The pressure dependence of the metastable-state lifetimes in antiprotonic ${}^3\mathrm{He}$ atoms $\overline{p}{-}^3{\mathrm{He}}^{+}$ was studied using the recently observed laser resonance transition $(n,\mathcal{l})=(36,33)\mathrm{}\to \mathrm{}(35,32)\mathrm{}$ in the $\upsilon =n-\mathcal{l}-1=2$ cascade. To this end antiprotons from the Low Energy Antiproton Ring (LEAR) at CERN were extracted in approximately 200-ns-long bunches with about ${10}^8\hspace{0.5em}\overline{p}$ per bunch and stopped in a ${}^3\mathrm{He}$ gas target. Time spectra of delayed annihilation products were taken with the help of a Čerenkov counter with a gated photomultiplier. Under our experimental conditions, $p=136\mathrm{}-690\mathrm{mbars}$ and $T=5.8\mathrm{}\mathrm{K},$ the lowest metastable level $\mathrm{}(36,33)\mathrm{}$ was found to be much shorter lived than the corresponding state (37,34) in the $\upsilon =2\mathrm{}$ cascade of $\overline{p}{-}^4{\mathrm{He}}^{+}.$ It was strongly quenched at pressures above 500 mbars, whereas the higher states in the cascade remained nearly unaffected. This enabled us to investigate the $\mathrm{}(37,34)\mathrm{}\to \mathrm{}(36,33)\mathrm{}$ transition by resonant laser deexcitation. Its wavelength was found to be $\lambda =524.155\mathrm{}\pm 0.004\mathrm{nm},$ only 6 ppm lower than the theoretical value given by Korobov and Bakalov [Phys. Rev. Lett. 79, 3379 1997)] after including relativistic corrections.

The need for a reassessment of nuclear magnetic dipole moments is prompted by recent experiments on the ground-state hyperfine structure in highly charged hydrogenlike systems which are sufficiently sensitive to probe QED effects. This work gives an overview of the magnetic dipole moments for the nuclei of interest, i.e., ${}^{165}\mathrm{Ho},$ ${}^{185,187}\mathrm{Re},$ ${}^{203,205}\mathrm{Tl},$ ${}^{207}\mathrm{Pb},$ and ${}^{209}\mathrm{Bi}.$ It is found that the present uncertainties in the nuclear magnetic dipole moment limit the interpretation of the accurate experimental hyperfine structures for these systems.

We have studied the two-center effect (TCE) in ion-atom ionization by measuring the energy and angular distributions of the double-differential cross sections (DDCSs) ${(d}^2\sigma /d\varepsilon d\Omega )$ of the low-energy electrons emitted in a collision of He atoms with 2.5 MeV/nucleon ${\mathrm{C}}^{6+}$ ions. The electrons with energies between 0.1 and 300 eV were detected for 13 different emission angles between $15°$ and $160°.$ From the measured DDCSs we have deduced the single differential cross sections such as $d\sigma /d\Omega$ and $d\sigma /d\varepsilon$ and the total cross section. The data have been compared with the continuum distorted wave eikonal initial state calculations with H-like and Hartree-Fock-Slater wave functions for the initial and final state of the electron and first Born calculations. The forward-backward asymmetry parameter also has been deduced to study the TCE in detail. In the following paper [Lokesh C. Tribedi et al., Phys. Rev. A 58, 3626 (1998)] we have explored the collision dynamics by deducing the momentum distributions of the electron, the recoil ion, and the projectile.

We have used the energy and angular distributions of the low-energy electron emission cross sections from the preceding paper [Lokesh C. Tribedi et al., Phys. Rev. A 58, 3619 (1998)] to derive the doubly differential final-state longitudinal momentum distributions of the electrons, recoil ions, and projectiles in ion-atom ionization for ${\mathrm{C}}^{6+}+\mathrm{H}\mathrm{e}.$ The complementary nature of the electron spectroscopy and the recoil-ion momentum spectroscopy have been investigated using a formulation based on three-body kinematics to explore the ionization dynamics in detail. The influence of the three-body ionization as well as the binary-encounter processes on the recoil-ion (and projectile) longitudinal momentum distributions has been investigated. The separation of the soft- and hard-collision branches of recoil-ion distributions is an important feature of the present technique. The present method also allows one to determine cross sections for very large electron momenta. The single- differential distributions are also derived by numerical integration of the double-differential distributions. The first Born approximation, the continuum distorted wave eikonal initial state, and the classical trajectory Monte Carlo calculations are used to explain the data.

