Symmetry-resolved photoabsorption spectra measured at the C and O K edges of CO have demonstrated the symmetry decomposition of the conventional photoabsorption spectra. These spectra have been compared with theoretical calculations in the K-shell ionization continua. Definite assignments for the symmetries have been given in the photon-energy regions below and above the K-shell ionization thresholds.

X-ray photoionization of hydrogen in the presence of a low-frequency laser field is studied using an approach which partially takes into account the radiative stimulated corrections to both bound and continuum states. Special attention is paid to the cases in which one low-frequency photon is exchanged (absorbed or emitted) between the atomic system and the laser field. It is demonstrated that atomic ‘‘dressing’’ effects are important close to the ionization threshold.

Formulas for the first-order corrections in α to the dipole form for the angular distribution for atomic photoionization are derived within a nonrelativistic central-potential model. The results are expressed in terms of two parameters γ and δ and simple expressions for the angular distribution for unpolarized, polarized, and partially polarized light are given. Calculations for a number of rare-gas subshells are presented and compared with previous calculations.

The recently developed Residuum method [Tal-Ezer, Kosloff, and Cerjan, J. Comput. Phys. 100, 179 (1992)], a Krylov subspace technique with variable time-step integration for the solution of the time-dependent Schrödinger equation, is applied to the frequently used soft Coulomb potential in an intense laser field. This one-dimensional potential has asymptotic Coulomb dependence with a ‘‘softened’’ singularity at the origin; thus it models more realistic phenomena. Two of the more important quantities usually calculated in this idealized system are the photoelectron and harmonic photon generation spectra. These quantities are shown to be sensitive to the choice of a numerical integration scheme: some spectral features are incorrectly calculated or missing altogether. Furthermore, the Residuum method allows much larger grid spacings for equivalent or higher accuracy in addition to the advantages of variable time stepping. Finally, it is demonstrated that enhanced high-order harmonic generation accompanies intense field stabilization and that preparation of the atom in an intermediate Rydberg state leads to stabilization at much lower laser intensity.

The energy differential cross sections for electron-impact ionization of H- and He-like ions have been calculated using a distorted-wave Born exchange approximation. The calculated results are well fitted to a two-parameter Gaussian function. These results from the fitting, which reproduce the calculated cross sections with an average error of about 4%, are in good agreement with experimental results.

Ionization cross sections differential in the angle and energy of the detected electrons were measured for electron impact on hydrogen gas at 200, 500, 1000, and 1500 eV. Measurements were made from 15°–150° and from a few eV to T-I where T is the primary electron energy and I the ionization potential of hydrogen. The results are compared to earlier measurements and to available calculations.

Electron-impact ionization cross sections have been calculated for ${\mathrm{Kr}}^{24+}$ and ${\mathrm{Kr}}^{25+}$. The contributions of excitation-autoionization and resonant excitation double autoionization (REDA) were included in addition to the cross sections for direct ionization of a 3s electron. For ${\mathrm{Xe}}^{43+}$ the REDA contributions were extended to two times the ionization threshold. The calculations were performed using the relativistic distorted-wave and the multiconfiguration Dirac-Fock methods. For ${\mathrm{Kr}}^{25+}$ the indirect processes enhance the ionization cross section by a factor of 4. For ${\mathrm{Kr}}^{24+}$ the total cross section is about 3.5 times the direct-ionization cross section. The REDA process produces numerous strong resonances and contributes about 20% to the average ionization cross section for ${\mathrm{Kr}}^{24+}$ and ${\mathrm{Kr}}^{25+}$ and 15% to the average ionization cross section for ${\mathrm{Xe}}^{43+}$.

An extensive study of the photoionization of metastable He(2 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$) between the N=2 and N=3 thresholds is presented in this paper. We have calculated the partial 1sεp, 2sεp, 2pεs, 2pεd, and total photoionization cross sections, as well as the β asymmetry parameters that permit one to obtain the differential cross sections. A complete set of resonance parameters for the first twelve ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ doubly excited states is provided. In particular, the partial widths of these resonances are presented. Our results show that this photoionization process leaves preferentially the ${\mathrm{He}}^{+}$ ion in a 2s state and that the resonance structure is much more pronounced than in photoionization from the ground state. Also, the photoelectrons are ejected preferentially in the direction of the polarization axis of the incident radiation field. Electron-correlation properties of the doubly excited states responsible for the resonant peaks observed in the spectra are discussed in detail.

