A comparative study of electron degradation spectra and yields for various species in gaseous and solid ${\mathrm{H}}_2$O is carred out by using the rigorous Spencer-Fano theory and the continuous-slowing-down approximation (CSDA). As input we use cross-section data given by Hayashi [in Atomic and Molecular Data for Radiotherapy, Proceedings of an IAEA Advisory Group Meeting, Vienna, June 1988, Report No. IAEA-TECDOC-506 (International Atomic Energy Agency, Vienna, 1989), p. 193] for the gas and by Michaud and Sanche [Phys. Rev. 36, 4672 (1987)] for the solid. Vibrational excitation is the dominant mechanism of the slowing down of the electron in both gas and solid phases at intermediate energies of 8–2 eV. Rotational excitation for the gas and phonon excitation for the solid, which share the same origin of dynamics, are the second important mechanism. The general trends of the electron degradation spectra are similar in the two phases. However, details of the spectra differ notably from one another. Because the energy dependence of some of the cross sections is complex, the CSDA fails to reproduce even a local average of the Spencer-Fano degradation spectrum, and gives yields of various products appreciably different from those evaluated from the Spencer-Fano degradation spectrum.

A simple mathematical technique is introduced to establish the connection between the universality of diatomic potentials and correlation property of the Graves-Parr parameters ${\mathrm{\beta }}_1^2$ (=${\mathit{a}}_1^2$/Δ) and ${\mathrm{\beta }}_2$ (=${\mathit{a}}_2$/Δ) [Phys. Rev. A 31, 1 (1985)]. On the basis of this universal feature of diatomic potentials, an accurate expression of the ground-state reduced potential valid for all diatomic molecules has been obtained.

The K-shell photoionization cross section of argon near the threshold has been obtained with the multiconfiguration Hartree-Fock method for continuum wave functions. The electron correlation and the core-relaxation effects that are very important in this process have been taken into account in the calculation very accurately in the ab initio manner through the configuration-interaction procedure. The present results are compared with the experimental measurements and are found to be in excellent agreement. The two-electron resonances that have been observed immediately above the ionization threshold have been identified and assignments have been made to these based on accurate calculations.

The wave-packet simulation (WPS) method for calculating the time a tunneling particle spends inside a one-dimensional potential barrier is reexamined using the Feynman path-integral technique. Following earlier work by Sokolovski and Baskin [Phys. Rev. A 36, 4604 (1987)], the tunneling (or traversal) time ${\mathit{t}}_{\mathrm{pack}}^{\mathit{T}}$ is defined as a matrix element of a classical nonlocal functional between two states that represent the initial and transmitted wave packets. These states do not lie on the same orbit in Hilbert space; as a result, ${\mathit{t}}_{\mathrm{pack}}^{\mathit{T}}$ is complex-valued. It is shown that Re${\mathit{t}}_{\mathrm{pack}}^{\mathit{T}}$ reduces to the standard WPS result, ${\mathit{t}}_{\mathrm{phase}}^{\mathit{T}}$, for conditions similar to those employed in the conventional WPS analysis. Similarly, Im${\mathit{t}}_{\mathrm{pack}}^{\mathit{T}}$ is shown to contain information about the energy dependence of the transmission probability. Under semiclassical conditions, Im${\mathit{t}}_{\mathrm{pack}}^{\mathit{T}}$ reduces to the well-known Wentzel-Kramers-Brillouin expression for the tunneling time. It is shown there are different definitions for the traversal time of a classical moving object, whose size is comparable to the width of the region of interest. In the quantum case, these different definitions correspond to different ways of analyzing the WPS experiment. The path-integral approach demonstrates that the tunneling-time problem is one of understanding the physical significance of complex-valued off-orbit matrix elements of an operator or functional. The physical content of complex-valued tunneling times is discussed. It is emphasized that the use of complex tunneling times includes real-time approaches as a special case. Nevertheless, there is a limitation in the description of tunneling experiments using tunneling times, whether real or complex. The path-integral approach does not supply a universal traversal time, analogous to a classical time, that can be used in quantum situations. It is demonstrated that the often expressed hope of finding a well-defined and universal real tunneling time is erroneous.

