The electronic structure of the ground state and some excited states of neutral atoms with the nuclear charge numbers $1<~Z<~10$ and their singly positive ions are investigated by means of our two-dimensional mesh Hartree-Fock method for strong magnetic fields $0.5<~\gamma <~10000.$ For $\gamma =10\mathrm{}000$ the ground-state configurations of all the atoms and ions considered are given by fully spin-polarized configurations of single-electron orbitals with magnetic quantum numbers ranging from $m=0\mathrm{}$ to $m=-N+1,$ where N is the number of the electrons. Focusing on the fully spin-polarized situation, we provide critical values of the magnetic-field strength for which crossovers with respect to the spatial symmetries of the ground state take place. It is found that the neutral atoms and singly charged positive ions with $2<~Z<~5$ have one fully spin-polarized ground-state configuration, whereas for $6<~Z<~10$ one intermediate fully spin-polarized configuration with an orbital of ${2p}_0$ type occurs.

The photoionization cross section $\sigma ,$ spin-polarization parameters P and Q, and the angular-distribution asymmetry parameter $\beta$ are calculated for the $7s$ state of francium for photon energies below 10 eV. Two distinct calculations are presented, one based on many-body perturbation theory and another based on the model potential method. Although predictions of the two calculations are similar, the detailed energy dependences of the photoionization parameters from the two calculations differ. From the theoretical p-wave phase shifts, we infer quantum defects for $p_{1/2}$ and $p_{3/2}$ Rydberg series, permitting us to calculate positions of experimentally unknown p states in francium.

We are reporting on a quantum-mechanically complete experimental and theoretical study of electron transfer from nonisotropic initial states to nonisotropic final states. The processes under consideration are the charge-exchange reactions ${\mathrm{H}}^{+}+\mathrm{Na}{(3p}_{\pm 1})\to \mathrm{H}{(2p}_{0,\pm 1})+{\mathrm{Na}}^{+}$ at a projectile energy of 1 keV, requiring, for a complete description, the determination of a minimum of five real parameters. Two of the three off-diagonal matrix elements and the trace of the density matrix (the total differential cross section) have been directly determined from an angular correlation analysis of the experiment. In parallel, atomic-orbital coupled-channel calculations have been performed. The results are used for the final analysis of the density-matrix elements, and allow a complete extraction of all amplitudes and phases from the experimental data.

We present a theoretical treatment of macroscopic quantum self-trapping (MQST) and quantum coherent atomic tunneling in a zero-temperature two-species Bose-Einstein condensate system in the presence of the nonlinear self-interaction of each species, the interspecies nonlinear interaction, and the Josephson-like tunneling interaction. It is shown that the nonlinear interactions can dramatically affect the MQST and the atomic tunneling, and lead to the collapses and revivals (CR) of the population imbalance between the two condensates. The competing effects between the self-interaction of each species and the interspecies interaction can lead to a quenching of the MQST and a suppression of the CR and the Shapiro-like steps of the atomic tunneling current. It is revealed that interatomic nonlinear interactions can induce coherent atomic tunneling between two condensates even though interspecies Josephson-like tunneling coupling does not exist.

Reverse-saturable absorbers are of considerable interest for optical limiting. Using the electric dipole perturbation, we derived the rate equation for a five-level system describing reverse-saturable absorbers. Traditional theories for the propagating laser beam in these materials are expressed in terms of the optical intensity. However, with the introduction of high-power short-pulsed lasers, the propagation of light in these materials may be subject to nonlinear phenomena such as self-focusing and self-phase modulation. Furthermore, conventional theories treat the laser light as a continuous wave or as a very broad temporal pulse in which dispersive effects are neglected. In order to incorporate these other nonlinear or dispersive effects, and therefore determine their influence in reverse-saturable absorbers, we derived an equation for the propagation of the electromagnetic field, rather than the intensity, coupled to the rate equations for a five-level system. We also coupled our theory to experimentally measurable parameters for these materials and detailed the various physical approximations used to obtain the rate equations.

We demonstrate that near-threshold decay processes may be accelerated by repeated measurements. Examples include near-threshold photodetachment of an electron from a negative ion, and spontaneous emission in a cavity close to the cutoff frequency or in a photon band-gap material.