The rate coefficient for the laser-induced associative-ionization reaction of ultracold Na in an optical trap is calculated as a function of the trap laser detuning from the Na${(}^2$${\mathit{S}}_{1/2}$,F=2)→Na${(}^2$${\mathit{P}}_{3/2}$,F=3) resonance. Aided by a knowledge of the excited-state potential-energy curves and of the nature of the free-atom optical-pumping process, we propose the following mechanism, which we call photoassociative ionization (PAI): 2Na${(}^3$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$)${\to }^{\mathrm{ħ}\mathrm{\omega }}$${\mathrm{Na}}_2^{\mathrm{*}}$($0_{\mathit{g}}^{\mathrm{-}}$ and $1_{\mathit{g}}$)${\to }^{\mathrm{ħ}\mathrm{\omega }}$${\mathrm{Na}}_2^{\mathrm{*}\mathrm{*}}$($1_{\mathit{u}}$)→${\mathrm{Na}}_2^{+}$+${\mathit{e}}^{\mathrm{-}}$. Maxima in the calculated PAI rate coefficient occur at detunings that are simultaneously one-photon resonant with bound levels of the long-range $0_{\mathit{g}}^{\mathrm{-}}$ state and two-photon resonant with the lower rotational levels of a vibrational level lying 9 GHz below the dissociation threshold of the autoionizing $1_{\mathit{u}}$ state. The calculated PAI spectrum (PAI rate coefficient versus trap-laser detuning) displays a series of broad peaks between -0.5 and -4 GHz detuning and a cutoff at -5 GHz detuning, as does the experimentally measured spectrum of Lett et al. [Phys. Rev. Lett. 67, 2139 (1991)].

The broad widths of the peaks in the PAI spectrum is due in part to the orientation averaging of the collision vector with respect to the electric-field vector and to the optical pumping of the ${\mathrm{Na}}_2$→${\mathrm{Na}}_2^{\mathrm{*}}$ rovibronic transition. The calculated PAI rate coefficient at -0.6 GHz detuning is a factor of 4 higher than the experimental value. Fine-structure-changing transitions play a role in the doubly excited states, because of the $1_{\mathit{u}}$ avoided crossing, but not in the intermediate states, because they have gerade symmetry and the only states that have been shown to undergo fine-structure-changing transitions with large probability have ungerade symmetry. Several predictions based on the proposed model and suggestions for future experiments are discussed.

Absolute cross sections for the collisional ionization and the total destruction of hydrogen and helium atoms in low-lying Rydberg states colliding with Ar, Xe, and ${\mathrm{N}}_2$ have been measured over the energy range from 1.0 to 30 keV/amu. The independent-particle model fails to explain the behavior of the Rydberg atom’s outer electron in ionization collisions below 3 keV/amu for He (12≤n≤15) on Xe and ${\mathrm{N}}_2$, but not on Ar. This represents an observation of the model’s failure in collisions of Rydberg atoms with neutral perturbers in this energy range and shows that at low energies, where a molecular picture of the core-target scattering is valid, the Rydberg electron is sensitive to details of the molecular interactions between the target and the Rydberg atom’s core.

Electron-impact cross sections for excitation of the 7 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$, 8 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$, 5 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$, and 6 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ states of rubidium have been measured from threshold to 1000 eV. The optical-excitation-function method has been employed in a crossed atom- and electron-beam apparatus. Relative, total (cascade including) experimental cross sections are made absolute by comparison with the known total cross section of the Rb D1 line. Total excitation cross sections are compared with theoretical calculations employing first Born approximation and theoretical branching ratios. Born cross sections for the 7 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ and 8 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ states are obtained from the literature, while Born cross sections for the 5 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$, 6 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$, and all cascading states are calculated in this paper. At high energies, the measured total ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ state cross sections show 1/E behavior and converge to first Born theory; for E>100 eV, experiment and theory agree within 6.7% for 5 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ and within 3.7% for 6 ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$. The total cross sections for the ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ states do not converge to Born theory even at 1000 eV, and it is shown that this cannot be attributed to cascading. At low energies, ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ state total excitation cross sections have similar shapes with sharply peaked maxima at about 0.9 eV above threshold. After cascading is corrected using first Born theory, estimated experimental cross sections for direct excitation of higher fine-structure states of rubidium are given.

