We report results of a systematic study of the energy-field dependence of the four elementary periodic orbits of the electron motion of the hydrogen atom in crossed magnetic-electric fields from weak to strong perturbation, up into the continuum regime. We find the classical dynamics of the crossed-field atom to be intimately connected to that of the Sun-Earth-Moon three-body problem in Hill’s approximation, exhibited by striking similarities of the four elementary orbits of the atomic and celestial systems in their energy-field dependence. © 1996 The American Physical Society.

The time-dependent mean-field approximation for two dynamically coupled subsystems is extended to include correlation effects between the subsystems, allowing for decorrelation processes to develop in the reduced density matrices. The extended scheme is formulated in terms of the truncation to M terms of the Schmidt decomposition of the full density matrix. This M natural orbitals truncation scheme is compared to the exact numerical solution for a system of two coupled anharmonic oscillators in a factorized initial state. It is found that the approximation M=3 gives a good approximation to the exact results over several characteristic times of the system. © 1996 The American Physical Society.

We present a nonlocal kinetic-energy functional which includes an anisotropic average of the density through a symmetrization procedure. This functional allows a better description of the nonlocal effects of the electron system. The main consequence of the symmetrization is the appearance of a clear shell structure in the atomic density profiles, obtained after the minimization of the total energy. Although previous results with some of the nonlocal kinetic functionals have given incipient structures for heavy atoms, only our functional shows a clear shell structure for most of the atoms. The atomic total energies have a good agreement with the exact calculations. Discussion of the chemical potential and the first ionization potential in atoms is included. The functional is also extended to spin-polarized systems. © 1996 The American Physical Society.

It has long been assumed that electron correlation effects are relatively unimportant for describing dipole-bound anionic states. It is shown here that this assumption is incorrect: high-level electronic structure calculations on the dipole-bound anion states of ${\mathrm{CH}}_3$CN, ${\mathrm{C}}_3$${\mathrm{H}}_2$, and ${\mathrm{C}}_5$${\mathrm{H}}_2$ reveal that for these species a large fraction of the electron binding energy derives from the dispersion-type interaction between the loosely bound electron and the neutral molecule. The predicted values of the electron affinities of the dipole bound states of ${\mathrm{CH}}_3$CN and ${\mathrm{C}}_3$${\mathrm{H}}_2$, 108 and 173 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, are in excellent agreement with the recent experimental results, 93 ${\mathrm{cm}}^{\mathrm{-}1}$ and 171±50 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. The predicted value for ${\mathrm{C}}_5$${\mathrm{H}}_2$ is 614 ${\mathrm{cm}}^{\mathrm{-}1}$. © 1996 The American Physical Society.

The Kohn-Sham effective potentials for selected lithium iso-electronic series (Z=3–10) and for the nitrogen atom have been obtained. Configuration interaction and the spin-unrestricted Hartree-Fock (SUHF) densities are used as input and the Kohn-Sham effective potentials for spin-majority and -minority channel are deduced, from which the exchange and correlation contributions are obtained. Accurate values are obtained for such quantities as exchange and exchange-correlation energies, Kohn-Sham orbital energies, and kinetic contributions to exchange and correlation energies. Comparison is made between the exchange potentials obtained from SUHF densities and from optimized effective potential calculations. The structure of the correlation potentials is discussed, and emphasis is put on how the potentials behave as functions of the electron density and the spin density. © 1996 The American Physical Society.

The semiclassical expression for the spectral density (Gutzwiller’s trace formula) can be viewed as a Fourier expansion in terms of classical (quasi)periodic orbits. The inverse Fourier transform of the full quantum spectral density therefore not only gives the periodic orbit spectrum in the semiclassical limit, but may also provide a tool to characterize the quantum spectrum. We investigate the scaled energy Fourier transform spectrum of the hydrogen atom and doubly excited states of helium. For exactly solvable hydrogenic Rydberg spectra we study the ‘‘window’’ effect due to the finite length of the spectrum, the influence of resonance width, and of quantum defects. In helium a large number of periodic orbits including previously unknown orbits can be identified. © 1996 The American Physical Society.

