After making a general description of the properties of macroscopic superpositions of collective atomic states, we present a detailed analysis of a method for their experimental realization recently proposed by C.C. Gerry and R. Grobe [Phys. Rev. A 56, 2390 (1997)] which is based on cavity QED techniques involving the manipulation of electromagnetic and matter fields. Specific conditions and limitations of the procedure are discussed.

Ultraviolet-visible fluorescence $(300\mathrm{}\hspace{0.5em}\mathrm{nm}<~\lambda <~500\hspace{0.5em}\mathrm{nm})$ of argon is investigated in the Ar $2p$ excitation regime $(240\mathrm{}\hspace{0.5em}\mathrm{eV}<~E<~260\hspace{0.5em}\mathrm{eV})$ using monochromatic synchrotron radiation for primary excitation. Fluorescence excitation spectra and dispersed fluorescence spectra at constant photon energy are reported as well as time-resolved dispersed fluorescence profiles. Various radiative processes are assigned, indicating that these processes are primarily due to the relaxation of ${\mathrm{Ar}}^{+}.$ A distinct state-selective behavior is found for some fluorescence excitation spectra, whereas others show no state selectivity at all. Time-resolved experiments give further insight into this state-selectivity and excited-state lifetimes. The results indicate that excited cation states are either directly formed as a result of primary Ar $2p$ excitation and subsequent electronic relaxation or via subsequent radiative cascades. The present results are consistent with earlier work on resonant Auger spectroscopy, extending the dynamic range of processes occurring in the Ar $2p$ regime into the nanosecond time regime.

We present a $B$-spline-based multichannel $K$-matrix method following an extension from a $B$-spline-based configuration-interaction approach. Theoretical procedures are presented in detail for its application to photoionization from two-electron and divalent (e.g., alkaline-earth) atoms. Numerical examples, including doubly excited photoionization spectra above the $N=2\mathrm{}$ threshold from the He $1s2s{}^{1}S$ metastable state, are also given.

The simultaneous electron capture and the ejection of another electron by which a doubly charged He ion and a Ps atom are produced in a single collision between a positron and a He atom is investigated theoretically. The angular distributions of the Ps atom as well as of the ejected electron are studied in the low-and intermediate-(100–500 eV) energy regime with respect to the threshold energy for the transfer ionization process. The electron-electron correlation effect which mainly governs such two-electron transition processes has been taken into account in both the initial and final channels. The fully differential cross sections for different dynamics reveal some structures which are to be verified by the future experiments.

The photoabsorption process in atomic uranium has been investigated experimentally and theoretically in the 15–150-eV region. Using the dual laser plasma technique, the $6p$ photoabsorption spectrum has been recorded while for the first time the $5d$ region has been remeasured photoelectrically using both photoabsorption and photoion spectroscopy. Interpretation of the photoabsorption spectra is supported by Hartree-Fock calculations which take into account spin-flip decay and the interaction of many discrete states with many continua. The $6p$ spectrum is entirely dominated by spin-orbit split $6\overrightarrow{p}6d$ transitions. The $5d$-subshell photoabsorption is shown to consist predominantly of discrete $5\overrightarrow{d}5f$ excitations; here the electrostatic and spin-orbit interactions are comparable in strength.

We present one-dimensional particle-in-cell simulations of multielectron dissociative ionization (MEDI) of diatomic and linear triatomic molecules exposed to an intense subpicosecond laser pulse. Our method allows us to deal with all the electrons and nuclei explicitly, and to follow the temporal evolution of MEDI. The simulation results confirm that ionization probability is strongly enhanced at a certain critical internuclear distance. This occurs when the internal potential barriers surpass the external one and electrons are localized in higher potential wells. The mechanism of enhanced ionization in a diatomic and a triatomic molecule can be understood in a similar way, though all three potential wells participate in it for the triatomic case. Nevertheless, the occurrence of ionization is not necessarily localized at the critical distance. In addition, our simulations show that there is no significant difference in the final charge state of ion fragments between diatomic and linear triatomic molecules. The relative smallness of the critical distance in our results may suggest that not only the ionization taking place preferably near the critical distance, but also the non-Coulombic nature of the molecular potential curves, cause the experimentally observed kinetic-energy defects.

