The exotic helium atom, consisting of a helium nucleus ${\mathrm{He}}^{2+}$, an electron ${\mathit{e}}^{\mathrm{-}}$, and a massive, negatively charged particle ${\mathit{X}}^{\mathrm{-}}$, such as an antiproton (p¯) or a kaon (${\mathit{K}}^{\mathrm{-}}$), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei ${\mathrm{He}}^{2+}$ and ${\mathit{X}}^{\mathrm{-}}$, if ${\mathit{e}}^{\mathrm{-}}$ is in a low-lying orbital and ${\mathit{X}}^{\mathrm{-}}$ is in a highly excited orbital. Such states are generally formed in the capture of ${\mathit{X}}^{\mathrm{-}}$ by helium. The Born-Oppenheimer (BO) separation of the ${\mathit{e}}^{\mathrm{-}}$ motion and the ${\mathrm{He}}^{2+}$-${\mathit{X}}^{\mathrm{-}}$ motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different ${\mathit{X}}^{\mathrm{-}}$ and different isotopes of ${\mathrm{He}}^{2+}$. A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational and high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes τ of highly excited states formed immediately after the capture of ${\mathit{X}}^{\mathrm{-}}$ are of the order of μsec for ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$-p¯-${\mathit{e}}^{\mathrm{-}}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$-p¯-${\mathit{e}}^{\mathrm{-}}$, and those for ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$-${\mathit{K}}^{\mathrm{-}}$-${\mathit{e}}^{\mathrm{-}}$ are about half the values for the antiprotonic helium.

The effect of the electronic charge polarization by ${\mathit{X}}^{\mathrm{-}}$ on τ is explained quantitatively in a simple picture in terms of the molecular dipole moment. Low vibrational, high rotational states are described as circular or near-circular states in the atomic picture. They are stable against Auger decay, which the nonadiabatic perturbation on the BO states induces. An infinite series of bound Σ electronic states of exotic helium negative ions (${\mathrm{He}}^{2+}$-${\mathit{X}}^{\mathrm{-}}$-${\mathit{e}}^{\mathrm{-}}$)-${\mathit{e}}^{\mathrm{-}}$ exists for any ${\mathrm{He}}^{2+}$-${\mathit{X}}^{\mathrm{-}}$ distances R beyond 0.819 a.u. in the BO approximation. An infinite Π series exists for R>3.811 a.u., and an infinite Δ series for R>9.532 a.u.

We have measured the angular distributions of electrons ejected from doubly excited autoionizing Rydberg 3pnd J=1 and 3 states of magnesium, where n=10–18. To excite the aligned 3snd ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ states of Mg, we used two dye lasers crossed with an atomic beam. A third laser, linearly polarized in the same direction as the other two, excited the atoms in the 3snd states to the 3pnd states. The angular distributions of ejected electrons are given by dσ/dΩ=${\mathit{tsum}}_{\mathit{k}}$${\mathit{a}}_{\mathit{k}}$${\mathit{P}}_{\mathit{k}}$(cosθ), where k=0,2,4,6. We measured the angular distribution parameters ${\mathit{a}}_{\mathit{k}}$ as a function of the energy of the third laser photon using an electron detector and a ${\mathrm{Mg}}^{+}$ ion detector. We compare our results with the predictions of an R-matrix multichannel quantum-defect theory. We find good agreement between experiment and theory.

The Balslev-Combes theorem states that upon scaling the internal degrees of freedom in the Hamiltonian by a complex factor η=exp(iθ), the resonances are isolated from the other states in the continuum, which are rotated into the lower half complex-energy plane by the angle of 2θ. On the basis of this theorem, the resonance positions and lifetimes in atomic, molecular, and nuclear systems have been calculated. It is reported here that, due to the finite-matrix approximation, the rotating continuum, ${\mathit{E}}_{\mathit{c}}$(θ), shows a structure when Im[${\mathit{E}}_{\mathit{c}}$(θ)] is plotted versus Re[${\mathit{E}}_{\mathit{c}}$(θ)]. This structure is a fingerprint of the resonances. Moreover, the density of states of the rotating continuum, ${\mathrm{\rho }}_{\mathit{c}}$, does not show the expected monotonic behavior as ‖${\mathit{E}}_{\mathit{c}}$‖ is increased but has ‘‘holes’’—i.e., a drastic decrease of ρ—when ‖${\mathit{E}}_{\mathit{c}}$(θ)‖=‖${\mathit{E}}_{\mathrm{res}}$‖. These ‘‘holes’’ gradually disappear as θ and/or the basis set is increased. An illustrating numerical example is given.