We report on a numerical study of the density matrix functional introduced by Lieb, Solovej, and Yngvason for the investigation of heavy atoms in high magnetic fields. This functional describes exactly the quantum mechanical ground state of atoms and ions in the limit when the nuclear charge Z and the electron number N tend to infinity with N/Z fixed, and the magnetic field B tends to infinity in such a way that B/${\mathit{Z}}^{4/3}$→∞. We have calculated electronic density profiles and ground-state energies for values of the parameters that prevail on neutron star surfaces and compared them with results obtained by other methods. For iron at B=${10}^{12}$ G the ground-state energy differs by less than 2% from the Hartree-Fock value. We have also studied the maximal negative ionization of heavy atoms in this model at various field strengths. In contrast to Thomas-Fermi type theories atoms can bind excess negative charge in the density matrix model. For iron at B=${10}^{12}$ G the maximal excess charge in this model corresponds to about one electron. © 1996 The American Physical Society.

We have measured five-photon decay of orthopositronium (o-Ps) and obtained the branching ratio of o-Ps→5γ to o-Ps→3γ as [2.$2_{\mathrm{-}1.6}^{+2.6\mathrm{}}$(stat.)±0.5(syst.)]×${10}^{\mathrm{-}6}$, being consistent with the lowest-order QED calculation. Hence the five-photon decay process does not account for the discrepancy between the experimentally obtained decay rate for o-Ps and the theoretically calculated decay rate for o-Ps→3γ. © 1996 The American Physical Society.

Charge-conjugation (C) invariance demands that ortho- and parapositronium decay into an odd number and an even number of photons, respectively. For the test of the C invariance, we searched for a C-violating process, i.e., four-photon decay from orthopositronium, using a multi-γ-ray spectrometer. No C-violating process was observed leading to the upper limit as 2.6×${10}^{\mathrm{-}6}$ at 90% confidence level for the branching ratio of four- to three-photon decays from orthopositronium. © 1996 The American Physical Society.

The effect of molecular dissociation on the exchange-correlation Kohn-Sham potential ${\mathit{v}}_{\mathrm{xc}}$ has been studied by the construction of ${\mathit{v}}_{\mathrm{xc}}$ from the ab initio correlated density ρ for the monohydrides XH (X=Li, B) at several bond distances R(X—H). The molecular dissociation manifests itself in the formation of a characteristic peak of ${\mathit{v}}_{\mathrm{xc}}$ in the bonding region. The partially integrated conditional probability amplitude Φ(${\mathit{s}}_1$, ${\mathit{x}\to }_2$,..., ${\mathit{x}\to }_{\mathit{N}}$|${\mathit{r}\to }_1$) has been used to analyze the behavior of ${\mathit{v}}_{\mathrm{xc}}$ by means of a partitioning into various components: the potential of the exchange-correlation hole ${\mathit{v}}_{\mathrm{xc}}^{\mathrm{hole}}$, the kinetic component ${\mathit{v}}_{\mathit{c},\mathrm{kin}}$, and the ‘‘response’’ component ${\mathit{v}}_{\mathrm{resp}}$. These components have been constructed from ab initio correlated first- and second-order density matrices. The peak of ${\mathit{v}}_{\mathrm{xc}}$ in the bonding region has been represented as a combination of the corresponding peak of ${\mathit{v}}_{\mathit{c},\mathrm{kin}}$ and the positive buildup of ${\mathit{v}}_{\mathrm{resp}}$ around the more electronegative atom H. Using the conditional amplitude analysis, the asymptotical expressions have been obtained for ${\mathit{v}}_{\mathrm{resp}}$ and its positive buildup for the general case of a heteroatomic molecule AB. The dependence of the Kohn-Sham energy characteristics such as the kinetic energy of noninteracting particles ${\mathit{T}}_{\mathit{s}}$, the kinetic part of the exchange-correlation energy ${\mathit{T}}_{\mathit{c}}$, and the energy of the highest occupied molecular orbital ${\mathrm{\epsilon }}_{\mathit{N}}$ on the bond distance has been studied. The results obtained have been compared with those for the homoatomic two-electron ${\mathrm{H}}_2$ molecule. © 1996 The American Physical Society.

