The behavior of the generalized oscillator strength for the Lyman-Birge-Hopfield transition as a function of the transferred momentum is examined, using the Born-Ochkur-Bonham (BOB) approximation with correlated wave functions. It is shown that the experimental behavior can be well described in the region of small and medium ${\mathit{k}}^2$ values, once orbital relaxation and electronic correlation effects are properly taken into account. For large ${\mathit{k}}^2$ values the calculations deviate from the experiments due to the failure of the BOB approximation to describe this region.

Resonant-transfer and excitation cross sections for ${\mathrm{Ca}}^{\mathit{q}+}$ (q=16–19) in collisions with ${\mathrm{H}}_2$ and He have been calculated in impulse approximation using the multiconfiguration Dirac-Fock method in intermediate coupling with configuration interaction. The predicted charge-state dependence of the cross sections for K-shell excitation agrees well with experimental observations. Including the spin-orbit interaction removes the remaining discrepancies between nonrelativistic Hartree-Fock theory and experiment.

An analytic expression is presented for the off-shell $T$-matrix elements for scattering from the spherical hard-core plus square-well potential. Various analytical properties of the off-shell $T$ matrix, such as symmetry and unitarity, are examined. The typical behavior of the off-shell $T$ matrix is shown numerically. It is found that the behavior strongly depends on the three independent quantities, i.e., the energy and the initial and final momenta, which can be off the energy shell. These results, combined with multiple-scattering theory, can be used to study high-energy many-body collision processes, such as collision-induced dissociation.

Electron-impact excitation of the [3/2${]}_1^0$ 6s state of xenon has been studied at 30 eV and over a range of scattering angles up to 50°. Linear and circular polarization correlation measurements show very good agreement with distorted-wave Born-approximation calculations. For the experimental parameters under consideration, the excitation is demonstrated to be completely coherent. Hyperfine depolarization of the excited-state charge cloud is found to be an important effect.

We consider the hypothesis that the repulsive interaction V in van der Waals–type systems is proportional to the charge-density overlap integral S. This hypothesis is shown to be accurate for interatomic interactions, with a proportionality coefficient V/S∼10 a.u. in most cases. It is related to previous results of Kim, Kim, and Lee [Chem. Phys. Lett. 80, 574 (1971)] and Nyeland and Toennies [Chem. Phys. Lett. 127, 172 (1986)]. The hypothesis is consistent with the effective-medium theory result for immersion of an atom or ${\mathrm{H}}_2$ into jellium. For the interatomic problem, the computed values of the decay lengths (${\gamma }^{\mathrm{-}1}$) of V are simply related to those (${\mathrm{\beta }}^{\mathrm{-}1}$) of the asymptotic atomic charge density. This model provides a theoretical basis for an improved combining rule for the minimum position ${\mathit{R}}_{\mathit{A}\mathit{B}}$ of the total interaction ${\mathit{V}}_{\mathit{t}\mathit{o}\mathit{t}}$ between unlike species. It also explains why this quantity depends systematically on the ionization ratio ${\mathit{I}}_{\mathit{B}}$/${\mathit{I}}_{\mathit{A}}$ of the interacting species.

The system studied is a two-level particle whose translational motion is one dimensional and restricted to the half line (0,∞). The internal degrees of freedom of the particle are weakly coupled to the translational motion via a coupling including both adiabatic and nonadiabatic parts. We explore the idea of describing the interaction times of this scattering system in terms of Hermitian sojourn-time operators. Treating the coupling as a perturbation, we show that the first-order perturbation theory can be consistently formulated in terms of appropriate sojourn-time operators. The mean values of these operators give mean sojourn time (dwell time) of the particle in spatial regions or, more generally, in certain subspaces of the state space of the system. The perturbed S operator and also the effect of the perturbation on the change of observables due to the scattering are, in first-order approximation, fully expressible in terms of the sojourn-time operators. In particular, these operators fully determine the first-order approximation to the inelastic scattering probabilities. The relation to the case of scattering on the whole line (-∞,∞) is briefly discussed.