We analyze how nonclassical features of squeezed radiation (in particular, the sub-Poissonian noise) are degraded when it is transmitted through an amplifying or absorbing medium with randomly located scattering centra. Both the cases of direct photodetection and of homodyne detection are considered. Explicit results are obtained for the dependence of the Fano factor (the ratio of the noise power and the mean current) on the degree of squeezing of the incident state, on the length and the mean free path of the medium, the temperature, and on the absorption or amplification rate.

The phenomenon of “collapse” of $5d$ and $5f$ orbitals in the ${\mathrm{Yb}}^{+}$ isoelectronic sequence occurring when the increasing Coulomb and exchange attractions overcome centrifugal repulsion is studied in this paper using both the relativistic model-potential and single-configuration Dirac-Fock techniques. As before for xenon and cesium sequences, valence-core electron exchange is found to play an essential role in the collapse phenomenon. Furthermore, the collapse greatly enhances the sensitivity of the properties of the orbitals that undergo this phenomenon to small changes in the effective potential, so that even much more subtle effects such as nonlocal exchange effects or core-valence electron correlation (core polarization) became more than usually important.

The local properties and accuracy of the positronium negative-ion $\left({\mathrm{Ps}}^{-}\right)$ ground-state wave functions obtained by the stochastic variational method (SVM) and by direct solution of the Schrödinger equation with the help of the correlation-function hyperspherical-harmonic method (CFHHM) are studied and compared. Though the energy, calculated by both methods, agrees to up to ten digits, the amplitudes of the values of the operator $D=H\Psi /E\Psi -1,$ characterizing local deviation of the wave function from its true value, in all of the coordinate space in the SVM are consistently larger (by up to five orders of magnitude) than in the CFHHM, despite the fact that the SVM observables except $〈\delta \left({\mathbf{r}}_k\right)〉$ converge to significantly more digits than the CFHHM observables for their respective selected bases.

We present a general formulation of the optical forces on a three-level atom coupled to two coherent light beams. Analytical expressions for the forces are derived using a master equation formulation in the intense field limit. All three configurations such as Raman, V shape, and cascade are discussed with special values of the detuning parameters for analytical simplicity and transparency.

Using a formal scattering theoretical approach, we develop a nonperturbative quantum electrodynamics theory to describe high-order harmonic generation (HHG). This approach recovers the semiclassical interpretation that HHG results from the recombination of photoelectrons, ionized by the laser field, with the parent ions, and gives the same phenomenological cutoff law. The HHG emission rate is expressed as an analytic closed form. We also discuss the connection between HHG and the above threshold ionization from the formal scattering viewpoint.

We investigate atomic population dynamics of two Bose-Einstein condensate (BEC) states that are in different traps or in the same trap and are optically coupled through a common excited state by Raman laser pulses. For large interatomic interactions, almost all of the atoms that are initially in one of the traps (or one of the internal states) are either completely transferred into the other state or stay in the initially occupied state after the laser pulses, and the atoms (populations) are never distributed to both condensate states at the same time. Applications to the atomic logic gate and to output couplers of the atom laser are pointed out.

Three-photon-resonant excitation and subsequent ionization of xenon at high concentrations has been studied utilizing both Bessel and Gaussian beams. Comparisons of analogous results with the two modes of focusing are made. The excitation-ionization profiles can be analyzed on the basis of the well-studied interference effect involving combined contributions of three- and one-photon transition amplitudes in resonant atomic excitation. We show that the principal features of the excitation problem with Bessel beams can be simulated by a simple picture involving crossed plane-wave beams. Using this approach it has been shown that pressure-broadened and shifted lines that result from the interference-based cooperative Lorentzian line shift constitutes the main pressure-dependent peak of off-resonance excitation profiles produced in Bessel beams.

