Both relativistic energy-consistent small-core ab initio pseudopotential and fully relativistic density-functional all-electron calculations have been carried out by exploiting the presently available highest computational capability for the first to fourth ionization potentials as well as the $\mathrm{df}$ $[{\Delta }_{\mathrm{df}}\hspace{0.5em}=\hspace{0.5em}{E(f}^n{d}^1{s}^2{)-E(f}^{n+1\mathrm{}}{d}^0{s}^2)$ ($n=0\mathrm{}-13$ for Ac–No)] and $\mathrm{fd}$ $[{\Delta }_{\mathrm{fd}}\hspace{0.5em}=\hspace{0.5em}{E(f}^n{d}^2{s}^2{)-E(f}^{n+1\mathrm{}}{d}^1{s}^2)$ ($n=0\mathrm{}-13$ for Th–Lr)] excitation energies for the whole series of actinide atoms. The calculated ionization potentials might be useful to guide future experimental measurements.

The lifetimes of the ${1s}^{2}2s2p{}^3{P}_2$ level of Ar XV and ${1s}^2{2s}^{2}2p{}^2{P}_{3/2}$ of Ar XIV have been measured using metastable ${\mathrm{Ar}}^{14+}$ and ${\mathrm{Ar}}^{13+}$ ions produced by an electron cyclotron resonance ion source, which were subsequently separately captured into a Kingdon ion trap. The lifetime results are τ(Ar XV, 2s2p ${}^3{P}_2)=13.4\mathrm{}(7)\mathrm{}\mathrm{ms}$ and τ(Ar XIV,$2p{}^2{P}_{3/2})=9.12\mathrm{}(18)\mathrm{}\mathrm{ms}.$ Transition rates derived from the measured lifetimes differ significantly from both relativistic and nonrelativistic calculations of the $2s2p{}^3{P}_1{–}^3{P}_2$ $M1\mathrm{}$ transition rate of Ar XV, but are in reasonable agreement with calculations for the $2p{}^2{P}_{1/2}{–}^2{P}_{3/2}$ $M1\mathrm{}$ rate of Ar XIV.

The uv spectra of ${\mathrm{H}}_3,$ ${\mathrm{D}}_2\mathrm{H},$ and ${\mathrm{D}}_3$ emitted by a neutralized ion beam have been measured in the wavelength range 200–400 nm. The spectra are continuous and show two maxima at 240 and 310 nm. Numerical simulations of the uv spectra, taking ab initio potential surfaces and transition moments as a starting point, explain their bimodal structure as arising from radiative decay into the two Jahn-Teller sheets of the ground-state potential surface. Two different lifetimes ${\tau }_1$ and ${\tau }_2$ for the lower sheet and the upper sheet of the Jahn-Teller adiabatic ground-state surface, respectively, were introduced and varied when fitting the simulated spectra to the experimental ones. These lifetimes have to be interpreted as average values for different vibrational states with angular momentum quantum numbers ranging from 0 to 7. The values are for ${\mathrm{H}}_3$ ${\tau }_1=3.5\mathrm{}\mathrm{fs}$ and ${\tau }_2=6.7\mathrm{}\mathrm{fs},$ for ${\mathrm{D}}_2\mathrm{H},$ ${\tau }_1=4.9\mathrm{}\mathrm{fs}$ and ${\tau }_2=8.2\mathrm{}\mathrm{fs},$ and for ${\mathrm{D}}_3,$ ${\tau }_1=5.9\mathrm{}\mathrm{fs}$ and ${\tau }_2=8.7\mathrm{}\mathrm{fs}$ and the error is estimated to be 0.5 fs. These results are consistent among each other, i.e., they show the expected dependence on the square root of the reduced masses. Furthermore, lifetimes for the uv emitting Rydberg states have been measured. Since lifetimes of Rydberg states of ${\mathrm{H}}_3$ and its isotopomeres are already known from analogous measurements in the visible spectrum, the $3s$ and $3d$ states could be identified as the main emitters of the uv radiation. Their mean lifetimes range from about 4 ns for ${\mathrm{H}}_3,$ 5 ns for ${\mathrm{H}}_2\mathrm{D},$ and 6 ns for ${\mathrm{D}}_2\mathrm{H}$ to 10 ns for ${\mathrm{D}}_3.$