The n distributions produced by charge transfer of ${\mathrm{Ar}}^{+}$ and ${\mathrm{Na}}^{+}$ ions in a target of Na(nl) Rydberg atoms were extensively measured at intermediate velocities. The 60–2100-eV ions bombarded a laser-excited atomic-beam target. The projectiles were neutralized by capture into Rydberg states of Ar and Na and were analyzed by field ionization in an inhomogeneous-field detector whose response over states and energies was carefully mapped. The choice of initially prepared Na states, 24d, 25s, 28d, 29s, 33d, and 34s, allowed a comparison of l=0 and ≥2 targets at nearly equal binding energies over a range of reduced velocity ṽ=0.187 to 1.95. Capture populates m sublevels broadly, not merely m≊0. Overlapping contributions from adiabatic and diabatic modes of field ionization were accommodated in the analysis, which used a maximum-entropy-principle parametric form to fit the observed final-state distributions. The peak of the distributions, ${\mathit{n}}_{\max }$, shifts upward from a value less than the initial state ${\mathit{n}}_{\mathit{i}}$ to a value one to three units higher than ${\mathit{n}}_{\mathit{i}}$ at ṽ between 0.7 and 0.9 and ultimately shifts downward below ${\mathit{n}}_{\mathit{i}}$ as ṽ is further increased. The distributions become significantly sharper where the maximum upward shift occurs. Two ratios were defined to express the widths of final-state distributions in relative terms, one measuring the spread of orbital kinetic energy and the other the spread of Bohr-orbit velocity. By these ratios a universal behavior over energies, states, and projectile species is observed, and small differences between l=0 and ≥2 targets may be seen. A theoretical understanding of the present results, which span velocities where both molecular and perturbative theories are normally used, will require a quantal formulation that models the free-ranging response that is a hallmark of the high-quantum-number limit.

Translational energy spectra have been measured on ${\mathrm{N}}^{2+}$ product ions resulting from 2-keV charge-stripping collisions between ${\mathrm{N}}^{+}$ ions and He. The projectile ${\mathrm{N}}^{+}$ ion beam is produced under varying ion source conditions so as to contain different fractions of a long-lived metastable component. By measuring the endoergic energy defect for charge stripping from ground-state and excited-state projectiles at different electron energies in the ion source, the quantal identity of the ${\mathrm{N}}^{+}$ metastable state is deduced to be either 2s2${\mathit{p}}^2$2p’ ${}_{}{}^1{}_{}{}^{}$D°, which lies 17.9 eV above the 2${\mathit{s}}^2$2${\mathit{p}}^2$ ${}_{}{}^3{}_{}{}^{}$P ground electronic state, or 2${\mathit{s}}^2$2p3s ${}_{}{}^1{}_{}{}^{}$P°, whose excitation energy is 18.5 eV.

Classical phase-space models of the hydrogen molecule and helium are utilized to study shell and subshell electron-capture processes in collisions of fully stripped ions on charge states from 1 to 15 with ${\mathrm{H}}_2$ and He at intermediate impact energies (20–200 keV/amu). Autoionization after double electron capture is approximated and is found to be an important channel for high projectile charges. The resulting (n,l) distributions for proton impact of He and ${\mathrm{H}}_2$ as well as total capture cross sections are found to be in very good agreement with experimental data. l distributions for high projectile charges are also analyzed for n levels for which autoionization is negligible. It is found that electrons are captured preferentially to large l values.

We report the results of an ab initio treatment of electron-impact excitation cross sections for the process ${\mathit{e}}^{\mathrm{-}}$+${\mathrm{N}}_2^{+}$(X${\mathrm{}}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$)→${\mathit{e}}^{\mathrm{-}}$+${\mathrm{N}}_2^{+}$(B${\mathrm{}}^2$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$) from 5.0 to 40 eV. The dynamically important low-order partial wave components of the scattering amplitude were obtained from multistate close-coupling calculations with correlated target wave functions. The calculations were carried out using a modification of the complex Kohn variational method. The higher-order partial-wave components, which are essential for computing converged cross sections for optically allowed transitions such as this, were efficiently evaluated by an asymptotic Coulomb-Born approximation. Of the widely differing sets of experimental cross sections reported for this process, our calculations are in excellent agreement with the most recent measurements.