The strengths of individual 1s2l2l’ resonances for dielectronic recombination of heliumlike iron are measured on an electron-beam ion trap using high-resolution x-ray spectroscopy. An energy resolution ΔE/E of better than 0.1% is achieved by monitoring the exit channel of the recombination process. Dielectronic-resonance strengths are measured relative to the cross section for nonresonant radiative electron capture of heliumlike Fe xxv, and an overall uncertainty of 20% is achieved. Good agreement with theory is found for the strongest resonances; however, less good agreement is found for the weaker resonances. The results indicate that further theoretical and experimental efforts are necessary to reliably use weak dielectronic satellite lines in plasma diagnostics.

The total photoionization cross sections of F, Cl, Br, and I are calculated near the ${\mathit{ns}}^2$${\mathit{np}}^4$ thresholds. They show excellent agreement with experiment for all of the halogens except I whose cross section is only in fair agreement with experiment. At the ${}_{}{}^1{}_{}{}^{}$S threshold of Cl our results agree with experiment and with the highest of the previous calculations. These results suggest that atomic theory is now capable of reproducing the dynamics of all the open-p-shell atoms with Z<54, at energies near the lower ionization thresholds. We identify a type of mixing which goes beyond the usual recoupling frame transformation and is not present in alkaline-earth atoms, nor in rare-gas atoms.

By using a kinetic model, we can easily resolve the observed effective lifetimes of the K ${\mathit{n}}^2$S (n=8–11) and n ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ (n=6–8) states in the presence of the ${\mathrm{H}}_2$ gas. Thereby, in accordance with the Stern-Volmer equation, the radiative lifetimes and the collisional deactivation cross sections of these states can be determined. The lifetimes observed in this work agree satisfactorily with results reported elsewhere. The cross section for the n ${}_{}{}^2{}_{}{}^{}\mathrm{S}$ state increases with increasing n, whereas that for the n ${}_{}{}^2{}_{}{}^{}\mathrm{D}$ state tends to decrease. A qualitative explanation for this n dependence is given.

We have studied the production of ${\mathrm{H}}^{\mathrm{-}}$ by fast ${\mathrm{H}}_3^{+}$ ions (4≤v≤7 a.u.) colliding with He, Ne, Ar, and Xe atoms. The negative-ion yields were measured as a function of target pressure and from them were extracted the ${\mathrm{H}}^{\mathrm{-}}$ production cross sections, the maximum yields, and the pressure values associated with these maxima. We observed that the maximum yield is about the same for all gases and velocities in the range under consideration. This fact reveals an almost identical scaling for the ${\mathrm{H}}^{\mathrm{-}}$ production, the ${\mathrm{H}}_3^{+}$ destruction, and the ${\mathrm{H}}^{\mathrm{-}}$ destruction cross sections. The ratio of the ${\mathrm{H}}^{\mathrm{-}}$ production and the ${\mathrm{H}}_3^{+}$ destruction cross sections was also verified to assume essentially the same value for all measurements. The cross sections were well fitted by a semiempirical version of the free collision model.

State-dependent cross sections and rate coefficients for the charge-transfer process ${\mathrm{N}}^{4+}$+H→${\mathrm{N}}^{3+}$(2s3l)+${\mathrm{H}}^{+}$ for l=s,p,d in the collision-energy range from 0.1 eV to 8 keV are presented. A close-coupled quantum-mechanical method is employed, and it is argued that the molecular-state expansion method without translation factors is a valid low-energy approximation for charge-transfer processes. We show that the method does not suffer from the electron-origin problem. The spin-coupled valence-bond method is used to construct both ab initio potential curves and adiabatic coupling elements. The calculated cross sections exhibit variations with energy, which we propose are a manifestation of Stueckelberg oscillations. The cross sections are compared with measured total cross sections for electron capture by ${\mathrm{N}}^{4+}$ ions in collisions with hydrogen. Agreement is excellent in the collision-energy range from 4.1 to 182 eV/amu.

We develop a simple Green’s function for δ-function potentials with a hard core, and use it to calculate polarizabilities of negative halogen ions and multiphoton-detachment cross sections for these ions. The predictions indicate an influence of a resonance for two- and three-photon cross sections, and yield approximately universal relative amplitudes as functions of a scaled electron energy.

A hybrid approach to electron scattering from polar molecules that uses a recently introduced algebraic rotating-frame approach is presented. The high partial waves are taken from the rotating-frame calculation, while the low partial waves are taken from coupled-channel calculations. The method is applied to electron scattering from a diatomic molecule with large dipole moment and compared with a recent hybrid approach that uses the albegraic-eikonal approximation.