It was earlier reported [Phys. Rev. Lett. 78, 199 (1997)] that long-lived excited states of positronium can be formed in crossed electric and magnetic fields at laboratory field strengths. Unlike the lower-lying states that are localized in the magnetically distorted Coulomb well, these long-lived states which can possess a lifetime up to many years are localized in an outer potential well that is formed for certain values of the pseudomomentum and magnetic field. The present work extends the original analysis and studies the dependence of the spectrum as a function of field strength and pseudomomentum over a wide range of parameters. We predict that in the limit of large pseudomomentum, the ground state of a positronium atom in a magnetic field will become delocalized; for strong fields, the binding energy of this state is quite large, resulting in a ground state that is both stable against direct annihilation and against ionization by low frequency background radiation.

The line shapes of saturation spectroscopy in optically thick two-level media are calculated for atomic and molecular transitions. Line narrowing and distortion effects due to propagation are presented. For Doppler-broadened media it is shown that, due to multiphoton processes, the natural linewidth limits the ultimate narrowing that can be reached. Thus, up to ${\alpha }_{0}L=20,$ only when the rate-equation approximation is valid subhomogeneous linewidths are expected.

We present measurements of the effects of spin exchange on the nuclear tensor polarization of atomic deuterium gas. Measurements of the nuclear polarization have been performed at several values of the magnetic field. In these measurements, only the gas density was changed, leaving all other experimental parameters constant. The results are in qualitative agreement with a model, based on the spin-exchange mechanism proposed by Purcell and Field. The results are relevant for nuclear and particle-physics experiments, using nuclear polarized hydrogen or deuterium targets, as well as for laser optical pumping applications of ${}^1\mathrm{H}$ and ${}^2\mathrm{H}.$

The effect of electron-vibrational coupling on the decay of a metastable electronic state is treated by a real-time path-integral method. The electronic resonance is described within the framework of the projection operator formalism of scattering theory. The effect of the bath is taken into account by the Feynman-Vernon influence functional technique. In this formulation, neither Born-type nor Markov-type approximations are invoked. The numerical evaluation of the time-discretized path integral is made possible by a recursive partial summation technique. This approach, which has previously been formulated for scattering amplitudes, is generalized to population probabilities that are given by a forward-backward double path integral. The performance of the method is demonstrated for model systems describing a $d$-wave shape resonance, which is linearly coupled to a bath with Ohmic spectral function. The effect of the bath is investigated as a function of coupling strength and temperature.

Ab initio potential-energy curves and coupling matrix elements of the ${}^{1,3}\Sigma$ and ${}^{1,3}\Pi$ molecular states involved in the collision of the ${\mathrm{Si}}^{4+}$ multicharged ion on helium have been determined by means of configuration-interaction methods. The total and partial electron capture cross sections have been determined using a semiclassical approach in the 2-eV–60-keV laboratory energy range. A sharp resonance has been exhibited for a 45.455-eV collision energy.

We apply the hyperspherical close-coupling method to the elastic and spin-flip processes in $p+p\mu$ and $e^{-}+\mathrm{Ps}$ (or equivalently, $e^{+}+\mathrm{Ps})$ scattering at low collision energies. The spin-spin interactions $V_S$ are included explicitly in the Hamiltonian, producing the correct hyperfine-splitting energy $\Delta \epsilon$ in the separated-atom limit. A comparison is made with a simpler method in which the effect of $V_S$ is represented only by the symmetry of the wave functions with respect to the interchange of the two identical particles. Explicit inclusion of $V_S$ is found to be of vital importance for the calculation of the spin-flip cross sections for collision energies lower than $\Delta \epsilon$ for both $p+p\mu$ and $e^{-}{(e}^{+})+\mathrm{Ps}$ collisions. The elastic cross sections are less sensitive to $V_S,$ but those for $p+p\mu$ are still affected by $V_S$ for collision energies lower than $\Delta \epsilon$.

The scattering of low-energy electrons by polar molecules is considered. The excitation to the rotational states in the sudden approximation and the effect of the Coulomb dipole force to all orders are taken into account. Analytic expressions for the scattering amplitudes are obtained. These are valid for arbitrarily large dipole moments of the molecule. Differential and momentum-transfer cross sections have been calculated for several molecules and compared with experimental data, with the Born approximation and with coupled-channels calculations.

