The mutlichannel Schwinger variational formalism in the momentum space has been used to investigate elastic scattering and the ground-state positronium formation process in the positron-hydrogen collisions in the Ore gap region, 6.8–10.2 eV. The s-wave results obtained by employing a correlated discrete basis set are found to be in agreement with existing accurate Kohn variational results of Humberston [Can. J. Phys. 60, 591 (1982)].

A method for computing the overlap integral of two Slater orbitals of arbitrary l and integer n≥l on arbitrary centers is developed. The expressions permit the elimination in a simple way of the numerical difficulties that arise when nonlinear parameters are close together. The complexity of a calculation using the results is discussed and compared with other approaches. Numerical examples and qualitative estimates are given to indicate the accuracy of the method.

The absorption oscillator strengths for a number of transitions among LS states and the fine-structure levels calculated using extensive configuration interaction in the wave functions are reported. The relativistic results are obtained adopting an intermediate-coupling scheme through the use of the terms of the Breit-Pauli Hamiltonian. In general, there is very good agreement between length and velocity forms of the oscillator strengths and the energies obtained using these wave functions compare very well with the observed values. The comparison of the oscillator strengths calculated in the present work with the other available calculations shows good agreement. Also reported are accurate results for many astrophysically important transitions involving quintet states. It is noted that spin-spin and spin-other-orbit terms of the Breit-Pauli Hamiltonian have considerable effect on the f values for the intercombination transitions.

The Feshbach projection method and the generalized saddle-point technique have been used to obtain energies of ${}_{}{}^{4,2}{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ and ${}_{}{}^{4,2}{}_{}{}^{}$${\mathit{D}}^{\mathit{o}}$ Feshbach resonances in ${\mathrm{He}}^{\mathrm{-}}$, Li, and ${\mathrm{Be}}^{+}$. The results from both methods confirm each other. Positions and Auger energies of 31 resonances (3 in ${\mathrm{He}}^{\mathrm{-}}$, 14 in Li, and 14 in ${\mathrm{Be}}^{+}$) lying between the 2 ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and 3 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ thresholds are given. These data can serve as a basis for identification of those resonances in future experiments. Some suggestions for such experiments are made.

Doubly excited ${}_{}{}^{1,3}{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ resonant states in positronium ions ${\mathrm{Ps}}^{\mathrm{-}}$ are calculated by using a method of complex-coordinate rotation. Resonance parameters (both resonance positions and widths) for doubly excited states associated with the n=2 and 3 thresholds of positronium atoms are evaluated using elaborate Hylleraas-type functions. In addition to Feshbach-type resonances lying below the Ps thresholds, we have also identified two ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ shape resonances lying above the n=3 Ps threshold.

The atomic effects on the β-decay ft value of tritium are discussed for three atomic systems: the tritium atom and the ions ${\mathrm{T}}^{+}$ and ${\mathrm{T}}^{\mathrm{-}}$. The effect of exchange between bound and continuum electrons in the decay of the tritium atom is shown to lead to an increase in the phase-space integral f by 0.15% relative to the f value for ${\mathrm{T}}^{+}$. This corrects an error in earlier work which suggested that the inclusion of exchange effects led to a decrease in the f value for this decay. Differences in atomic-electron screening between the T atom and the ${\mathrm{T}}^{+}$ ion are shown to lead to a reduction in f for the T atom by 0.41%, significantly larger than previous estimates of screening effects on f. The combined atomic effects are shown to lead to a difference in f between ${\mathrm{T}}^{+}$ and the T atom of 0.30%. The effect of these changes in f on a recent evaluation of the ratio of the weak-interaction coupling constants, ${\mathit{G}}_{\mathit{A}}$/${\mathit{G}}_{\mathit{V}}$, is discussed.

The first-order functional-sensitivity densities δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{\mathit{i}\mathit{j}}$(R) from close-coupling calculations are used for a quantitative probe of the role of structure in crossing diabatic curves used to model nonadiabatic collisions. Application to the excitation of Ne by ${\mathrm{He}}^{+}$ shows a region of significance for δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{12}$(R) as a prominent Gaussian-like profile around the crossing point (${\mathit{R}}^{\mathrm{*}}$) in accord with the δ(R-${\mathit{R}}^{\mathrm{*}}$) idealization of the Landau-Zener-Stueckelberg (LZS) theory. Similarly, the densities δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{11}$(R) and δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{22}$(R) mimic dδ(R-${\mathit{R}}^{\mathrm{*}}$)/dR-type behavior with one being the negative of the other in the neighborhood of ${\mathit{R}}^{\mathrm{*}}$, in qualitative agreement with the LZS theory. However, all three sensitivity profiles identify a much broader area of importance for the curves than the loosely defined avoided-crossing region. Also, although the sensitivities themselves decrease with increasing energy, the domain of importance of the curves increases. Examination of the functional-sensitivity densities δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{\mathit{i}\mathit{j}}$(R) for the chemi-ionization collision Na+I→${\mathrm{Na}}^{+}$+${\mathrm{I}}^{\mathrm{-}}$ reveals regions of potential-function importance very different from that predicted by the LZS theory.