Fast electron velocity spectra have been measured by means of the multidetector ARGOS (actually located at Laboratorio Nazionale del Sud, Catania) in a large angular range for atomic collisions induced by a $77A-\mathrm{MeV}$ ${}^{40}\mathrm{Ar}$ beam on an Al target. The results are in a fair agreement with a recently proposed relativistic billiard ball model and quasielastic scattering approximation calculations. However, the experimental binary-encounter peak velocities are found to be significantly lower than theoretically predicted. The data also show evidence for an excess of fast electrons in the backward direction.

We present a detailed theoretical analysis of the noble-gas nuclear-spin relaxation due to the anisotropic magnetic-dipole hyperfine interaction between the noble-gas nucleus and alkali-metal valence electron vis à vis the already well-understood (spin-conserving) isotropic magnetic-dipole hyperfine interaction in alkali-metal–noble-gas systems. We find that, for all pairs in which the noble gas is not helium, the predicted spin-relaxation rate from the anisotropic interaction does not exceed 2.5% of the rate from the isotropic interaction, thereby not appreciably limiting the maximum noble-gas nuclear polarization attainable via spin-exchange collisions with polarized alkali-metal atoms. For alkali-metal–helium pairs, we predict that the anisotropic interaction has a slightly larger relative effect, perhaps limiting the nuclear polarization to $\sim 95\%$ of the electronic polarization in the Rb-${}^3\mathrm{He}$ system; however, our confidence in the helium results is limited by a lack of knowledge of the interatomic potentials necessary for the calculation.

High-resolution O $1s$ photoelectron spectra of the ${\mathrm{CO}}_2$ molecule have been measured in the region of the resonant features above the O K edge. The cross sections and asymmetry parameters were determined for the vibrational components of the O $1s$ photoelectron line. The vibrational fine structure is completely dominated by the antisymmetric stretching mode, which arises due to vibronic coupling. The O $1s$ line is accompanied by a rich satellite structure. Auger electron spectra have been measured at the energies of the continuum resonances and used as an indirect probe for multielectron excitations. The shoulder on the low-energy side of the $4{\sigma }_u$ shape resonance is due to double excitations that decay via spectator processes.

Using multichannel quantum-defect theoretical methods, we have performed highly correlated $R$-matrix calculations for the photoionization excitation of neon in the (50–53)-eV photon energy range, where numerous doubly excited resonances are found. By incorporating spin-orbit effects through an intermediate-coupling frame transformation, we have reproduced experimental observance of the significant breakdown of $\mathrm{LS}$ coupling in resonant angular distributions and fine-structure partial cross sections. Extreme sensitivity to the target structure in this weak process and strong interactions between neighboring Rydberg series of resonances make a detailed comparison for every resonance difficult, but many of the interesting spin-orbit features, beyond fine-structure splitting of series, are revealed. This method permits a quantitative analysis of $\mathrm{LS}$-forbidden contributions to complex photoionization spectra and should prove useful for studying spin-orbit effects in light atomic systems.

This paper investigates the possibility of suppressing the ionization rate in a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms. Suppression of the ionization is critical for the possibility of reaching Bose-Einstein condensation with such atoms. We estimate the relevant long-range interactions for the system, consisting of electric quadrupole-quadrupole and dipole-induced dipole terms, and develop short-range potentials based on the ${\mathrm{Na}}_2$ singlet and triplet potentials. The autoionization widths of the system are also calculated. With these ingredients we calculate the ionization rate for spin-polarized and for spin-isotropic samples, caused by anisotropy of the long-range interactions. We find that spin polarization may allow for four orders of magnitude suppression of the ionization rate for Ne. The results depend sensitively on a precise knowledge of the interaction potentials, however, pointing out the need for experimental input. The same model gives a suppression ratio close to unity for metastable xenon in accordance with experimental results, due to a much increased anisotropy in this case.

An expression of the density of the generalized oscillator strength (GOS) for electron impact ionization of atomic hydrogen using a classical trajectory to describe the state of the ejected electron is presented with the help of its linear-response representation in the framework of the first Born approximation (FBA). Based on the reproduction of the GOS density of the FBA, the target polarization and the postcollision interaction (PCI) effect are included within the expression. The target polarization expressed by the decreasing of the initial expectation momentum of the atomic electron decreases the GOS density for small energy transfer E and moves the Bethe ridge toward higher momentum transfer K for larger $E.\mathrm{}$ The PCI effect should not be remarkable for the GOS density.