Relativistic distorted-wave method was used to calculate the electron-impact excitation processes for Fe XXIII. Collision strengths for the resonant transitions and transitions between excited levels within the first 46 levels of Fe XXIII are reported. Multiconfiguration Dirac-Fock wave functions with 133-level configuration expansion were used to describe the target-ion states accurately. The distorted-wave potentials with semiclassical exchange potential were used to determine the continuum orbitals. The collision strengths for some of the transitions in $n=2\mathrm{}–3$ were previously calculated only by a nonrelativistic distorted-wave method. If they are compared with our results, significant differences are found. The reason for these differences is explained. The accuracy of the collision strength for the transition from the ground state to ${2s2p}^3{P}_1$ is reestimated among different methods. Because of the accurate target states used and the fully relativistic effect caught in the atomic structure and relativistic distorted-wave method used in the collision dynamics, the collision strengths provided in the present calculation should be accurate and reliable. Also, we report collision strengths for some of the $n=3\mathrm{}–3$ transitions in this paper. These electron impact excitation results have various applications in solar flare spectra, plasma modeling and diagnostics, and in achieving population inversion in the development of x-ray lasers.

Potential scattering of free-electron wave packets is considered in the framework of the nonstationary quantum-mechanical theory. To characterize the efficiency of this process, the average angle of scattering is calculated in the lowest (first and second) orders of perturbation theory in the potential. In the approximation of a well-localized, narrow, and nonspreading wave packet, the first-order term is shown to agree with the classical angle of scattering, linear in the potential. This term is shown to vanish at a very large size of the initial wave packet (much larger than the size of a target). An analogy between the potential scattering of wave packets and electron-light ponderomotive scattering is discussed.

Cross sections for electron loss of ${\mathrm{He}}^{+}$ and ${\mathrm{C}}^{3+}$ projectile ions on atomic and molecular hydrogen were measured in the $0.5-\mathrm{MeV}<~E<~3.5-\mathrm{MeV}$ energy range. The measurements of the ${\sigma }_{{\mathrm{H}}_2}/{\sigma }_{\mathrm{H}}$ cross section ratios were made by using an absolutely calibrated hydrogen furnace. The influence of different models describing the form factor of molecular hydrogen is examined by means of the ratio ${\sigma }_{{\mathrm{H}}_2}/{\sigma }_{\mathrm{H}},$ which is sensitive to the model adopted, particularly in the case of ${\mathrm{He}}^{+}$ projectiles. The general behavior of the measured data is analyzed within the independent electron approximation including the electron loss (screening and antiscreening contributions), electron capture, and target ionization channels. The connection between the antiscreening mode and the electron impact ionization of multicharged ions is explored.

In the present study we consider the radiation emitted during ${\beta }^{\pm }$ decay and a few related problems of importance in applications. The so-called initial inner bremsstrahlung is emitted by the accelerated ${\beta }^{\pm }$ particle at very short distances from the maternal nucleus. Then, the ${\beta }^{\pm }$ particle moves with constant velocity through the electronic shells of the maternal atom (ion) and emits the inner bremsstrahlung. Later the ${\beta }^{\pm }$ particle leaves the maternal atom (ion) and begins to pass (with the same constant velocity) through the surrounding atoms and emits the usual bremsstrahlung. Likewise, both the maternal and surrounding atoms can be excited and even ionized either by the instantaneous change of the nuclear charge or by such a penetrating ${\beta }^{\pm }$ particle. Then the excited atoms transit to the ground state emitting the so-called atomic transition radiation, which may be observed during or after ${\beta }^{\pm }$ decay. Our present analysis includes the inner bremsstrahlung and inner atomic transition radiation, i.e., both continuous and discrete radiation from the maternal ${\beta }^{\pm }$-decaying atom (ion).

