The angular distributions of electrons ejected from collisionally excited ${\mathrm{O}}^{\mathrm{-}}$ ions have been measured in order to test the previous assignments of excited states for this ion. The angular distributions support the interpretation that the two dominant peaks in the electron-energy spectrum represent the autodetachment of the 2${\mathit{p}}^3$${(}^2$D)3${\mathit{s}}^2$ ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}^{\mathit{o}}$ state, decaying to the ${}_{}{}^3{}_{}{}^{}$P and ${}_{}{}^1{}_{}{}^{}$D states of the oxygen ground-state configuration, with an experimental branching ratio of 1.58±0.08, and not that two different autodetaching states are populated, as previously assumed. Theoretical calculations are in good agreement with the experimental findings.

The Livermore electron-beam ion trap has been used to measure cross sections for electron-impact ionization of lithiumlike Ti, V, Cr, Mn, and Fe. The measurements were made using two variations of an x-ray technique that correlates ionization events with dielectronic-recombination photons. The cross sections were determined at one energy corresponding to approximately 2.3 times threshold. Ionization cross sections are measured (i) relative to the theoretical cross section for radiative recombination onto the n=2 shell for lithiumlike ions and (ii) relative to the theoretical cross section for direct excitation of the lithiumlike line q [1s2s${(}^3$S)2p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$→1${\mathit{s}}^2$2s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$]. The uncertainties in the ionization cross sections are estimated to be about 10% from the experiment together with a systematic uncertainty from the choice of normalization, which is 3% for radiative recombination and 10% for direct excitation. Good agreement is found with relativistic distorted-wave calculations.

A single-particle potential for the calculation of photoexcited electron orbitals is introduced. This potential is defined to include exactly all of the first-order electron correlations that appear in the diagrammatic perturbation series of the dipole polarizability and that contain potential corrections in the intermediate state. The angular coefficients that are associated with this potential can be written in the form of an average term plus a correction to the average. It is demonstrated that the correction to the average contribution is dependent only upon the initial-state coupling of the atom. Additionally, a trivial relationship exists between the correction to the average contributions for conjugate initial-state configurations. The angular coefficients are tabulated for all of the ${\mathit{s}}^{\mathit{n}}$, ${\mathit{p}}^{\mathit{n}}$, and ${\mathit{d}}^{\mathit{n}}$ initial-state LS couplings that are possible. Supplementary analytic relationships are also presented.

One of the most interesting, important, and problematic components of interaction potentials for electron-atom and -molecule scattering arises from many-body effects in the near-target region. Such ‘‘core-polarization’’ effects are of particular concern for vibrational-excitation calculations, where these short-range bound-free correlation and nonadiabatic velocity-dependent effects have remained resistant to rigorous treatment, being represented instead by approximations or model potentials. In order to provide guidance for assessing such potentials and insight into the nature of these many-electron effects, we have investigated the sensitivity to core polarization of total, momentum-transfer, rotational-excitation, and vibrational-excitation e-${\mathrm{H}}_2$ cross sections. The sensitivity analysis for the latter cross section also comments on a long-standing, severe discrepancy [most recently documented in S. J. Buckman et al., Phys. Rev. Lett. 65, 3253 (1990)] between cross sections determined in various crossed-beam experiments and by transport analysis of swarm data for this simplest electron–neutral-molecule system.

A detailed Monte Carlo simulation model has been developed to study the absorption of soft x rays and the subsequent behavior of the resulting electrons under the influence of an applied electric field in gaseous xenon. All relevant physical processes are included from the initial photoionization through the subsequent decay of the residual ion to the scattering and drift of the electrons resulting from the interactions with the background gas. Details are provided for the cross sections and decay rates employed as well as the criteria used to terminate the simulation, depending on the information or results required. Examples of its use in modeling gaseous radiation detectors are included.