The Greenberger-Horne-Zeilinger (GHZ) effect provides an example of quantum correlations that cannot be explained by classical local hidden variables. This paper reports on the experimental realization of GHZ correlations using nuclear magnetic resonance (NMR). The NMR experiment differs from the originally proposed GHZ experiment in several ways: it is performed on mixed states rather than pure states, and instead of being widely separated, the spins on which it is performed are all located in the same molecule. As a result, the NMR version of the GHZ experiment cannot entirely rule out classical local hidden variables. It nonetheless provides an unambiguous demonstration of the “paradoxical” GHZ correlations, and confirms the predictions of quantum mechanics.

The damped harmonic oscillator is a workhorse for the study of dissipation in quantum mechanics. However, despite its simplicity, this system has given rise to some approximations whose validity and relation to more refined descriptions deserve a thorough investigation. In this work, we apply a method that allows us to diagonalize exactly the dissipative Hamiltonians that are frequently adopted in the literature. Using this method, we derive the conditions of validity of the rotating-wave approximation (RWA) and show how this approximate description relates to more general ones. We also show that the existence of dissipative coherent states is intimately related to the RWA. Finally, through the evaluation of the dynamics of the damped oscillator, we notice an important property of the dissipative model that has not been properly accounted for in previous works, namely the necessity of new constraints to the application of the factorizable initial conditions.

We present a consistent framework of coupled classical and quantum dynamics. Our result allows us to overcome severe limitations of previous phenomenological approaches, such as evolutions that do not preserve the positivity of quantum states or that allow one to activate quantum nonlocality for superluminal signaling. A “hybrid” quantum-classical density is introduced, and its evolution equation derived. The implications and applications of our result are numerous: it incorporates the back-reaction of quantum on classical variables, and it resolves fundamental problems encountered in standard mean-field theories, and remarkably, also in the quantum measurement process; i.e., the most controversial example of quantum-classical interaction is consistently described within our approach, leading to a theory of dynamical collapse.

We theoretically study specific schemes for performing a fundamental two-qubit quantum gate via controlled atomic collisions by switching microscopic potentials. In particular we calculate the fidelity of a gate operation for a configuration where a potential barrier between two atoms is instantaneously removed and restored after a certain time. Possible implementations could be based on microtraps created by magnetic and electric fields, or potentials induced by laser light.

The development and initial results of a method for the determination of differential cross sections for electron scattering by molecular oxygen are described. The method has been incorporated into an existing package of computer programs which, given spectroscopic factors, dissociation energies and an energy-loss spectrum for electron-impact excitation, determine the differential cross sections for each electronic state relative to that of the elastic peak. Enhancements of the original code were made to deal with particular aspects of electron scattering from ${\mathrm{O}}_2,$ such as the overlap of vibrational levels of the ground state with transitions to excited states, and transitions to levels close to and above the dissocation energy in the Herzberg and Schumann-Runge continua. The utility of the code is specifically demonstrated for the “6-eV states” of ${\mathrm{O}}_2,$ where we report absolute differential cross sections for their excitation by 15-eV electrons. In addition an integral cross section, derived from the differential cross section measurements, is also reported for this excitation process and compared against available theoretical results. The present differential and integral cross sections for excitation of the “6-eV states” of ${\mathrm{O}}_2$ are the first to be reported in the literature for electron-impact energies below 20 eV.

Calculations are made of the resonance contribution to electron-impact excitation of Li-like ${}^{57}\mathrm{Fe}$ and H-like ${}^{14}\mathrm{N}$ ions to the upper hyperfine levels that produce $\mathrm{mm}$ lines of likely astrophysical interest. The resonance contribution is found to be very important for ${}^{57}\mathrm{Fe},$ but rather unimportant for ${}^{14}\mathrm{N}.$ Inclusion of the effect of radiative decay is found to be slightly significant for ${}^{57}\mathrm{Fe},$ but insignificant for ${}^{14}\mathrm{N}.$

A joint theoretical and experimental investigation on $e^{-}-{\mathrm{CH}}_4$ elastic scattering in the intermediate energy range is reported. More specifically, calculated cross sections in the (1–500)-eV incident energy range, as well as measured absolute cross sections from 100 to 500 eV, are presented. A complex optical potential consisting of static, exchange, correlation-polarization, plus absorption contributions is used to describe the electron-molecule interaction. The Schwinger variational iterative method combined with the distorted-wave approximation is applied to calculate the scattering amplitudes. The experimental absolute elastic differential cross sections are determined using the relative flow technique. In general, our calculated results are in very good agreement with our experimental data and with other experimental and theoretical results available in the literature. Our study has shown the significant role played by the absorption effects on the calculated cross sections for incident energies of 50 eV and above. The importance of the nonspherical components of the $e^{-}-{\mathrm{CH}}_4$ interaction potential has also been verified in our calculations.