The x-ray mass attenuation of ${}_{48}\mathrm{Cd},$ ${}_{49}\mathrm{In},$ ${}_{50}\mathrm{Sn},$ and ${}_{51}\mathrm{Sb}$ in the energy regime of the $L$-subshell edges has been measured. For a comparison of the data of neighboring elements, these were scaled to ${}_{47}\mathrm{Ag}.$ The scaled data were compared with theoretical calculations of photoionization cross sections by Scofield, which use the common single electron approach. The comparison reveals minor but significant deviations between measurement and calculation: The measured cross sections are smaller than the prediction in the regime between the $L_3$ and $L_2$ edges, they have a flatter slope in the regime between the $L_2$ and $L_1$ edges, and they exhibit a decrease just above the $L_3$ and $L_2$ edges. All observed deviations can be explained as electron correlation effects originating from a polarization of the whole electron cloud by the ionizing radiation, since they are qualitatively reproduced by comparative calculations of the ionization process either omitting (independent particle approach) or including (in the linear response approximation) the electron correlations. However, the comparative calculations quantitatively overestimate the electron correlation effects.

We develop an explicitly time-dependent theory for one-step resonant excitation-deexcitation processes of core electron states in diatomic molecules. Emphasis is placed on a conceptual picture demonstrating how the effective time of the formation of the spectra—which is influenced by the bandwidth of the exciting radiation, by the excitation of the molecule being resonant or off resonant (detuned), and by the actual core hole lifetime—changes the appearance of the deexcitation electron spectra. Explicit time-dependent model calculations for three final states each of $1s$-hole excited ${\mathrm{N}}_2$ (including one spectator decay) and $\mathrm{C}1s$–hole excited CO allow demonstration of the various consequences for the spectral shapes which derive from these influences. In particular, off-resonance excitation is shown to shorten the effective time of the spectrum formation below the lifetime of the core-excited state leading to the recently observed collapse of the vibrational structure in the spectrum. Our calculated spectra also demonstrate the influences of the relative positions and shapes of the potential curves involved. On resonance, the nodal structure of the vibrational wave functions of the core-excited state is reflected in the shapes of the spectator decay spectra of ${\mathrm{N}}_2$ with a soft final state interatomic potential.

The electric dipole, quadrupole, and octupole polarizabilities of a helium atom and the associated dispersion coefficients for the interaction of two helium atoms are calculated for the atomic $n=2\mathrm{}$ singlet and triplet states, respectively, $\mathrm{He}\left(2\mathrm{}{}^{1}S\right)$ and $\mathrm{He}\left(2\mathrm{}{}^{3}S\right)$. The calculations utilize highly accurate, variationally determined, wave functions composed of doubled basis sets. The effects of retardation due to the finite speed of light are calculated for the dispersion potentials of two $\mathrm{He}\left(2\mathrm{}{}^{1}S\right)$ or two $\mathrm{He}\left(2\mathrm{}{}^{3}S\right)$ atoms and for the atom-surface interaction potentials of a $\mathrm{He}\left(2\mathrm{}{}^{1}S\right)$ or a $\mathrm{He}\left(2\mathrm{}{}^{3}S\right)$ atom with a perfectly conducting or a dielectric wall.

Cross sections for electron capture from pair production by 10.8-GeV/nucleon $(\gamma =12.6)$ bare ${\mathrm{Au}}^{78+}$ ions have been measured in Au, Ag, Cu, and Al targets. These measurements can be compared with calculations used to predict beam lifetimes in the Relativistic Heavy Ion Collider at Brookhaven National Laboratory and Large Hadron Collider at CERN. Experiment and calculations are compared.

Using simple kinetic theory and geometric considerations, a plausible explanation for the observation of anomalously narrow linewidths in time-of-flight dissociative-recombination experiments is presented. The widths of these peaks were previously unexplainable. The explanation suggests that the very subthermal molecules that are responsible for these dissociative-recombination reactions are formed by associative-ionization reactions.

Experimental measurements of the projectile angular distributions for 2.5-keV ${\mathrm{Ar}}^{8+}$ ions capturing one to five electrons from a gas-phase ${\mathrm{C}}_{60}$ target are presented. The number of captured electrons was determined by demanding a coincidence between the scattered projectile and a charge-state-analyzed intact ${\mathrm{C}}_{60}$ recoil ion. The results are compared to calculations based on a dynamical classical overbarrier model. Good agreement is obtained only if the influence on the projectile trajectory by the large polarizability of the ${\mathrm{C}}_{60}$ target is taken into account, thereby making the collective dielectric response of the cluster target observable in a scattering experiment.