The impact-parameter dependence of electronic screening effects is investigated within the framework of the time-dependent semiclassical perturbation theory concerning excitations. A multipole expansion is used for different screening potentials to determine time-dependent matrix elements which are suitable for first- or higher-order impact-parameter calculations. For numerical matrix elements, the present method provides a complete solution. Analytical formulas for 1s-2s and 1s-2p excitations are obtained by using hydrogenic wave functions. Our integrated cross sections are equivalent to those determined within the framework of the plane-wave Born approximation. It is shown that difficulties may arise in reproducing the present results by means of existing approximate factorization and scaling formulas.

Electron-impact excitation cross sections have been calculated using the impact-parameter method for X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$→B ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ and X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$→C ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$ transitions for both ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ molecules as a function of incident energy and of vibrational quantum number ${\mathit{v}}_{\mathit{i}}$. The results show that the cross sections initially increase monotonically with increasing ${\mathit{v}}_{\mathit{i}}$ and follow the opposite trend for higher ${\mathit{v}}_{\mathit{i}}$ values.

Photodetachment from negative ions in a static electric field has been studied using ${\mathrm{Cl}}^{\mathrm{-}}$ and ${\mathrm{S}}^{\mathrm{-}}$. These experiments were done with greater precision than previous studies and allow a quantitative comparison of data with theory. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam velocity. A pulsed dye-laser beam, perpendicular to the ion beam, photoneutralized the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms. Detachment below threshold and oscillations on the cross section above threshold were observed near the ${\mathrm{S}}^{\mathrm{-}}$ threshold at 16 269.5 ${\mathrm{cm}}^{\mathrm{-}1}$ and near the ${\mathrm{Cl}}^{\mathrm{-}}$ threshold at 29 138.3 ${\mathrm{cm}}^{\mathrm{-}1}$. The phase of the oscillations in the data is in good agreement with predictions for s-wave photodetachment in a static electric field, but the amplitude of the oscillations is found to be slightly reduced.

The probability of Kr 1s photoionization alone and accompanied by 4p, 3d, 3p, 2s, and 2s4p excitations has been calculated as a function of photon energy in the vicinity of the respective thresholds; pertinent energies have been computed including relativistic, quantum-electrodynamic, and relaxation effects. Sharp features from two-electron excitations are expected in absorption spectra only if at least one of the electrons undergoes a transition to a bound state; this becomes less probable in inner shells. An absorption-spectrometry experiment on Kr has been performed with synchrotron radiation; results generally confirm the predictions but also point toward refinements that are required in the theory and the need for improvements in techniques and synchrotron-radiation sources.

Ab initio calculations are performed on the ${\mathit{e}}^{\mathrm{-}\mathrm{-}}$${\mathrm{He}}_2^{+}$ complex using the R-matrix technique. A two-state approximation is utilized in which the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ and the A ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ electronic states of ${\mathrm{He}}_2^{+}$, in the close-coupling expansion, are represented by truncated configuration-interaction (CI) wave functions obtained from multiconfiguration-self-consistent-field–complete-active-space-SCF (MCSCF-CASSCF) calculations. A (4s,2p,2d) Slater basis is used to perform bound-state calculations at SCF, full CI, and MCSCF-CASSCF levels, to gauge the quality of wave functions at various bond separations. Elastic cross sections are presented for selected bond lengths in the range 1.5–4.0${\mathit{a}}_0$, for the ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ total symmetry. ${\mathrm{He}}_2^{\mathrm{*}}$(1${\mathrm{\sigma }}_{\mathit{g}}$1${\mathrm{\sigma }}_{\mathit{u}}^2$npσ and nfσ ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$) resonances are detected at each separation and fitted to Breit-Wigner formulas in order to determine their energies and autoionization widths. In addition, low-lying ${\mathrm{He}}_2^{\mathrm{*}}$(1${\mathrm{\sigma }}_{\mathit{g}}^2$1${\mathrm{\sigma }}_{\mathit{u}}$ns,nd, and ${\mathit{ng}}^3$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$) bound states are also calculated at several geometries.

