The Los Alamos Bright Source laser was used to create a silicon plasma by irradiation of a solid target. A very-high-resolution curved crystal spectrograph was used to detect the emitted x rays. The satellite lines due to the decay of doubly excited states that accompany the Lyman-α lines were observed. The 2p3l-1s3l radiative transitions in He-like Si were identified. In addition, all the components of the 2l2p ${}_{}{}^3{}_{}{}^{}$P–1s2l ${}_{}{}^3{}_{}{}^{}$L transitions were resolved. The spectra were compared to various collisional-radiative calculations with good results.

We report an experimental and theoretical study of the hyperfine structure (hfs) in various metastable states in ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ ii. Hyperfine structures of five levels in Nb ii have been measured using a combination of the laser-rf double resonance and laser-induced fluorescence methods in a collinear laser–ion-beam geometry. Theoretically, for J=2, a multireference calculation of energies and hfs based on a relativistic configuration-interaction methodology of the lowest ten levels in the (4d+5s${)}^4$ manifold is reported. The average energy error is 450 ${\mathrm{cm}}^{\mathrm{-}1}$. Many of the hyperfine constants show large changes from the Dirac-Fock values and the magnetic dipole constant has a 4% accuracy for the one J=2 level measured. We have also identified all the core-valence and core-core effects that dominate the energy differences and hfs.

An alternative approach is introduced for adiabatic and nonadiabatic calculations in the ABA Coulomb systems from ${\mathrm{H}}_2^{+}$ to the positronium negative ion ${\mathrm{Ps}}^{\mathrm{-}}$ (${\mathit{e}}^{\mathrm{-}}$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$) so that they are treated as molecular systems. The adiabatic vibrational solutions point out the most relevant ‘‘nuclear’’ configurations that should appear in the construction of the nonadiabatic wave function and reveal the importance of the AA vibrations at this stage. Applications provide evidence for the separability of the ground state of ${\mathrm{Ps}}^{\mathrm{-}}$ and give energy values for six S bound states of ${\mathrm{\mu }}^{+}$${\mathit{e}}^{\mathrm{-}}$${\mathrm{\mu }}^{+}$ that seem not to be reported so far.

Using high-resolution and high-efficiency high-purity Ge Gamma-X detectors, relative intensities of K x rays of elements with Z=79–82 have been very accurately (uncertainty ≊1%) determined. Comparison of the data with other experimental works and theoretical calculations have been done. It is seen that a theory with the exchange effect taken into consideration is quite adequate to explain the radiative transition rates for the elements measured in the present work.

X-ray spectra of 2p-nd transitions with 4≤n≤18 in molybdenum (Z=42) charge states around neonlike (${\mathrm{Mo}}^{32+}$) have been observed from Alcator C-Mod plasmas. Accurate wavelengths (±0.1 mÅ) have been determined by comparison with neighboring argon and chlorine lines with well-known wavelengths. Line identifications have been made by comparison to ab initio atomic-structure calculations, using a fully relativistic, parametric-potential code, and agreement between measured and theoretical wavelengths is good. Calculated wavelengths and oscillator strengths are presented for the strongest transitions with upper levels n between 4 and 14 to the lower levels n=2 in the four charge states ${\mathrm{Mo}}^{30+}$–${\mathrm{Mo}}^{33+}$. Effects of configuration interaction have been observed in the intensities of lines with nearly degenerate energy levels.

The Stark shift of the (6s${)}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$→(6s6p${)}^1$${\mathit{P}}_1$ transition in barium was measured using a laser to excite an atomic beam as it traversed a uniform electric field and a field-free region. The laser frequency was scanned across the transition while fluorescence produced by the radiative decay of the excited state was detected. The frequency scan was calibrated using an acousto-optic modulator which frequency shifted the laser by a specified amount. The Stark-shift rate was found to be -28.532±0.060 kHz/(kV/cm${)}^2$. The hyperfine splitting of the ${}_{}{}^{135,137}{}_{}{}^{}\mathrm{Ba}$ F=1/2 level relative to ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$ of the (6s6p${)}^1$${\mathit{P}}_1$ state was determined to be -549.47±0.12 MHz, where the minus sign indicates that the ${}_{}{}^{138}{}_{}{}^{}\mathrm{Ba}$ transition is at a lower frequency.

If an accurate ground-state electron density ${\mathrm{\rho }}_0$ for a system is known, it is shown from calculations on atoms that a strikingly good estimate for the total electronic energy of atoms is provided by the formula E[${\mathrm{\rho }}_0$]=${\mathit{tsum}}_{\mathit{i}}$${\mathrm{\varepsilon }}_{\mathit{i}}$-(1-1/N)J[${\mathrm{\rho }}_0$], where N is the number of electrons, J[${\mathrm{\rho }}_0$] is the classical Coulomb repulsion energy for ${\mathrm{\rho }}_0$, and the ${\mathrm{\varepsilon }}_{\mathit{i}}$ are the Kohn-Sham orbital energies determined by the Zhao-Morrison-Parr procedure [Phys. Rev. A 50, 2138 (1994)] for implementation of the Levy-constrained search determination of the Kohn-Sham kinetic energy. The surprising accuracy of this formula is attributed to the fact that the exchange-correlation functional is equal to -J/N plus a functional that behaves as if it were approximately homogeneous, of degree 1 in the electron density. A corresponding exact formula is given, and various approximate models are constructed.