M-shell x-ray production cross sections are reported for ${}_{72}{}^{}{}_{}{}^{}\mathrm{Hf}$, ${}_{73}{}^{}{}_{}{}^{}\mathrm{Ta}$, ${}_{74}{}^{}{}_{}{}^{}\mathrm{W}$, ${}_{75}{}^{}{}_{}{}^{}\mathrm{Re}$, ${}_{76}{}^{}{}_{}{}^{}\mathrm{Os}$, ${}_{77}{}^{}{}_{}{}^{}\mathrm{Ir}$, ${}_{78}{}^{}{}_{}{}^{}\mathrm{Pt}$, ${}_{79}{}^{}{}_{}{}^{}\mathrm{Au}$, ${}_{83}{}^{}{}_{}{}^{}\mathrm{Bi}$, and ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}$ bombarded by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ions of energy 0.8–4.0 MeV. The measured cross sections are compared with the predictions of the semiclassical and first-order Born approximations and the calculations of the perturbed-stationary-state (PSS) theory that accounts for energy-loss (E), Coulomb deflection (C), and relativistic (R) effects (ECPSSR). The ECPSSR theory gives the best overall description of the measured data, although systematical discrepancies are found in the low-velocity region. Apart from deficiency of the available M-shell Coster-Kronig factors and fluorescence yields near or above ${\mathit{Z}}_2$=74, where strong ${\mathit{M}}_4$-${\mathit{M}}_5$${\mathit{N}}_{6,7}$ Coster-Kronig transitions become energetically forbidden, the increasing underestimation of the data by the ECPSSR theory with decreasing projectile velocities is genuine. In fact, we have found previously [Pajek et al., Phys. Rev. A 42, 261 (1990)] the same discrepancy for identical target elements bombarded by protons at comparably low velocities.

Total cross sections for state-selective electron capture in collisions between ions and alkali-metal atoms have been calculated by means of a three-body classical-trajectory Monte Carlo (CTMC) method using model potentials to describe the electron–ionic-core interactions. Calculations have been performed for ${\mathrm{Na}}^{+}$-Na(28d) collisions and for ${\mathrm{N}}^{5+}$ and ${\mathrm{Ar}}^{8+}$-Cs(6s) collisions. The collision velocity range corresponds to 0.5≲${\mathit{v}}_{\mathit{p}}$/${\mathit{v}}_{\mathit{e}}$≲2, where ${\mathit{v}}_{\mathit{p}}$ is the projectile velocity in the laboratory frame and ${\mathit{v}}_{\mathit{e}}$ is the initial orbital velocity of the electron bound to the alkali-metal core. In the case of ${\mathrm{Na}}^{+}$+Na(28d) collisions, calculations of the final n,l,m distributions show the importance of the electron-capture cross sections into states with m>1. For the case of multiply charged ion–Cs(6s) collisions, a predominance of electron capture to nearly circular states (large l values) is predicted for cross sections near the maximum of the n distribution. When the ${\mathit{e}}^{\mathrm{-}}$-${\mathrm{Cs}}^{+}$ interaction is described by a realistic model potential, the CTMC calculations are found to be in good agreement with recent measurements of the final n values that are predominantly populated after single-electron capture.

Relativistic-distorted-wave methods have been used to calculate electron-impact-ionization cross sections for ${\mathrm{Xe}}^{43+}$. Cross sections were calculated for direct ionization of the 3s electron and the n=2 inner-shell electrons. Electron-impact-excitation cross sections were calculated for transitions from the n=2 subshells to the n=3 and 4 levels of the ion. Detailed Auger and radiative rates were calculated for the intermediate excited states and these were used to determine the excitation-autoionization and resonant-excitation double-autoionization contributions to the ionization cross section. Our results are in excellent agreement with reported experimental results for incident electron energies between 8 and 15 keV. Recent measurements using the electron-beam ion trap are in qualitative agreement with our results for energies between 4 and 8 keV.