The chemi-ionization cross section is about ten times more sensitive to the ionic curve than the covalent curve. Also, the domain of sensitivity of the ionic curve is larger compared to that of the covalent curve. The density δ${\mathrm{\sigma }}_{12}$(E)/δ${\mathit{V}}_{12}$(R) for chemi-ionization shows that the area of maximum potential significance is not at the crossing point itself but the regions bracketing it on both sides. Also, the dominant sign dependence of the coupling sensitivity is unexpectedly negative. The results offer other observations about the domain of validity of the intuitive pictures rooted in the LZS theory. The significance of these results to the inversion of inelastic cross-section data is briefly discussed.

This paper considers the collisional excitation of O${(}^1$D) modeled by the crossing of two valence 1 ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ curves dissociating to O${(}^3$P)+O${(}^3$P) [${\mathit{V}}_{11}$(R)] and O${(}^3$P)+O${(}^1$D) [${\mathit{V}}_{22}$(R)] which in turn are further crossed by the ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{g}}^{}{}_{}{}^{}$ Rydberg curve dissociating to O${(}^3$P)+O${(}^5$S) [${\mathit{V}}_{33}$(R)]. The role of structure in the potential curves and coupling matrix elements is quantitatively probed by the first-order functional-sensitivity densities δ ln${\mathrm{\sigma }}_{12}$(E)/δ ln${\mathit{V}}_{\mathit{i}\mathit{j}}$(R) of the excitation cross section ${\mathrm{\sigma }}_{12}$(E) obtained from close-coupling calculations. The results reveal that, in spite of the well-separated nature of the crossing between the two valence curves from their crossings with the Rydberg potential curve, the excitation cross section ${\mathrm{\sigma }}_{12}$ displays considerable sensitivity to the Rydberg curve ${\mathit{V}}_{33}$(R) at all energies in the range 3.0–9.0 eV. For relative collisional energies corresponding to the higher closely spaced vibrational energy levels of the Rydberg state, the excitation cross section is found to be much more sensitive to the Rydberg state than to the two valence states themselves.

At all energies, the sensitivity of the excitation cross section ${\mathrm{\sigma }}_{12}$ to the coupling ${\mathit{V}}_{12}$(R) between the valence states is much larger than the sensitivity to the couplings ${\mathit{V}}_{13}$(R) or ${\mathit{V}}_{23}$(R) with the Rydberg state. At higher energies, the large increase in the sensitivity of the cross section to the Rydberg potential is mirrored by a similar increase in sensitivity to its coupling ${\mathit{V}}_{23}$(R) with the upper valence state. Due to the weak coupling between the three curves, a qualitative similarity exists between the sensitivity profiles and those predicted by the Landau-Zener-Stueckelberg (LZS) theory. Quantitative departures witnessed in earlier work are, however, more pronounced for the multilevel curve crossings investigated here. Implications of the results for attempts to extend the LZS-type treatment to multilevel curve crossings and for functional-sensitivity-based algorithms for the inversion of cross-section data are discussed.

Electron-excitation cross sections are reported for the 3s ${}_{}{}^2{}_{}{}^{}$S→3p ${}_{}{}^2{}_{}{}^{}$P(h,k) resonance transition in ${\mathrm{Mg}}^{+}$ at energies from threshold (4.43 eV) to approximately 9 times threshold (40.0 eV). The electron-energy-loss merged-beams technique used in these measurements is described in detail. In addition, the method of separating contributions of the elastically scattered (Coulomb) and the inelastically scattered electrons in the present ${\mathrm{Mg}}^{+}$ case and previously reported ${\mathrm{Zn}}^{+}$ results [Phys. Rev. Lett. 67, 30 (1991)] is described. Comparisons in the experimental energy range are made for ${\mathrm{Mg}}^{+}$ with the two five-state close-coupling theoretical calculations carried out herein, and with other published close-coupling, distorted-wave, and semiempirical calculations. The present ${\mathrm{Mg}}^{+}$ cross sections and ${\mathrm{Zn}}^{+}$ cross sections from earlier measurements are tabulated.

Photodetachment from negative ions initially bound in s states has been studied in a static electric field. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam direction. A pulsed dye-laser beam, perpendicular to the ion beam, photodetached the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms with a gated channeltron detector. Polarization-dependent electric-field oscillations were oberved on the p-wave detachment cross section of ${\mathrm{Au}}^{\mathrm{-}}$ near the threshold at 18 620.2 ${\mathrm{cm}}^{\mathrm{-}1}$. A simple theory for calculating the oscillatory structure is presented. Good agreement is found between the data and the theory, for laser polarizations both parallel to and perpendicular to the static electric field.