The development of a family of functionals of the electronic density and its first and second derivatives is described. These functionals contain correct contributions to the exchange energy from regions far from the nuclei of an atom or molecule, and simultaneously give rise to a Kohn-Sham potential that is asymptotically correct in these regions. However, numerical studies show that none of the functionals can give accurate exchange energies, and the reasons for this deficiency can be traced to an instability with respect to density fluctuations arising from the use of the Laplacian of the density.

In three-level atoms where one level is the ground state and one is a metastable state, even low-intensity resonance radiation can lead to bleaching. This effect causes the radiation trapping to become nonlinear and thus greatly complicates the simulation of such a system. We set up equations to model an optically excited three-level atomic vapor in a two- or three-dimensional cylindrical cell; the model includes the effects of radiation trapping, particle diffusion, and quenching. We then develop a fast and efficient approximate method for the solution of these equations. Results are compared to Monte Carlo simulations; agreement within 15% is achieved.

The cross sections of antihydrogen formation in the ground state and n=2 level by the impact of antiprotons on the ground state of positronium have been calculated under the framework of the eikonal approximation for incident energy of 30–1000 keV. The excited-state capture cross sections are quite appreciable and are even larger than the ground-state cross sections for impact energies ≤75 keV. The total eikonal cross sections (σ=${\mathrm{\sigma }}_{1\mathit{s}}$+${\mathrm{\sigma }}_{2\mathit{s}}$+${\mathrm{\sigma }}_{2\mathit{p}}$) are always higher than the corresponding first-order Born approximation cross section throughout the present energy span.

With a wall-stabilized arc source, we have measured the relative transition probabilities of 100 lines of neutral nitrogen in the visible and near-infrared spectrum and have normalized our data to an absolute scale utilizing four recent lifetime results. We estimate that the expanded uncertainties of our data are in the range of ±11–15 %. For a number of 3s-3p and 3p-3d multiplets, we have measured complete sets of lines and observe that most 3p-3d multiplets show considerable departure from LS coupling, while the 3s-3p multiplets adhere to it within ±20%. Agreement between intermediate coupling calculations and our experimental data for the 3p-3d multiplets is noticeably better, but significant differences are still encountered for two multiplets originating from the 3d ${}_{}{}^4{}_{}{}^{}$P level.

Near an absorption edge, x-ray emission cannot be treated separately from the absorption process itself; a scattering formalism must be used. Experimental data have been recorded showing x-ray emission from xenon following excitation by tunable synchrotron radiation below and above the ${\mathit{L}}_3$ absorption edge. Complete data sets are presented for Xe L${\mathrm{\alpha }}_{1,2}$ and L${\mathrm{\beta }}_{2,15}$ emission from 10 eV below to 40 eV above the ${\mathit{L}}_3$ edge. In accord with the resonant-inelastic-scattering model, the observed x-ray emission evolves from resonant Raman scattering into characteristic fluorescence as the excitation energy is scanned from below to above the absorption edge.

We report a study of the connections between classical dynamics and quantum structure based on the lithium Stark spectrum. The system undergoes a transition to chaos as the energy or field is increased. We employ scaled-energy spectroscopy and closed-orbit theory to relate the photoabsorption spectrum to classical orbits. Bifurcations of closed orbits provide a semiclassical interpretation of the spectrum as it evolves from a simple sinusoidal modulation at positive energy to a discrete spectrum below the classical ionization limit. The system is compared with the closely related, but completely regular, hydrogen Stark system. The differences between their spectra and the development of chaos in lithium can be understood in terms of scattering of recurrences from one closed orbit to another by the alkali-metal core.

Variational calculations using multiconfiguration interaction wave functions are carried out on the 1s2s2${\mathit{p}}^2$${(}^5$P) and the 1s2${\mathit{p}}^3$${(}^5$S) states for the Be i isoelectronic sequence from Z=3–10. For each species, the upper bound, the nonrelativistic energy, the transition wavelength, the oscillator strengths, the radiative lifetime, the fine structure, the hyperfine parameters, and coupling constants are reported. The relativistic corrections and the mass polarization are included. The upper bounds and the nonrelativistic energies obtained are much lower than the previously published values for each member of the Be i isoelectronic sequence. The calculated fine-structure results are compared with the existing theoretical and experimental data. The predicted results of the hyperfine structure in this investigation provide useful data in the experimental analysis of fine-structure transitions between the 1s2s2${\mathit{p}}^2$${(}^5$P) and the 1s2${\mathit{p}}^3$${(}^5$S) states for berylliumlike isoelectronic sequence spectra.