Dielectronic capture processes in highly charged uranium ions are studied in the isolated-resonance, distorted-wave approximation. A fully relativistic Dirac-Fock method is employed to calculate resonant transfer and excitation cross sections for collisions of ${\mathrm{U}}^{90+}$ and ${\mathrm{U}}^{89+}$ with ${\mathrm{H}}_2$ and dielectronic recombination cross sections associated with the 2p→2s stabilizing transition of ${\mathrm{U}}^{89+}$. A semirelativistic method based on the previously developed autostructure code is found to give cross sections in good agreement with the Dirac-Fock method in all cases. When the Dirac-Fock method is further extended to include the effect of the two-body Breit interaction, substantial changes are found in certain low-energy cross sections associated with K-shell resonant transfer and excitation, but little change is found in the Δn=0 dielectronic recombination cross sections.

Atomic collision processes that proceed through electron correlation are examined in the context of the electron-impact ionization of atomic ions. Contributions to the ionization cross section from simultaneous double excitation followed by autoionization are calculated for ${\mathrm{Li}}^{+}$ in a close-coupling approximation. Contributions from simultaneous double excitation with capture followed by simultaneous double autoionization, which proceed through triply excited states, are also calculated for ${\mathrm{Li}}^{+}$ using high-order perturbation theory. Both calculated cross sections are then compared with recent high-resolution crossed-beams experimental measurements.

The doubly excited states of the two-electron atom (or ion) are considered in the case when one electron is excited much more than the other. The characteristic separation of the strongly excited electron from the atomic nucleus significantly exceeds that for the weakly excited electron. The electron-electron interaction can be approximated by the dipole term in its multipole expansion. The inner electron velocity is assumed to be much larger than that of the outer electron, which permits assigning a fixed principal quantum number for the inner electron. The same approach is extended to the general Coulomb three-body problem with the particles of arbitrary mass and charge. For the three-body states with symmetry S and ${\mathit{P}}^{\mathit{e}}$, the quantum problem reduces to the three-term recursion relations, which means that the system is effectively one dimensional. The slow evolution of the classical particle trajectories under the influence of interaction is described. It is shown that, depending on the parameters of the system, two types of librations or rotation can be realized. The semiclassical quantization rules (analogous to the well-known Bohr-Sommerfeld rules) are deduced. For S and ${\mathit{P}}^{\mathit{e}}$ states they differ only by the substitution of a half-integer quantum number by an integer. Particularly simple results are obtained in the harmonic approximation, which is valid in the vicinity of the effective-potential curve extreme.

We have investigated the 6${\mathit{p}}_{1/2,3/2}$np; J=0, 1, and 2 doubly excited, autoionizing series in atomic barium, that is, nine of the ten series of the 6pnp configuration, for n between 10 and 28. Experimental results are analyzed from two complementary points of view. First, we have studied the energy-level structure within a given n multiplet. Second, we have performed simple multichannel-quantum-defect-theory analysis of the 6${\mathit{p}}_{1/2,3/2}$np;J series, for each J value. We find that the jk coupling scheme is the most appropriate for describing the resonance states in the long-range region.

Low-energy electron-hydrogen scattering data are calculated within the quasiparticle approach to Faddeev-type integral equations. The simple phenomenological polarization potentials employed in former applications of this formalism, however, are replaced by the more sophisticated parameter-free Temkin-Lamkin potential. Despite the fact that, in contrast to the former calculations, there is no longer a fitting parameter at our disposal that could be used to optimize the results, the best previous values are reproduced or even slightly improved.

Cross sections for emission of Rydberg transitions in collisions of ${\mathrm{Ar}}^{9+}$ ions with He, Ne, and Ar atoms have been measured at an impact energy of 180 keV. Transitions from levels n=6,...,11 and l=5,...,9 following multielectron capture and cascading processes have been observed in fast ${\mathrm{Ar}}^{7+}$, ${\mathrm{Ar}}^{6+}$, ${\mathrm{Ar}}^{5+}$, and ${\mathrm{Ar}}^{4+}$ ions. Emission of recoil ions, attributed to transitions in Heii, Ne ii–iv, and Arii–iv give an additional proof of the capture of several electrons by the projectile. Doubly excited Rydberg states, also observed in Arviii, are due to a multicapture process. Cross sections for multicapture with stabilization of two electrons in Rydberg states are compared to those determined by coincidence measurements.