We have measured coincidences between neutral H atoms and Lyman-α photons for collisions between 50-keV protons and noble gases as a function of the projectile scattering angle. The coincidences are dominated by capture to the 2p state of the projectile. While the total cross sections depend strongly on the target, the shape of the angular distribution of the differential cross sections was found to depend only weakly on that parameter. The data indicate that electrons are captured predominantly from the outermost shell of the target atom for the collision systems studied here.

The total cross sections of chlorofluoromethane molecules were measured in the 75–4000-eV range. A comparison is given with existing experimental and theoretical data. The present data and previous measurements in the 15–50-eV range were fitted with a Born-like formula. This simple fit allows the cross sections to be reproduced within experimental errors. Phenomenological double Yukawa scattering potentials can be derived from the formula. A short discussion for the total-cross-section partitioning is given for ${\mathrm{CF}}_4$, ${\mathrm{CF}}_2$${\mathrm{Cl}}_2$, and ${\mathrm{CCl}}_4$.

Experimentally determined single-electron-capture cross sections are presented for 100–1500-keV ${\mathrm{C}}^{+}$ impact on He, Ne, Ar, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, CO, ${\mathrm{NH}}_3$, ${\mathrm{CH}}_4$, ${\mathrm{C}}_2$${\mathrm{H}}_6$, and (${\mathrm{CH}}_3$${)}_2$ NH gaseous targets. For the atomic targets, the single-capture cross sections were found to merge as the impact energy increased. For the molecular targets, the relative changes in the cross sections exhibit little or no dependence on the impact energy. For these systems and in the energy range considered here, the single-electron-capture cross sections were found to scale remarkably well with the ‘‘size’’ of the molecular target, namely with the square of the bond length for the diatomic targets and with the number of atoms comprising the polyatomic targets.

A quantum-mechanical calculation of charge-transfer cross sections for the forward (endothermic) and backward (exothermic) collision processes ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$${+}^4$He ⇄ ${}_{}{}^3{}_{}{}^{}\mathrm{He}$${+}^4$${\mathrm{He}}^{+}$ in the threshold energy region has been carried out by using a two-state close-coupling treatment where the charge-transfer energy defect, approximately 1.3 meV, is the isotopic term difference between the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ binding energies. The cross sections show numerous shape and orbiting resonances, which can be labeled with predissociation rotational and vibrational quantum numbers. From these cross sections, the forward and backward rate coefficients have been calculated. The present results, particularly the ratio of forward and backward rate coefficients, agree well with measurements of Schauer, Jefferts, and Dunn [Phys. Rev. A 42, 5332 (1990)].

The Kα and Kβ x-ray spectra of molybdenum bombarded by 5.5-MeV/amu ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ions were measured with high resolution. In such heavy-ion–atom collisions, multiple ionization of the M and L shells of the target atoms is extremely likely to occur, resulting in a complex structure of the observed spectra. The ${\mathit{L}}^{\mathit{n}}$-satellite structure was resolved, whereas M vacancies produce only a shift and a broadening of the lines so that it is not possible to obtain in a direct way information about the M-hole distribution. We thus propose a method for the analysis of x-ray spectra of multiply ionized atoms. In this method the measured Kα and Kβ spectra are simultaneously analyzed as a sum of Kα${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{\mathit{m}}$ and Kβ${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^{\mathit{m}}$ components, respectively, with theoretically determined profiles. A binomial distribution of holes in the M shell is assumed and the M-shell ionization probabilities are treated as adjustable parameters. The profiles are constructed as sums of Voigt functions, whose positions and heights are determined by extensive multiconfiguration Dirac-Fock method calculations. If, in a simultaneous fit to the Kα${\mathit{L}}^0$ and K${\mathrm{\beta }}_{1,3}$${\mathit{L}}^0$ lines, just one parameter ${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$ describing the M-shell ionization at the moment of the K x-ray transition is used, the experimental data are not reproduced in an entirely satisfactory way (${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$=0.19±0.02). Therefore, two parameters, one common for the 3s and 3p subshells ${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$ and the other for the 3d subshells, were introduced and a much better fit to both lines (${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$=0.17±0.02, ${\mathit{p}}_{3\mathit{d}}^{\mathit{X}}$=0.23±0.02) was obtained.