Photoion and photoelectron data for hollow lithium states were recently reported with increased spectral resolution. In order to reproduce and identify the important resonances due to ${2ln}^{\prime }{l}^{\prime }{n}^{″}{l}^{″}$ ${}^2{P}^o$ autoionizing states new photoionization calculations have been performed using the R-matrix code with a 29-term target representation for incident photon energies between 140 and 167 eV. Excellent agreement between theoretical and measured results over partial or total photoionization cross sections confirms the quality of the theoretical model. A determination of the resonance positions for each series converging to a ${2l2l}^{\prime }$ ${}^{1,3}L$ threshold of Li is proposed.

We use our recently developed explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core-excited states in diatomic molecules to investigate the $\mathrm{O}\hspace{0.5em}1s$ core-hole decay electron spectra of ${\mathrm{O}}_2.$ Looking at the changes of the spectra as the exciting radiation frequency is tuned to and away from the excitation resonance allows us to follow the time evolution of the nuclear degrees of freedom of the molecule in the short-living core-excited state. We explicitly demonstrate that here, in contrast to other diatomic molecules such as ${\mathrm{N}}_2$ or CO investigated earlier, the relative softness of the intramolecular bond in the core-excited state allows us to interpret changes in some gross features of the decay spectra with varying photon energy in terms of the classical time evolution in the excited state. In particular, the low-energy tail of the electron spectra, due to the fraction of the core-excited state molecules surviving longest, gets visibly suppressed if the exciting radiation is detuned away from the x-ray-absorption resonance.

The subject of body frames and their singularities for three-particle systems is important not only for large-amplitude rovibrational coupling in molecular spectroscopy, but also for reactive scattering calculations. This paper presents a geometrical analysis of the meaning of body frame conventions and their singularities in three-particle systems. Special attention is devoted to the principal axis frame, a certain version of the Eckart frame, and the topological inevitability of frame singularities. The emphasis is on a geometrical picture, which is intended as a preliminary study for the more difficult case of four-particle systems, where one must work in higher-dimensional spaces. The analysis makes extensive use of kinematic rotations.

Four-atom systems may soon be subject to state-to-state reactive scattering calculations and understanding body frames and their singularities will be an important part of this effort. This paper examines body frames in four-atom systems, building on a geometrical analysis of the nine-dimensional configuration space and the six-dimensional internal space. Kinematic rotations are an important tool in this analysis. A central role is played by the “kinetic cube,” the space of all asymmetric top shapes related by kinematic rotations. The singularities, multiple branches, and connectivity of the principal axis frame are examined in detail and related to the topology of the kinetic cube. The principal axis frame has singularities on all symmetric top shapes, both oblate and prolate, of both chiralities. A version of the Eckart frame, however, has singularities only on prolate symmetric top shapes of one chirality. Frame singularities are inevitable in the four-body problem and no other frame has a smaller singular set than the Eckart frame.

We report an observation of the formation of muonium $\left[\mathrm{Mu}\equiv \right({\mu }^{+}{e}^{-})$ bound state] with kinetic energies between 1 and 40 keV on 500-nm-thick solid argon, xenon, and nitrogen $\left({\mathrm{N}}_2\right)$ layers. The thin films are deposited on a $250-\mu \mathrm{m}$-thick aluminum target which is bombarded with a 3.6-MeV ${\mu }^{+}$ beam. We measured the charge state of the muons emerging from the layers as a function of exit energy. The solid layers investigated are of potential interest for the efficient moderation of ${\mu }^{+}$ down to energies of 10 eV for use as a source for a low-energy ${\mu }^{+}$ beam. For the cryogenic insulators investigated, we find that the measured energy-dependent neutral fraction of the exiting muon beam is well interpreted by velocity scaling of known cross sections of protons for electron capture and electron loss down to muon energies of 1 keV. The experimental results are well reproduced by a Monte Carlo simulation. The total fraction of muonium in the exiting beam is found to be $2\times {10}^{-3}$ for argon and nitrogen as well as for the aluminum substrate without deposited layer, and about 20% less than this for xenon.