In a recent paper, Plante et al. [Phys. Rev. A 49, 3519 (1994)] presented relativistic all-order many-body calculations for n=1 and n=2 states of heliumlike ions with nuclear charge in the range Z=3–100. They were, however, unable to obtain converged solutions for the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ states for low Z (Z≤6). We demonstrate in the present work how application of the ‘‘extended model space formulation’’ by Lindgren [J. Phys. B 7, 2441 (1974)] leads to convergence also for these cases.

Various energetic, geometric, and other properties for the ground and first vibrationally excited S states (L=0) of Coulomb three-body systems having unit charges are obtained by means of accurate exponential wave functions. The mass dependence of the properties for these systems has a relatively smooth form for the ground states. The results presented should be useful as a basis for further investigation of these three-body systems. The prethreshold (weakly bound) exotic nonsymmetric systems (${\mathit{d}}^{+}$${\mathit{t}}^{+}$${\mathit{p}}^{\mathrm{-}}$, ${\mathrm{\mu }}^{+}$${\mathrm{\pi }}^{+}$${\mathrm{\mu }}^{\mathrm{-}}$, and ${\mathrm{\mu }}^{+}$${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$) considered have sharp cluster structures, i.e., the lighter positive ion moves in the field of the neutral pair (e.g., ${\mathit{d}}^{+}$ in ${\mathit{d}}^{+}$${\mathit{t}}^{+}$${\mathit{p}}^{\mathrm{-}}$ moves in the field of ${\mathit{t}}^{+}$${\mathit{p}}^{\mathrm{-}}$ ).

Using an accurate ab initio interaction potential and converged close-coupling calculations to describe collision dynamics, we have calculated line-mixing effects for isotropic Raman spectra of ${\mathrm{D}}_2$ in He. Predicted asymmetries in the profiles of isolated lines caused by this line-mixing agree in sign and approximate magnitude with those recently observed in spectra of pure ${\mathrm{D}}_2$ which were attributed to this effect, lending support to this interpretation.

The radiative decay of double K-shell vacancies in zirconium (Z=40), molybdenum (Z=42), and palladium (Z=46) was observed using a high-resolution transmission crystal spectrometer. The samples were ionized by the impact of 100-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ ions. From the well resolved ${\mathit{K}}^{\mathrm{-}2}$→${\mathit{K}}^{\mathrm{-}1}$${\mathit{L}}^{\mathrm{-}1}$ transitions, the energies, linewidths, intensity ratios, and double K-shell ionization cross sections were deduced. The experimental energies and linewidths of the hypersatellites are compared with results of Dirac-Hartree-Slater calculations and with multiconfiguration-Dirac-Fock calculations, both taking into account the Breit interaction. For the extracted double K-shell ionization cross sections a comparison is made with the semiclassical approximation predictions within the independent-particle model.

Dispersion energies between molecules involving one in an electronically excited state are calculated using fourth-order perturbation theory within the framework of the multipolar form of quantum electrodynamics. There are significant differences between the energies for these cases and those where both molecules are in their ground states. The calculations are performed within the electric-dipole approximation for the interaction of the molecules with the electromagnetic field. The energies found are valid for all separations beyond the electronic overlap region. The results of previous investigations are shown to be incomplete and the origin of the incompleteness is traced to the neglect of certain real-photon contributions. The energies obtained in this paper are in agreement with our earlier calculations based on a form of response theory. They are made up of two types: one resulting from virtual-photon exchange and the other from real photons. The virtual-photon term has the same structure as the Casimir-Polder potential for ground-state molecules. The real-photon term is a polynomial in the inverse intermolecular separation ${\mathit{R}}^{\mathrm{-}1}$, in contrast to the modulated contributions in the previous incomplete investigations. The asymptotic forms for the total energy are discussed. The far-zone behavior is dominated by the real-photon term and shows an ${\mathit{R}}^{\mathrm{-}2}$ dependence. The near-zone behavior shows an ${\mathit{R}}^{\mathrm{-}6}$ dependence, arising in the multipolar formalism from both virtual- and real-photon exchange.

We describe two simple models for the determination of electron binding energies (${\mathit{E}}_{\mathit{b}}$) of several dipole-bound negative ions from field-detachment experiments and charge-transfer formation-rate-constant measurements in collisions between cold neutral molecules and Rydberg atoms. These models are able to correctly reproduce the experimental data, from which we deduce two compatible sets of ${\mathit{E}}_{\mathit{b}}$ values. A curve-crossing model for calculating anion formation rate constants allows us to qualitatively understand the sharply peaked shapes of the experimental curves and to derive a simple quantitative law between ${\mathit{E}}_{\mathit{b}}$ values and principal Rydberg quantum numbers at which anion formation rate constants are maximum.