The potential-energy curves of the low-lying ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }$ states of the cation ${\mathrm{NHe}}^{2+}$ are calculated and it is shown that the lowest state of each symmetry has a potential well inside a repulsive Coulomb barrier that contains several long-lived vibrational levels. Estimates are made of the cross sections of the charge-transfer processes ${\mathrm{N}}^{2+}$+He→${\mathrm{N}}^{+}$+${\mathrm{He}}^{+}$ which occur through transitions at avoided crossings of the molecular states.

We calculated differential cross sections for excitation of the 3s ${}_{}{}^5{}_{}{}^{}$S°, 3p ${}_{}{}^5{}_{}{}^{}$P, 3d ${}_{}{}^5{}_{}{}^{}$D°, 3d ${}_{}{}^3{}_{}{}^{}$D°, and 3s’ ${}_{}{}^3{}_{}{}^{}$D° states of atomic oxygen by electron impact using the R-matrix method in the energy region from 13.9 to 50 eV. We included twelve LS states (2${\mathit{p}}^4$ ${}_{}{}^3{}_{}{}^{}$P, 3s ${}_{}{}^{5,3}{}_{}{}^{}$S°, 3p ${}_{}{}^{5,3}{}_{}{}^{}$P, 4s ${}_{}{}^{5,3}{}_{}{}^{}$S°, 3d ${}_{}{}^{5,3}{}_{}{}^{}$D°, 4p ${}_{}{}^{5,3}{}_{}{}^{}$P, and 3s’ ${}_{}{}^3{}_{}{}^{}$D°) in the R-matrix expansion. The calculated results are generally in good agreement with the available experiments, except for the ${}_{}{}^3{}_{}{}^{}$P–3s ${}_{}{}^5{}_{}{}^{}$S° and ${}_{}{}^3{}_{}{}^{}$P–3d ${}_{}{}^3{}_{}{}^{}$D° transitions at 30 eV where calculated values are larger than the measurement.

We present a completely ${\mathit{L}}^2$ integrable method to evaluate continuum states in multichannel photoionization. The Feshbach theory is used to separate the resonant and nonresonant components of the wave function. The nonresonant continuum is built from discretized uncoupled channels, using a formal scattering theory. In practice, the coupling between channels is introduced in a purely algebraic way, by simply solving a system of linear equations. The advantage of the present approach is that it can be used irrespective of the interaction strength between the uncoupled states. We have applied the method to study photoionization of helium between the N=3 and 4 thresholds. The results are in good agreement with the existing experimental data.

The translational-energy-spectroscopy technique has been used to measure the apparent ionization-excitation functions for the metastable states 3 ${}_{}{}^4{}_{}{}^{}\mathrm{D}$, 3d ${}_{}{}^4{}_{}{}^{}\mathrm{F}$, 3d ${}_{}{}^2{}_{}{}^{}\mathrm{F}$, and 3d’ ${}_{}{}^2{}_{}{}^{}$F of an ${\mathrm{Ar}}^{+}$ ion beam extracted from an electron-impact ion source. The initial states were identified from the measured energy loss in the translational-energy spectra for single-electron stripping from ground-state and metastable ${\mathrm{Ar}}^{+}$ ions in collisions with ${\mathrm{O}}_2$. This technique provides an alternative method of selectivity in the study of electron-impact ionization and excitation processes.

In this paper we present a calculation of intensities and polarization of the fluorescence from the Na two ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ and ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ states, at λ=589.16 and 589.76 nm, respectively, produced by photodissociation of the Na-Ar van der Waals molecule. We have used a coupled-channel wave-packet method that includes all the relevant interactions and coherence between the collision channels [J. Chem. Phys. 95, 8124 (1991)]. We show that it is possible to obtain distinct insights into the dynamics of fine-structure-changing processes via such half-collision studies.