An approximation is derived for the amplitude for spontaneous emission of an arbitrary number of low-frequency photons during the collision of a charged particle with a (short-range) center of force. The result correctly reproduces the first four terms in an expansion of the amplitude in powers of the frequency. The evaluation of the first two terms requires a knowledge of the physical (on-shell) amplitude for radiationless scattering. At this level the approximation represents a generalization of Low’s theorem, reducing to his result for the case of single-photon bremsstrahlung. The next two terms involve as additional input the physical one-photon emission amplitude. The calculation is based on the Bloch-Nordsieck method for summing those contributions arising from the emission of soft photons in either the initial or final state of the scattering process. The derivation of the soft-photon approximation is facilitated by the introduction of a transformation to the length gauge. The possibility of extending this approach to allow for a more general class of scattering systems is briefly discussed.

The photoionization cross sections for double ionization of Ne by single photons have been measured from threshold to 280 eV. The results presented are in considerable variance with other published data. These cross sections are compared with those for ${\mathrm{N}}^{2+}$ and ${\mathrm{O}}^{2+}$. It is found that the ratio of the oscillator strength for double ionization to the total photoionization oscillator strength for each of these atoms is proportional to N-1, where N is the total number of electrons in the atom.

Measurements of the ratio of the rate coefficient for charge transfer between ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ to that for the reverse process have been made in the temperature range 8 K≲T≲80 K. A 1.1-meV difference in the ionization potential of these species leads to an endothermicity in the forward direction of this reaction, and these experiments constitute the only measurements of a threshold resulting from such a small endothermicity. In addition, a limited number of direct measurements of the rate coefficients for these processes have been made and are included here. The measurements were done in a cooled Penning trap using Fourier transform spectroscopy of the axial trapping frequency. The measurements are compared to theoretical estimates.

Measurements of the differential and total cross sections for production of free electrons are reported for collisions of 66.7–350-keV/u ${\mathrm{C}}^{+}$ ions with He. The experimental data are compared with theoretical calculations that include electron emission from both target and projectile using the classical-trajectory Monte Carlo method and the independent-electron approximation. Good agreement is observed between theory and experiment, and structures appearing in the electron spectra can be explained in terms of either, or both, target and projectile ionization.

Structures in nonhydrogenic photoionization cross sections for atoms or ions include shape resonances, closely associated with enhancement phenomena in the corresponding electron—residual-ion scattering process, which also depends on the atomic or ionic effective potential, and Cooper minima, associated with the nonhydrogenic shift in the overlap of initial- and final-state wave functions. Shape resonances occur near the energies at which the energy derivatives of the quantum defects are maximum. Here, taking Xe and Fe atoms as examples, we discuss how such nonhydrogenic features vary with increasing degrees of ionization.

We have measured the ${\mathit{X}}^1$${\mathrm{\Sigma }}^{+}$–B ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }$ bound-free molecular photodissociation profile of NaK under molecular-beam conditions. The dissociation results in excited ${\mathrm{K}}^{\mathrm{*}}$(4 ${}_{}{}^2{}_{}{}^{}$P). We have measured the ${\mathit{D}}_1$/${\mathit{D}}_2$ fine-structure branching ratio; this ratio is a strong function of wavelength ranging from near zero to ${\mathit{D}}_1$/${\mathit{D}}_2$≃25% as the laser is tuned through the bound-free absorption profile.

We have applied the Schwinger multichannel formulation [K. Takatsuka and V. McKoy, Phys. Rev. A 24, 2473 (1981)] to the elastic scattering of low-energy (1–30-eV) electrons by silane. Our results, obtained within the fixed-nuclei, static-exchange approximation, are in generally good agreement with recent differential, integral, and momentum-transfer cross-section measurements between 5 and 30 eV. At lower energies, our results reflect the known limitations of the static-exchange approximation. Near the shape resonance, the calculated differential cross section shows significant enhancement at high angles, suggesting an explanation for differences between integral cross sections determined from integrated differential data and those obtained from transmission measurements.