The importance and influence of alternative ionizing processes as electron-capture and rearrangement processes are discussed and the M-shell ionization probability at the moment of the collision (${\mathit{p}}_{\mathit{M}}$=0.18±0.02) is deduced from the ${\mathit{p}}_{\mathit{M}}^{\mathit{X}}$ value. It is shown that the differences between ${\mathit{p}}_{3\mathit{s}\mathit{p}}^{\mathit{X}}$ and ${\mathit{p}}_{3\mathit{d}}^{\mathit{X}}$ are mainly due to Coster-Kronig transitions, rather than due to different subshell ionization probabilities. The spectra were also analyzed with respect to the L-shell ionization on the basis of the calculated Kα${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^0$, K${\mathrm{\beta }}_{1,3}$${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^0$, and K${\mathrm{\beta }}_2$${\mathit{L}}^{\mathit{n}}$${\mathit{M}}^0$ components. The M-shell ionization was taken into account by taking a larger Gaussian width and by shifting the positions of the lines. The intensity yields of the L satellites were determined, the primary vacancy distribution was deduced, and the results from Kα and Kβ spectra are compared.

We have calculated dielectronic recombination (DR) rate coefficients for H-like silicon, calcium, and iron at plasma temperatures 0.1–10 keV. The calculation involves all DR processes 1s+ε${\mathit{l}}_{\mathit{c}}$→(2lnl’${)}^{\mathrm{*}}$→1sml’’+γ with n=2-∞. The effects of configuration interaction and spin-orbit coupling have been included for all states with n≤8; for higher-lying resonance states, the 1/${\mathit{n}}^3$ scaling was employed.

We discuss a recently proposed model for describing single-electron transfer in slow- or intermediate-energy-ion–${\mathrm{H}}_2$-molecule collisions and present results for the examples of ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, and ${\mathrm{C}}^{4+}$-ion impact. The dynamics of the two active electrons is given by a one-electron potential model, within the framework of the semiclassical close-coupling description, in conjunction with conservation of the norm of the total two-electron wave function. Considerable numerical simplifications occur when the length of the molecular axis is set to zero. It is argued that this model description of ion-molecule collisions is both efficient and remarkably accurate.

The S-wave (L=0) and P-wave (L=1) phase shifts and the S-wave and P-wave elastic and positronium-formation cross sections for positron scattering from hydrogen at energies below the first excitation threshold are calculated in the close-coupling approximation, using the algebraic variational method. Four target-state coupling schemes (H 1s–Ps 1s, H 1s– H 2s–Ps 1s, H 1s–H 2s–H 2p–Ps 1s, and H 1s–H 2s–H 2p¯–Ps 1s) have been considered for the calculation. It is found that the H 2p¯ target channel significantly affects the S-wave positronium-formation cross sections.

Cross sections and asymmetry parameters associated with the process of photodetaching an electron from the ${\mathrm{Li}}^{\mathrm{-}}$ ion via the process hν+${\mathrm{Li}}^{\mathrm{-}}$→Li(2 ${}_{}{}^2{}_{}{}^{}$S)+${\mathit{e}}^{\mathrm{-}}$(εp) have been measured at several photon energies including the region in the vicinity of the opening of the channel: hν+${\mathrm{Li}}^{\mathrm{-}}$→Li(2 ${}_{}{}^2{}_{}{}^{}$P)+${\mathit{e}}^{\mathrm{-}}$(εs). Strong coupling between these channels produces anomalous behavior in the ${}_{}{}^2{}_{}{}^{}$Sεp channel cross section. The measured cross sections are slightly lower than the theoretical predictions, but are in essential agreement within the combined uncertainties in the two values. A search has also been made for a ${}_{}{}^1{}_{}{}^{}$P° excited state of ${\mathrm{Li}}^{\mathrm{-}}$ in the vicinity of the 2 ${}_{}{}^2{}_{}{}^{}$P threshold. The saturation characteristics of the photodetachment of ${\mathrm{Li}}^{\mathrm{-}}$ and ${\mathrm{D}}^{\mathrm{-}}$ ions have been investigated over a range of laser powers.

Data on the production and dissociation of ${\mathrm{CO}}^{\mathit{Q}+}$ molecular ions (where Q=1 through 7) obtained by ion-ion coincidence time-of-flight measurements were analyzed to determine production yields, dissociation fractions, and branching ratios. A detailed comparison of the dissociation fractions for ${\mathrm{CO}}^{+}$ and ${\mathrm{CO}}^{2+}$ for several collision systems in the same perturbative regime revealed them to be quite similar, whereas the dissociation fraction for ${\mathrm{CO}}^{+}$ produced by valence-electron photoionization is a factor of 1.8 to 3.6 larger. The results for Q≥2 indicated a preference for dissociation channels leading to symmetric or nearly symmetric charge division. An enhancement of the total ionization yields for Q>4 was observed, and it suggests that electron transfer followed by LMM Auger decay plays an important role in determining the final charges of the dissociation products.