We present a study of the 1s2s ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_{1–}$1s2p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{0,2}$ transitions in ${\mathrm{Kr}}^{34+}$ and the 1${\mathit{s}}^2$2s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2–}$1${\mathit{s}}^2$2p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2,3/2}$ transitions in ${\mathrm{Kr}}^{33+}$. Wavelengths of two- and three-electron krypton ions have been accurately measured using the beam-foil technique. The limits of precision are discussed. Our results are compared with Z-expansion and multiconfigurational Dirac-Fock theories taking account of the two-electron quantum electrodynamics (QED) contributions we have calculated for high-Z ions. The agreement is satisfactory for heliumlike ${\mathrm{Kr}}^{35+}$. However, a sizable disagreement is found for the 2 ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{1/2–}^{}{}_{}{}^{}$2p ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{3/2}^{}{}_{}{}^{}$ fine structure of lithiumlike Kr$3^6$+, which could reflect the difficulty of calculating the QED corrections and retarded Breit interactions for three-electron ions.

M-shell x-ray production cross sections in ${}_{73}{}^{}{}_{}{}^{}\mathrm{Ta}$, ${}_{76}{}^{}{}_{}{}^{}\mathrm{Os}$, ${}_{79}{}^{}{}_{}{}^{}\mathrm{Au}$, ${}_{83}{}^{}{}_{}{}^{}\mathrm{Bi}$, and ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}$ bombarded by ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions of energy 0.6–3.0 MeV are reported. The data are compared with the predictions of the semiclassical and the 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 description of the measured cross sections, although a systematical underestimation of the data is observed in the low-velocity region. For tantalum, uncertainties of the available M-shell Coster-Kronig factors and fluorescence yields are indicated, as they have been noted previously for ${\mathit{Z}}_2$≊74 elements, bombarded by protons and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ions [Pajek et al., Phys. Rev. A 42, 261 (1990); 42, 5298 (1990)]. Using average M-shell fluorescence yields ω${\mathrm{¯}}_{\mathit{M}}$, we have obtained the scaled M-shell ionization cross sections, which were highly universal as a function of projectile velocity scaled to the mean M-shell orbital velocity. Finally, comparing our previously measured M x-ray production cross sections for ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions with the present data for ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions—taken at the same velocities—we try to test a description of the Coulomb deflection effect within the ECPSSR theory.

We have solved the collisional-radiative equations for atomic hydrogen and hydrogenlike ions (of nuclear charges 2 and 26) in a plasma and derived their excited-state populations for a wide range of plasma temperatures and densities. The populations of the higher-lying levels are well described by the Saha-Boltzmann equation. We refer to such plasmas as being in partial local thermodynamic equilibrium (LTE), and it is the purpose of this paper to present expressions by which it is possible to predict the lowest principal quantum number that meets the criterion of being within 10% of its Saha-Boltzmann value. We treat the ionizing plasmas, the recombining plasmas, and the plasmas in ionization balance as three separate cases. These criteria are then extended to the ground state to cover plasmas in complete LTE. Finally we discuss briefly the effect of opacity on these results and the extent to which they may be used for non-hydrogen-like systems. Throughout the paper care is taken to present physical pictures to explain the numerical results and comparisons are made with LTE criteria derived using more qualitative arguments by other authors.

We calculate the shifts of the ${\mathrm{Ly}}_{\mathrm{\alpha }}$, ${\mathrm{Ly}}_{\mathrm{\beta }}$, and ${\mathrm{H}}_{\mathrm{\alpha }}$ lines of ${\mathrm{He}}^{+}$ in a plasma, produced by electron scattering from radiating ions. Electron densities in the neighborhood of ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ and plasma temperatures from 1 to 6 eV are considered. The calculation is made in the impact approximation, and is based on a six-state close-coupling computation of the scattering matrices, into which an optical potential has been inserted. Small red shifts of the lines are obtained. The contribution of electron scattering to the linewidths is also considered.