The J-mixing process of alkali-metal atoms in thermal collisions with noble-gas atoms has been a nemesis due to the fact that the process is a complicated three-body collision process. Since current theoretical methods treat the collision process as a two-body one by ignoring the interaction between the perturber noble-gas atom and the alkali-metal atomic core, their results do not agree with experiments, especially in lower-n states and for heavier noble-gas atoms. Within the framework of an eikonal approximation in this paper, we have presented a theoretical description of the J-mixing process of alkali-metal atoms. Our present work shows that the direct contribution to the process from the interaction between the perturber atom and the alkali-metal atomic core is negligible, but the interaction makes its effect felt within a higher-order term which depends on the perturber-core total cross section ${\mathrm{\sigma }}_{\mathrm{tot}}^{\mathit{A}+\mathrm{-}}$B and the matrix element 〈${\mathit{r}}^{\mathrm{-}2}$${\mathrm{〉}}_{\mathit{n}\mathit{l}}$. The theoretical cross sections obtained for Cs and Rb atoms’ J-mixing processes exhibit a similar dependence on the principal quantum number n as the experimental results, expecially in the lower-n states and for the heavier noble-gas atoms.

Diagonal and real off-diagonal coherence elements of the density matrix for H(n=3) atoms produced in 20–100-keV electron-capture collisions of protons with Ar atoms are experimentally determined. Balmer-α light from the decay of H atoms from the (n=3) state to the (n=2) state is observed. The intensity and polarization of the light as a function of an axially symmetric electric field in the collision region are fitted to a numerical model of the H atom in an electric field in order to extract density-matrix elements. A new polarimeter, using a photoelastic modulator in conjunction with photon-counting techniques, is used in the experiment, and its efficacy is analyzed and compared to that of a rotating quarter-wave plate polarimeter previously used in similar experiments. The diagonal elements of the density matrix yield relative capture cross sections for the H(3l) angular-momentum substates, while the coherence terms are used to determine the dipole moment of the atoms produced. Results are compared to those for protons colliding with a He target and the differences are discussed.

Major features of the projectile-Auger-electron spectrum in the 220–300-eV energy range that are observed following 1-MeV/u collisions of Li-like carbon ions with helium target atoms are shown to be composites of closely spaced Auger lines that result from autoionizing (AI) transitions of excited ${\mathrm{C}}^{3+}$ configurations formed during the collision. From calculations of Auger-electron energies and Auger yields, a simulation of the Auger-electron spectrum is constructed and it is shown that a superposition of AI decays from LS-coupled doublets of the doubly excited ${\mathrm{C}}^{3+}$ 1s2snl configurations with n=2,3,4,...,∞, reproduces most of the observed structure. Remarks are made concerning relative intensities of the Auger peaks with respect to excitation and population probabilities, particularly in the case of the two doublet terms of the 1s2s2p configuration. An Auger peak of ≊265.0 eV in the projectile frame that could not be accounted for in the 1s2snl series is attributed to a possible two-electron process in which one of the projectile electrons is ionized and another is excited.

High-resolution translational energy spectra (0.2 eV at full width at half maximum) have been obtained for single-electron capture by 4-keV ${\mathrm{N}}^{2+}$ ions in collision with He, Ne, and Ar. Seven capture channels have been unambiguously identified for each of the He and Ne spectra, many of which have not been previously reported. This allows clarification of earlier studies. Twelve reaction channels have been identified for capture by ${\mathrm{N}}^{2+}$ in Ar, three of which involve excitation of the target product ${\mathrm{Ar}}^{+}$ ion. All the observed channels are spin conservative. Competing channels which violate spin conservation are seen to be rigorously suppressed.

Time-dependent wave-packet theory [B. Ritchie, Phys. Rev. A 45, R4207 (1992)] is applied to electron-molecule scattering. The Schrödinger equation is solved in time and space on a numerical grid, thus avoiding use of basis-set expansions. A plane-wave-centered, Gaussian wave packet is propagated through the region of the potential. Once the wave packet has passed outside the range of the potential, the differential cross section is calculated directly from the multidimensional wave function by use of the Huygens-Fresnel integral from optics.

By use of a theory due to Gasser, Tavard, and others, the leading first-order corrections to the spherically averaged atomic Compton profiles (CP’s) beyond the impulse approximation, for some inert closed-shell monatomic systems (He, Ne, Ar, and Kr), are obtained. This is accomplished by employing the Kohn-Sham technique of the density-functional theory, which leads to a self-consistent description, as opposed to the popular ‘‘effective-hydrogenic-potential’’ theories used in this context. The resulting corrections to the CP’s thus obtained are seen to compare well with the earlier effective hydrogenic estimates. The orbital corrections are seen to conform to an empirical rule put forth ealier by Gasser and Tavard [Phys. Rev. A 27, 117 (1983)].