We have carried out a relative measurement of the photoionization cross section of the 5d6p ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_1$ state of barium from the 6s ionization threshold to 875 ${\mathrm{cm}}^{\mathrm{-}1}$ (∼0.109 eV) above the 5${\mathit{d}}_{5/2}$ ionization threshold. The cross section was measured for two different final-state symmetries by varying the angle between the laser polarization vectors. Both photoionization cross sections contain ‘‘forbidden lines’’ which arise through a breakdown of the selection rules on J and ${\mathit{M}}_{\mathit{J}}$ due to the hyperfine interaction in the odd isotopes of barium. These experimental results are compared with jj-coupled eigenchannel R-matrix calculations and show good agreement. Approximate incorporation of the hyperfine effects into the calculations was necessary for a complete understanding of the measured photoionization cross sections. We found that for the odd isotopes (∼18% of natural barium) the excited 5d6p ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_1$ state was substantially depolarized by the effect of the hyperfine interaction.

Often the inclusion of electron-correlation effects in photoionization calculations is crucial; e.g., calculation of satellite cross sections and asymmetry parameters in core ionization of molecules. We have developed a computational approach to include the relaxation, multiconfiguration, and multichannel effects, in a multichannel configuration-interaction (MCCI) approximation, for the study of molecular photoionization. Results are presented of a detailed MCCI-Schwinger study of the C(1s) core-hole production in CO, including the formation of the lowest two π→${\mathrm{\pi }}^{\mathrm{*}}$ shake-up satellites. It is shown that the multichannel effects are more important in describing the satellites than the core-hole channel.

The Schrödinger equation for three charged particles in the continuum is considered in the region ${\mathrm{\rho }}_{\mathrm{\alpha }}$→∞, ${\mathit{r}}_{\mathrm{\alpha }}$/${\mathrm{\rho }}_{\mathrm{\alpha }}$→0, where ${\mathit{r}}_{\mathrm{\alpha }}$ is the distance between the particles β and γ, and ${\mathrm{\rho }}_{\mathrm{\alpha }}$ denotes the distance between the center of mass of the pair (β,γ) and particle α, α≠β≠γ. The asymptotic Schrödinger equation valid in this domain is found to have at least two types of solutions. The first, exact one which is of the familar product form however, does not connect to the known asymptotic expression for the solution of the original Schrödinger equation in the region where all interparticle distances ${\mathit{r}}_1$, ${\mathit{r}}_2$, and ${\mathit{r}}_3$ go to infinity. Therefore a second type of asymptotic wave function is derived which satisfies the asymptotic Schrödinger equation to leading order. It has a surprisingly simple form, being the product of asymptotic Coulomb distortion factors for the relative motion of the particles within each of the pairs (α,β) and (α,γ), and an ordinary two-particle scattering state for the third pair (β,γ) but belonging to an effective two-particle relative momentum which depends on the relative coordinate ${\mathrm{\rho }}_{\mathrm{\alpha }}$. This constitutes a genuine three-body effect. Based on this result, we present an expression for the asymptotic wave function which is the asymptotic solution of the three-charged-particle Schrödinger equation in all asymptotic regions: where all three interparticle distances are large where it goes over into the standard asymptotic wave function, as well as if only any two interparticle distances, say ${\mathit{r}}_{\mathrm{\beta }}$ and ${\mathit{r}}_{\gamma }$, are large and the third one satisfies the condition ${\mathit{r}}_{\mathrm{\alpha }}$/${\mathrm{\rho }}_{\mathrm{\alpha }}$→0.

The time that a transmitted particle interacts with a tunneling barrier is evaluated in one of the principal existing approaches by considering a small oscillatory perturbation added to the static barrier. The modulation frequency, at which a serious deviation from quasistatic behavior appears, is indicative of the traversal time. This approach is extended here, to a long region including the barrier, and utilizing a modulation potential which is spatially uniform throughout this region, except in the incident region ahead of the actual barrier, where it is made sufficiently smooth to avoid reflections. The resulting traversal time is ħ∂ ln[∂t/∂E]/∂E, where the complex transmission coefficient t(E) is defined as the ratio of the wave function at the far end to that at the point of incidence. This answer applies to arbitrary potentials and to all energies, including both propagation and tunneling in a single framework. This result is closely related to, but not identical to, that obtained by another earlier approach in which the potential is fixed, but the incident amplitude is modulated.