The relaxation dynamics of HCl, DCl, ${\mathrm{H}}_2\mathrm{S},$ and ${\mathrm{D}}_2\mathrm{S}$ following photoexcitation in the vicinities of the Cl and S K-shell thresholds $(\sim 2.8\mathrm{keV}$ for Cl, $\sim 2.5$ keV for S) were studied by means of ion time-of-flight mass spectroscopy. In all cases, the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the ${\mathrm{H}}^{+}$ peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, $\beta ,$ is approximately two for HCl, and is clearly positive for ${\mathrm{H}}_2\mathrm{S}.$

Ab initio close-coupling calculations are reported using the R-matrix method for photoionization cross sections $\left({\sigma }_{\mathrm{PI}}\right),$ oscillator strengths (f values), and energy levels (E) for oxygen ions: O I, O II, O III, O IV, O V, O VI, and O VII. Total, partial, and state-specific photoionization cross sections are computed and investigated in detail. Important features are found that should affect overall photoionization and recombination, especially at high temperatures and energies. Although these ions have been previously studied individually, present work aims at studying them as an isonuclear sequence and obtaining more accurate and complete results for applications. More extended eigenfunction expansions than in previous works, including $n=3\mathrm{}$ states, show correlation effects primarily manifested as additional groups of resonances in photoionization of oxygen ions such as O VI and O VII that are of importance in extreme ultraviolet and x-ray laboratory and astrophysical plasmas. Lifetimes of excited bound states are obtained from the present oscillator strengths and compared with experiments. The computed radiative atomic parameters, energy levels, oscillator strengths, and photoionization cross sections constitute a larger dataset than the Opacity Project data [M. J. Seaton, J. Phys. B 20, 6363 (1987)], with which comparisons are made.

The electron-impact multiple ionization and the subsequent dissociation of ${\mathrm{CO}}_2$ have been studied for electron energies from the threshold to 600 eV. The dissociation channels of up to quadruply ionized carbon dioxide molecules have been identified. The absolute cross sections for the ion-pair dissociation channels in double, triple, and quadruple ionization of ${\mathrm{CO}}_2$ have been obtained. The absolute cross sections of the other channels involving neutral fragments are also derived. The experiment shows that the total cross section of single, double, and triple ionization decreases by at least an order of magnitude as the ionization stage increases by 1. By studying the structure of the islands in the covariance maps, we have concluded that one of the ${\mathrm{CO}}_2^{2+}$ dissociation channels ${\mathrm{CO}}_2^{2+}\to {\mathrm{C}}^{+}{\mathrm{}+\mathrm{O}}^{+}\mathrm{}+\mathrm{O}$ is dominated by the process of secondary decay. The dissociation channel ${\mathrm{CO}}_2^{2+}\to 2{\mathrm{O}}^{+}\mathrm{}+\mathrm{C}$ is a concerted process. The three-body dissociation channels of triply ionized carbon dioxide are found to be dominated by concerted processes no matter how the charges are distributed on the fragments. The momentum distribution on the fragments can only be explained by the charge mobility during the dissociation process of the triply ionized molecules. The metastable decay of ${\mathrm{CO}}_2^{2+}$ is also observed directly.

In this work we describe a model for the electronic energy loss of bare ions at high velocities. Starting from first-order perturbation theory we propose a simple formula to calculate the impact-parameter dependence of the electronic energy loss for all impact parameters. The physical inputs are the electron density and oscillators strengths of the atoms. A very good agreement is obtained with full first-order calculations.

The electron capture cross sections of hydrogen-cluster ions ${\mathrm{H}}_n^{+}$ colliding with atomic helium have been measured in a large range of cluster size $(5\mathrm{}<~n<~35)$ for the same velocity ${(1.5v}_0,60\mathrm{keV}/u).\mathrm{}$ While the electron-capture cross section decreases from the ${\mathrm{H}}^{+}$ ion to the ${\mathrm{H}}_3^{+}$ one, the cluster electron-capture cross section is found to be independent of the cluster size and nearly equal to the capture cross section of the ${\mathrm{H}}_3^{+}$ ion. The electron capture by hydrogen clusters on a helium atom is a process involving only the ${\mathrm{H}}_3^{+}$ core of the cluster where the positive charge is localized. It appears that this very localized electron capture is not disturbed by the presence of molecules, up to 16, around the ${\mathrm{H}}_3^{+}$ core.