The Schwinger variational K matrix is used in the momentum space to calculate D- and higher-partial-wave phase shifts for the elastic scattering of positrons from atomic hydrogen below the pickup threshold in a discrete basis-set expansion, ‖${\mathit{u}}_{\mathit{n}}$〉 =‖[1-exp(-${\mathit{pr}}_1$)](1+${\mathit{r}}_1$/2)exp(i${\mathbf{k}}_{\mathit{i}}$⋅${\mathbf{r}}_1$)${\mathrm{\phi }}_{\mathit{i}}$(${\mathbf{r}}_2$) /(${\mathit{r}}_{12}$+a${)}^{\mathit{n}}$〉 (n=1, . . .N). The values of the Schwinger D-wave phase shifts are found to be in close agreement with the Kohn variational results as for the S- and P-wave phase shifts. To the best of our knowledge, there are no other variational calculations for the higher-partial-wave phase shifts for l≥3. We have made use of these Schwinger variational phase shifts to compute accurate elastic differential and total cross sections at several positron energies. The differential cross section exhibits a sharp minimum at an intermediate scattering angle for incident momenta ${\mathit{k}}_{\mathit{i}}$>0.3 a.u.

We show that the anisotropy of the photon excited Ar 2${\mathit{p}}_{3/2}^{\mathrm{-}1}$ns resonant Auger spectrum evolves smoothly into the anisotropy of the corresponding normal ${\mathit{L}}_3$MM Auger spectrum as the excitation energy is tuned across the ${\mathit{L}}_3$ ionization threshold. Our results show that the difference between the angular anisotropy parameters ${\mathrm{\alpha }}_2$ of the resonant and normal Auger transitions becomes smaller than 10% around n≳14.

Recent work has revealed that the frequency spectrum of the Casimir force is a function with discontinuities and large oscillations. This spectrum is derived from the Lifshitz theory, using a form of the theory that does not assume analyticity of the dielectric function. In the case of a perfect conductor, these oscillations almost exactly cancel. This leads to the possibility that one could upset this cancellation, and have the force between two slabs of material larger in magnitude than the Casimir force and possibly repulsive. In an approximation in which evanescent modes are ignored, it is shown that the force between two slabs may be expressed directly in terms of the reflection coefficients for light to be reflected from a single slab. This allows the force to be computed directly from empirical optical data.

A theory of the photodetachment of hydrogen negative ions in an external static electric field with the production of hydrogen in excited states is developed. We use a combination of the frame-transformation theory and the Green’s-function method. The cross section in nonzero fields is expressed in terms of the photodetachment matrix elements for zero field. The latter are calculated using the adiabatic hyperspherical representation. For energies close to the threshold we use a smooth extrapolation of the zero-field photodetachment matrix elements down to threshold taking care to enforce the proper threshold behavior of cross sections for nonzero fields. The ripples in the total cross section due to interference effects are not noticeable for incident photons linearly polarized along the static field direction, but are relatively well pronounced for the case of circularly polarized photons propagating along the static field. On the other hand, all partial cross sections corresponding to photodetached electrons having magnetic quantum number m’=0 (with respect to a quantization axis parallel to the static electric field) show significant ripple structure regardless of the light polarization. The partial cross section for detachment with excitation of the n=2, m=1 Stark substate by circularly polarized photons exhibits a particularly distinct ripple structure. All cross sections in the region of the shape resonance increase with increasing electric field in accordance with experimental observations. This effect is stronger in the case of linear polarization. We also predict a fairly stable value for the width of the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ shape resonance as a function of the electric-field strength over the range from 0.4 to 1 MV/cm. For electric-field strengths below 0.4 MV/cm, the ripple structure in the shape resonance makes it difficult to define a resonance width.

