose, and the perturbation expansion is complete to second order in the Coulomb interaction. In that way there is a comprehensive inclusion of correlation effects. Techniques to invert the integral equation to obtain photoionization cross sections from the computed polarizabilities are discussed, and results are given for the atomic systems neon and argon as test cases.

The dependence on the orientation of the intermolecular axis of cross sections for electron capture from molecular hydrogen by bare ions is calculated in a two-center approximation, which ignores multiple scattering between atomic centers and is compared to experiment. Electron-capture cross sections are calculated as functions of both projectile principal quantum number n and molecular orientation angles. Orientation-averaged cross sections are compared with recent measurements of ${\mathrm{O}}^{8+}$+${\mathrm{H}}_2$ at E=8, 10, 12.5, 16, and 20 MeV. Comparisons are also made between our calculations for electron capture into projectile n states by ${\mathrm{H}}^{+}$ from ${\mathrm{H}}_2$ and the available calculation for ns states from E=50 keV to 5 MeV. The orientation-dependence differential in the molecular-orientation angle is studied in dissociative ionization and excitation of the molecular ion following single-electron capture from ${\mathrm{H}}_2$ using a two-step independent-electron model. The calculated orientation dependence is in qualitative agreement with preliminary experimental observations as a function of the orientation angle done with ${\mathrm{O}}^{8+}$ ions at E=10 MeV, where capture to n>1 state is significant.

Theoretical intensity ratios of the Lβ, Lγ, Lη, L${\gamma }_1$, and L${\gamma }_{2,3,6}$ to the Lα line induced by proton impact on gold have been calculated from various L-subshell databases in the proton energy range 0.18–10 MeV. Good agreements are obtained between the measurements and the ratios computed by using the decay yields we present and the ionization cross sections by Cohen and Harrigan [At. Data Nucl. Data. Tables 33, 255 (1985)]. The latter were calculated in the plane-wave Born approximation with corrections for energy loss, Coulomb deflection, perturbed stationary state, and relativistic effects. The L-subshell coupling effects are not found.

Second-order Born cross sections for electron capture in relativistic collisions have been calculated without further approximations for a large number of projectile-target combinations and energies. Within the regime of high-Z target atoms and energies of a few GeV/u, the exactly evaluated cross sections are considerably larger than first-order cross sections, which already overestimate the experimental results. This shows that the Born expansion is not a suitable perturbation series in the charge and energy regime considered. At the same time it is demonstrated that previous evaluations using peaking approximations and αZ expansions are in error by more than an order of magnitude. For p+H collisions the differential cross section exhibits a peak at the classical relativistic Thomas angle.

Relativistic K-LL Auger transition rates are presented for nine ions in the neon isoelectronic sequence up to uranium. We employ the intermediate-coupling scheme including interchannel interactions. For neutral neon a comparison with experimental data is given. We demonstrate that intercontinuum interactions result in a remarkable redistribution of individual transition rates even in highly energetic transitions.