The photoionization cross sections and dynamic polarizabilities for the lithium atom and its positive ion are calculated using a discrete basis set to represent both the bound and the continuum states of these systems. Using this discrete-basis-set representation, we construct an approximation to the complex dynamic polarizability from which both the photoionization cross section and the frequency-dependent polarizability are extracted. The calculated results agree well with previous theoretical calculations but differ with experimental results, in the case of the neutral atom. The reason for the discrepancy observed among all the theoretical results and the experiments, in the case of the lithium atom, can be established in terms of the sum-rule criterion. When examined under this criterion, the experimental values furnish a total oscillator strength to the continuum states that exceeds the limit imposed by the oscillator strength sum rule S(0), suggesting that the experiments should be in error.

A spectroscopic technique for the accurate determination of multipole relaxation rates in ground- or near-ground atomic levels is described. The technique is based on the detection of Zeeman quantum beats in transmission geometry and yields relaxation rates for the destruction of both orientation and alignment. Collisional depolarization cross sections are reported for the 4${\mathit{f}}^6$6${\mathit{s}}^2$ ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_{\mathit{J}}$ (J=1–6) ground-term levels in samarium i by various rare-gas perturbers. For the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_1$ level the ratios of the cross sections for the destruction of orientation and alignment are unusually large (${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$≊1.6 for each of the rare-gas perturbers), whereas for the J=2–6 levels the reverse is true (${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$ in the range 0.39 to 0.46 for argon perturbers). These values of ${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$ are found to be in agreement with the predictions of a weak-collision model in which the collision interaction has an essentially pure quadrupole character. The observed cross sections for the ${}_{}{}^7{}_{}{}^{}$${\mathit{F}}_1$ level appear to be the first experimental realization of the simple theoretical result, ${\mathrm{\sigma }}_1$/${\mathrm{\sigma }}_2$=5/3, initially obtained by Omont [J. Phys. (Paris) 26, 26 (1965)].

We describe strong-field effects in a class of collisional energy-transfer processes occurring among atoms in a gas irradiated by a laser beam. The time evolution of the atomic system is evaluated by solving a system of differential equations, and by using the Magnus-Light expansion of the scattering matrix. Results from the two methods are then compared. The resulting spectrum is shifted in frequency towards the antistatic side, and the line shape becomes less asymmetric. All these theoretical predictions are based on the assumption that the motion of the atoms during the collision can be described by classical mechanics, with no appreciable change of the kinetic energy of the atoms.

Semiuniform off-shell wave functions and T-matrix elements are presented that remain valid even in the limit of equal incoming and outgoing momenta. Numerical comparisons are made with both primitive semiclassical and fully quantal predictions for scattering described by a Hulthen potential.

Stochastic fluctuations of coherent and incoherent components of the resonance fluorescence intensity induced by Wiener-Levy laser phase noise are investigated. Statistical properties of the atomic dipole moment for different values of the laser linewidth and different strength of the driving field are calculated. The stochastic Mollow spectra of atomic dipole fluctuations are derived. It is shown that these spectra exhibit a triplet structure which is purely classical and entirely laser-noise dependent.

Photoexcitation from the lowest metastable level of ${\mathrm{H}}_3$ is used to prepare d-electron Rydberg states with total angular momentum N=1 that are built on ${\mathrm{H}}_3^{+}$ cores with different rotational quantum numbers. These states are then photoexcited to high-n members of f-electron Rydberg series. Excitation is detected by monitoring ${\mathrm{H}}_3^{+}$ ions that are formed in field ionization and autoionization. The photoexcitation of the f-electron series allows a sensitive analysis of the molecular-ion-core configuration of the 3d states. We have analyzed the f-electron Rydberg series converging to the two lowest rotational levels of ${\mathrm{H}}_3^{+}$ and we have determined their quantum defects, ionization potentials, and their excitation intensity.