The semiclassical approximation is applied in second order to describe time ordering of two-step processes in energetic ion-atom collisions. Emphasis is given to the conditions for interferences between first- and second-order terms. In systems with two active electrons, time ordering gives rise to a pair of associated paths involving a second-order process and its time-inverted process. Combining these paths within the independent-particle frozen orbital model, time ordering is lost. It is shown that the loss of time ordering modifies the second-order amplitude so that its ability to interfere with the first-order amplitude is essentially reduced. Time ordering and the capability for interference is regained, as one path is blocked by means of the Pauli exclusion principle. The time-ordering formalism is prepared for papers dealing with collision experiments of single excitation [Stolterfoht et al., following paper, Phys. Rev. A 48, 2986 (1993)] and double excitation [Stolterfoht et al. (unpublished)].

The method of 0° Auger spectroscopy was applied to measure state-selective K-shell excitation in Li-like ${\mathrm{Ne}}^{7+}$ incident with 170 MeV on ${\mathrm{H}}_2$, He, ${\mathrm{CH}}_4$, Ne, and Ar. The high-resolution technique allows for the separate analysis of the states ${}_{\mathit{a}}{}^2{}_{}{}^{}$P and ${}_{\mathit{b}}{}^2{}_{}{}^{}$P whose dominant components are 1s(2s2p ${}_{}{}^3{}_{}{}^{}$P${)}^2$P and 1s(2s2p ${}_{}{}^1{}_{}{}^{}$P${)}^2$P, respectively. The results are used to study time-ordering and interference effects in the process of single excitation. For the state 1s(2s2p ${}_{}{}^3{}_{}{}^{}$P${)}^2$P it is found that the phenomenon of Pauli blocking inhibits the loss of time ordering, whereas dealignment effects diminish interferences between first- and second-order mechanisms. The 1s(2s2p ${}_{}{}^1{}_{}{}^{}$P${)}^2$P state involves partial loss of time ordering; however, non-spin-flip rules recover sufficient time ordering so that interference effects can be observed. The semiclassical approximation is used up to second order to provide conclusive evidence for the measured interference effects.

We have found symmetry relations which are generally applicable to most signals of phase-sensitive magnetic-resonance spectroscopy. One of them is a kind of reciprocal relation connecting the signals before and after magnetic-field reversal, and is based on a symmetry property of a system under an antiunitary transformation: time reversal. The relation tells us that the phase of a signal is closely related to the behavior of the signal when a static magnetic field is reversed. Another is a relation which originates from a symmetry property under a unitary transformation. From this, we can obtain information on a geometrical symmetry of a system. Some phenomena of phase-sensitive magnetic-sublevel spectroscopy are exemplified. The usefulness of these relations is also discussed.

Molecular emission bands accompanying all atomic laser transitions have been observed in a cw He-${\mathrm{Cd}}^{+}$ hollow-cathode laser. They appear as an asymmetrical broadening around the atomic laser transitions, which can extend as far as 90 nm for the case of blue emission. Their dependence on discharge conditions has been studied in depth under lasing conditions, and the results show that they are strongly related to the laser processes. These molecular bands do not originate from ${\mathrm{He}}_2$ and ${\mathrm{Cd}}_2$ dimers, but can be associated with (He-${\mathrm{Cd}}^{+}$${)}^{\mathrm{*}}$ complexes. The observation of molecular emission bands, under lasing conditions, suggests the existence of population mechanisms different from those previously accepted.

The goal of this study is to determine the strength of the dominant helical potential field, i.e., the chiral strength, of mirror-image molecules with a method which we call electron and positron circular dichroism, a matter-wave phenomenon postulated by many [P. S. Farago, J. Phys. B 13, L567 (1980); B. Ritchie, Phys. Rev. A 20, 1915 (1979); R. A. Harris and L. Stodolsky, J. Chem. Phys. 70, 2789 (1978); P. K. Kabir, G. Karl, and E. Obryk, Phys. Rev. D 10, 1471 (1974); V. G. Baryshevskii, Yad. Fiz. 4, 72 (1966) [Sov. J. Nucl. Phys. 4, 51 (1967)]]. Of five liquid chiral molecules, two were found to be left handed, and one was right handed and two poorly defined. The chiral strength varies between 48.9×${10}^{\mathrm{-}4}$ and 6.1×${10}^{\mathrm{-}4}$ eV.