A combined experimental and theoretical study of the polarizabilities of the ground-state zinc atom is presented. In our interferometric measurements we have obtained the frequency dependence of the linear dipole polarizability α(ω) between ω=0.05${\mathit{E}}_{\mathit{H}}$${\mathrm{ħ}}^{\mathrm{-}1}$ and 0.14${\mathit{E}}_{\mathit{H}}$${\mathrm{ħ}}^{\mathrm{-}1}$ (here ${\mathit{E}}_{\mathit{H}}$ is the Hartree energy). A detailed theoretical CCSD(T) study (coupled-cluster with single and double excitations and perturbational treatment of triple excitations) gives the static (zero-frequency) values of the linear dipole polarizability α, the second dipole hyperpolarizability γ, the quadrupole-quadrupole polarizability ${\mathrm{\alpha }}_2$, and the dipole-dipole-quadrupole polarizability B. From these studies we conclude with the static values of α=38.8(8)${\mathit{e}}^2$${\mathit{a}}_0^2$${\mathit{E}}_{\mathit{H}}^{\mathrm{-}1}$ (from experiment), α=39.2(8)${\mathit{e}}^2$${\mathit{a}}_0^2$${\mathit{E}}_{\mathit{H}}^{\mathrm{-}1}$ (from theory), γ=31(6)×${10}^3$${\mathit{e}}^4$${\mathit{a}}_0^4$${\mathit{E}}_{\mathit{H}}^{\mathrm{-}3}$, ${\mathrm{\alpha }}_2$=324.8(16.2)${\mathit{e}}^2$${\mathit{a}}_0^4$${\mathit{E}}_{\mathit{H}}^{\mathrm{-}1}$, and B=-2.37(24)×${10}^3$${\mathit{e}}^3$${\mathit{a}}_0^4$${\mathit{E}}_{\mathit{H}}^{\mathrm{-}2}$. © 1996 The American Physical Society.

Ab initio molecular-orbital methods using a newly constructed large, flexible basis set of the atomic-natural-orbital type and extensive treatments of electron correlation were used to accurately predict the ground-state potential-energy curves of ${\mathrm{Se}}_2$ and ${\mathrm{Se}}_2^{\mathrm{-}}$. In addition, the potential curves of low-lying electronically excited states of the ${\mathrm{Se}}_2^{\mathrm{-}}$ anion, which are coupled to the ground state through the electronic dipole operator, are determined. Calibration of the accuracy expected for the employed theoretical models is achieved by calculations on electronically excited states of neutral ${\mathrm{Se}}_2$, for which accurate experimental data are available. Besides reporting accurate predictions for spectroscopic constants, electron affinities, and transition matrix elements, particular emphasis is laid on the use of the computed dimer potential-energy curves for interpreting the recently investigated peculiar optical properties of diatomic selenium species embedded in solid host matrices. Further, the equilibrium geometries and relative stabilities of the ${\mathit{C}}_{2\mathit{v}}$ and ${\mathit{D}}_{3\mathit{h}}$ forms of the neutral ${\mathrm{Se}}_3$ cluster, the electron affinity of ${\mathrm{Se}}_3$, and the low-lying electronic states of ${\mathrm{Se}}_3^{\mathrm{-}}$ have been investigated theoretically.

The electron self-energy correction of the order α(Zα${)}^2$${\mathit{E}}_{\mathit{F}}$ to the ground-state hyperfine splitting in hydrogenic atoms is calculated using a semianalytical method. The correction is divided into three parts by introducing two auxiliary parameters. The low-energy part corresponds to the nonrelativistic limit, where photon energy is of the order m${\mathrm{\alpha }}^2$, and the effective hyperfine interaction is given by ${\mathrm{\delta }}^3$(r). In the middle-energy part electron and photon momenta are of the order mα and m, respectively. This part is calculated using on-shell electron form factors. The high-energy part corresponds to the S-matrix amplitude for the forward scattering. The final value does not depend on auxiliary parameters and amounts to ΔE=(α/π)(Zα${)}^2$${\mathit{E}}_{\mathit{F}}$×17.122. It is larger than the previous value of Sapirstein ∼15.10(29) and significantly alters theoretical predictions. © 1996 The American Physical Society.

We have measured the atomic transition probabilities of 80 spectral lines belonging to 3s-3p, 3p-3d, and 3p-4s multiplets of O II using a wall-stabilized arc. We applied recent lifetime results obtained for several 3p${\mathrm{}}^4$D levels to normalize our relative data to an absolute scale, and we estimate that the uncertainties of our data are in the range from 8% to 9%. The agreement of our multiplet values with recent advanced calculations is typically in the range from 5% to 15%. For many lines of the 3p-3d multiplets, the agreement of our experimental data with intermediate coupling calculations is much better than with LS-coupling data. © 1996 The American Physical Society.