In this paper, we undertake a feasibility study of improving the one-and-a-half-centered expansion (OHCE) method of Reading, Ford, and Becker [J. Phys. B 14, 1995 (1981); 15, 3257 (1982)]. We have explored the efficacy of an alternative method to evaluate the charge-transfer matrix elements and improved the estimated time dependence of the charge-transfer scattering amplitudes. More projectile states have been included in the calculations than used hitherto. A unitary matrix, U matrix, which can propagate the wave functions from -∞ to t, where t denotes time, has been constructed using the single-centered expansion (SCE) method. A complex basis set of nine radial s states and nine radial p states has been used in the expansion of trial wave functions for the target. Charge-transfer matrix elements have been evaluated by a Feynman integral technique; one numerical integral using Gaussian quadrature is needed. The radial parts of the matrix elements are stored on circles and used for all the impact parameters. In a OHCE calculation, we have to choose a function ${\mathrm{\beta }}_{\mathit{m}}$(z) to modulate the charge-transfer amplitudes. The only constraints on ${\mathrm{\beta }}_{\mathit{m}}$(z) are ${\mathrm{\beta }}_{\mathit{m}}$(-∞)=0 and ${\mathrm{\beta }}_{\mathit{m}}$(∞)=1. In this paper, ${\mathrm{\beta }}_{\mathit{m}}$(z) has been obtained from a SCE calculation. This ${\mathrm{\beta }}_{\mathit{m}}$(z) function increases gradually in the whole collision region.

It offers an improvement over the step function used in previous work. A computer code has been developed to include s and p states for the target and projectile. The calculations have been performed in the proton energy range from 30 to 250 keV. The charge transfer to the 1s state has been calculated and gives good agreement with the experimental data. The proton energy ranges have been extended from the 100 keV used in previous work to 250 keV. The charge-transfer cross sections to the 2p state fit the experimental data at 30 keV and are almost the same as those calculated using the four-state, two-centered expansion method proposed by Cheshire and Gallaher [J. Phys. B 3, 813 (1970)] and Shakeshaft [Phys. Rev. A 14, 1626 (1976)]. The results of the charge exchange to the 2s state are also in fairly good agreement with the measurements of Ryding [listed in Tawara, Kato, and Nakar, At. Data Nucl. Data Tables 32, 235 (1985)].

Relativistic total and single-differential cross sections of positron-impact ionization are calculated for the hydrogen atom and hydrogenlike ions for incident positron energies up to 10 times the ionization energy. Two different sets of distorting potentials are used to study the mutual screening of the outgoing positron and electron. Relativistic effects are investigated by taking the nonrelativistic limit. Results are compared with available theoretical and experimental data.

Infrared absorption spectra from metastable helium atoms (2s ${}_{}{}^3{}_{}{}^{}$S, 2s ${}_{}{}^1{}_{}{}^{}$S) and molecules (a ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$) were previously acquired by irradiating dense helium gas near 4.2 K with a pulsed proton beam [R. L. Brooks, J. L. Hunt, and D. W. Tokaryk, J. Chem. Phys. 91, 7408 (1989)]. The molecular spectrum was unusual because the observed rovibrational distribution within the a ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ state was far from thermal equilibrium. Three different rovibrational groups were observed: (i) v=0, N=1 (‘‘thermal’’ molecules); (ii) v=0, 9≤N≤21 (rotationally excited molecules); and (iii) 10≤v≤12, N=1 (vibrationally excited molecules). In this work, the time evolutions of members of these three molecular populations were studied both during irradiation and in the subsequent afterglow. In addition, the evolution of the 2s ${}_{}{}^3{}_{}{}^{}$S–2p ${}_{}{}^3{}_{}{}^{}$P atomic line was investigated. This study quantitatively explores the reaction dynamics of the metastable molecule in the gas phase near 4.2 K. Gas pressures between 100 and 750 Torr were used. Time-resolved data were taken with a transient-digitizer system and summed for several thousand cycles of the pulsed proton beam. The absorption measurements were converted to time-resolved number densities with the aid of theoretical transition moments. The analysis required that the data be fit to the solutions of sets of coupled differential equations with a nonlinear least-squares-fit routine. The results provide insight into the complicated reactions involved in generating the unusual molecular distribution and into the reactions between the metastable molecules, metastable atoms, and the background helium gas.

Doubly differential cross sections, in angle and energy, for state-selective single-electron capture from He and Ar by ${\mathrm{Ar}}^{\mathit{q}+}$ ions have been studied experimentally at laboratory impact energies between 25q and 100q eV (q=4 and 5, where q is the ion charge state) and scattering angles between 0° and 5°, by means of translational energy-gain spectroscopy. The translational energy-gain spectra show that only a few final states were selectively populated depending on the charge state of the projectile, ionization potential of the target, and weakly on the impact energy. A resonable description of the populated final states is described qualitatively in terms of the reaction windows, which are calculated using a multichannel Landau-Zener model. The general features of the differential cross sections are discussed in terms of a semiclassical curve-crossing model.

A general expression for predicting vibrational profiles of a molecular Auger spectrum is derived. The relationship between a ‘‘theoretical’’ cross section and an experimental spectrum is discussed and specific procedures for implementing cross-section calculations in the Born-Oppenheimer approximation are proposed. The carbon and oxygen K-LL Auger spectra of CO are reproduced on the entire energy range of interest and specific spectral regions are analyzed in greater detail. The results are compared with experimental data.