A quantum-mechanical close-coupled calculation of field-free collisions of cold metastable helium atoms has been performed. Theoretical predictions of the rate coefficients for elastic processes and losses due to the Penning and associative ionization processes are reported for temperatures ranging from 1 $\mu \mathrm{K}$ to 1 K. The sensitivity of the rate constants to uncertainties in the molecular potentials and autoionization widths has been investigated. The total ionization loss rates were found to be sensitive to the short-range form of the ${}^1{\Sigma }_g^{+}$ molecular potential and the autoionization widths, while the total elastic rate constants exhibit greater sensitivity to the short-range ${}^5{\Sigma }_g^{+}$ molecular potential.

We investigate, theoretically, the properties of an ideal trapped gas in a time-dependent harmonic potential. Using a scaling formalism, we are able to present exact analytical results for two important classes of experiments: free expansion of the gas upon release of the trap and the response of the gas to a harmonic modulation of the trapping potential. We present specific results relevant to current experiments on trapped fermions.

We analyze the separability properties of density operators supported on $C^2\otimes C^N$ whose partial transposes are positive operators. We show that if the rank of $\rho$ equals N then it is separable, and that bound entangled states have ranks larger than N. We also give a separability criterion for a generic density operator such that the sum of its rank and the one of its partial transpose does not exceed $3N.$ If it exceeds this number, we show that one can subtract product vectors until it decreases to $3N,$ while keeping the positivity of $\rho$ and its partial transpose. This automatically gives us a sufficient criterion for separability for general density operators. We also prove that all density operators that remain invariant after partial transposition with respect to the first system are separable.

Relative differential cross sections for excitation of vibrational levels of HF up to $v=4\mathrm{}$ by slow electron impact have been measured as a function of incident electron energy. Broad oscillatory structures converging to the dissociative attachment threshold, already known in the $v=\overrightarrow{0}3$ channel, have now also been observed in the $v=\overrightarrow{0}4$ channel at energies above 1.8 eV. They appear at different energies in the two final channels, revealing their boomerang character. They can be rationalized as a consequence of boomerang motion of the nuclei, initiated either by a broad repulsive ${\sigma }^{*}$ resonance, or resulting from broad overlapping vibrational Feshbach resonances. This observation complements results in other diatomic hydrides and reveals that boomerang structures are present in all members of the series $\mathrm{H}X$ $(X=\mathrm{H},$ F, Cl, Br), once thought to have no structures of vibrational origin in the cross sections at low energies, because of the extremely short lifetime (or absence) of ${\sigma }^{*}$ resonances. The shape of the $v=\overrightarrow{0}4$ cross section differs from those of the lower channels also near threshold—it has a weak shoulder at threshold followed by a rounded hump peaking ∼80 meV above threshold, whereas the cross sections for the $v=1\mathrm{}–3$ channels have (within resolution) vertical onsets and narrow peaks at thresholds.

The total and partial photoionization cross sections, branching ratios, and photoelectron angular-distribution asymmetry parameters have been calculated for atomic radon $\mathrm{}(Z=86\mathrm{})\mathrm{}$ for all subshells from the $6p$ valence shell down to the deep $n=3\mathrm{}$ subshells. The relativistic random-phase approximation, the relativistic random-phase approximation modified to include relaxation effects, and the relativistic random-phase approximation modified to include relaxation effects and Auger decay were all used to determine the relative importance of various many-body effects such as interchannel coupling, core relaxation, and Auger decay. Comparisons are made between the various theoretical models and experimental data for the total cross sections. Interchannel coupling among many channels was found to be important in calculations of the total cross sections for most shells, and relaxation effects were found to be substantial for the $n=5\mathrm{}$ shell.

We present a theoretical model for the recent experimental finding by Brunelle et al. [Phys. Rev. A 59, 4456 (1999)] of a reduction of average charge per ion in fast ${\mathrm{C}}_n$ clusters traversing thin carbon foils. This vicinage effect, due to the interaction of the electrons bound at each constituent ion with the neighboring ions, is described by the linear-response formalism, coupled with a statistical description of the cluster structure and the Brandt-Kitagawa variational theory for the bound electrons. The results obtained agree well with the experimental data.