Angular distributions of ${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$ from CO were measured, following vibrationally resolved C 1s excitations into the 2pπ, 3sσ, and 3pπ orbitals as well as into higher unresolved orbitals. A time-of-flight mass spectrometer, with a multihit-type position-sensitive anode, was used for the measurements. The anisotropy parameters (β) of ${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$ approach their theoretically expected values as the released kinetic energy in the fragmentation increases. The value of the β parameters remains constant for all vibrational states within each orbital. © 1996 The American Physical Society.

The Faddeev multiple-scattering formalism in second order is modified to treat lower projectile energies. The electronic part of the amplitude is evaluated using a Hartree-Fock approximation to the helium wave function and a consistent screened target potential for the active electron. Off-energy-shell scattering states appearing in the electronic part of the full amplitude are corrected for loss of normalization. The high velocity Faddeev nuclear-scattering contribution is normalized to the eikonally transformed electronic amplitude at large angles. An application to proton-helium collisions at 293 keV shows good agreement with the experimental data. The effects of the various approximations are studied. © 1996 The American Physical Society.

A general theory of slow-atom collisions is presented with special emphasis on the effects of nuclear statistics and atomic fine and/or hyperfine structures. Symmetry properties of the collision complex and correlations between the molecular states and the separated-atom states are carefully examined. The frame transformations between various angular momentum coupling schemes are derived, which, in combination with the multichannel quantum defect theory, provides a solid foundation for the computation and the physical interpretation of slow-atom collision processes. The theory reduces to those of Stoof et al. [Phys. Rev. B 38, 4688 (1988)] and Zygelman et al. [Phys. Rev. A 49, 2587 (1994); 50, 3920 (1994)] in their respective ranges of validity. © 1996 The American Physical Society.

The direct radiative recombination (RR) to n=3, 4, and 5 levels and the resonant dielectronic recombination (DR) cross section involving 2p→3d excitation of Sc-like barium ${\mathrm{Ba}}^{35+}$ and Ti-like barium ${\mathrm{Ba}}^{34+}$ are calculated and compared with a recent electron beam ion trap experiment at the National Institute of Standards and Technology. Assuming a fractional population of ${\mathrm{Ba}}^{35+}$ in the trap of about 30%, we obtain good agreement between theory and experiment for the cross section ratio σ(DR)/σ(RR)(n=4), as well as for σ(RR)(n=5)/σ(RR)(n=4). The result confirms again that the simple angular momentum averaged procedure can be effective in treating DR for heavy open-shell ions when the energy resolution is not high. A large, broad peak below 2 keV in the x-ray spectrum is being theoretically examined. © 1996 The American Physical Society.

Using light-induced drift as a tool to separate nuclear spin isomers, we have studied the ortho-para conversion process for gaseous ${}_{}{}^{13}{}_{}{}^{}\mathrm{CH}_3^{}{}_{}{}^{}$F interacting with ${\mathrm{O}}_2$, ${\mathrm{N}}_2$, ${\mathrm{CH}}_3$Cl, and ${\mathrm{CH}}_3$F itself. It is found that the conversion rate is significantly smaller for collisions with ${\mathrm{O}}_2$ or ${\mathrm{N}}_2$ than with ${\mathrm{CH}}_3$Cl or ${\mathrm{CH}}_3$F. The data show that intermolecular magnetic interactions do not play a leading role in the conversion. In order to test an alternative model based on mixing of states due to intramolecular interactions, pressure broadening has been measured for these systems. The observed correlation between conversion rates and broadening coefficients supports this mixing-of-states model. © 1996 The American Physical Society.

The x-ray spectra produced from 7.0-keV electrons in bombardment of thick targets of silver and gold have been measured using a Si(Li) detector at 90° to the incident electron-beam direction. The spectra were corrected for electron backscattering, energy loss, photon attenuation in the target, and for inherent filtration, allowing a determination of absolute intensity for x-ray emission. The experimental spectra have been compared with two models, (i) a modified Kramers formula, and (ii) a semiempirical expression. There is an overall agreement between the experiment and the predictions of the models both for the shape and for the absolute magnitude of the thick-target bremsstrahlung intensity to within 21%. © 1996 The American Physical Society.