Vibrational-resolved ion and ion-pair yield spectra of ${\mathrm{N}}_2$ and CO taken at their $1\overrightarrow{s}{\pi }^{*}$ resonance excitations reveal decay-channel-dependent core hole lifetimes. The linewidths of the yield spectra tend to be shortened when the number of electrons ejected in the deexcitation process increases, finally becoming narrower than the total ion yield or absorption natural lifetime width of the core-excited states of ${\mathrm{N}}_2$ and CO, respectively. In contrast to this, the linewidths of the yield spectra for the singly charged molecular ions, ${\mathrm{N}}_2^{+}$ and ${\mathrm{CO}}^{+},$ are shown to be broader than their corresponding natural linewidths. This linewidth shortening and broadening observed in the ion yield spectra are explained by the effect of different internuclear distances on the lifetime of an excited molecular state.

We report electron spectra obtained from grazing incidence collisions of ${\mathrm{He}}^{+}$ ions with an Al(111) surface at collision energies ranging from 200 eV up to a few keV. The part of the spectra due to potential electron emission is analyzed in terms of a rate equation model that uses transition rate constants for resonant charge exchange– and Auger-type processes, and interaction energies between the projectile and the surface. It is shown that, if recently obtained theoretical interaction energies and rate constants are used in the model, calculated and experimental electron spectra agree within experimental error. From this it is concluded that (i) the theoretical input information on the He/Al system is consistent with experiment and (ii) the rate equation model is a valuable tool for analyzing electron spectra.

The two-dimensional infrared-infrared-visible (IIV) sum frequency generation (SFG) is theoretically considered. IIV-SFG is a two-dimensional extension of the infrared-visible SFG, since it involves two vibrationally resonant infrared fields of which frequencies are experimentally controlled. An additional visible field is used to probe the vibrational coherence dynamics. This IIV-SFG technique can be used in studying intramolecular and intermolecular vibrational interactions in isotropic condensed phases as well as on surfaces and interfaces. Due to the selection rules that are quite different from the one-dimensional vibrational spectroscopies, the IIV-SFG signal intensity is determined by the vibrational anharmonic couplings and nonlinearities of the polarizability and dipole moments with respect to the vibrational coordinates. In particular, for a model system consisting of two neighboring molecules or subgroups in a macromolecule, how the spatial and structural information can be extracted from the IIV-SFG measurement is discussed.

The $p+\mathrm{H}$ collision in the presence of an intense laser field is studied by numerically solving the time-dependent one-dimensional Schrödinger equations. It is found that due to the enhanced ionization at the critical internuclear distance, the colliding particles can gain energy from the applied laser field. Also, the influences of the space distributions of the electronic and nuclear wave packets on the enhanced ionizations are discussed.

We study the dynamical response of a spinor Bose condensate under the influence of external magnetic fields. A rich set of phenomena are investigated such as stochastization in population evolution, metastability in spin composition, and dynamical localization in spin space.

We present a fully quantum-mechanical treatment of a single-mode atomic cavity with a pumping mechanism and an output coupling to a continuum of external modes. This system is a schematic description of an atom laser. In the dilute limit where atom-atom interactions are negligible, we have been able to solve this model without making the Born and Markov approximations. When coupling into free space, it is shown that for reasonable parameters there is a bound state which does not disperse, which means that there is no steady state. This bound state does not exist when gravity is included, and in that case the system reaches a steady state. We develop equations of motion for the two-time correlation in the presence of pumping and gravity in the output modes. We then calculate the energy spectrum of the steady-state output flux from the laser. We present a “self-consistent Markov approximation” which allows efficient calculation of this energy spectrum well above threshold.

The atomic tensor polarizability in a pulsed optical field can generate a phase-shift scaling quadratically with the interfering path number in a multiple beam Ramsey experiment. The phase can be interpreted as an internal atomic-state version of the electric (or scalar) Aharonov-Bohm effect. In the absence of classical forces, this nonlinear phase shift causes collapse and revival of the Airy-function-like interference pattern. The technique also holds promise for experiments testing for a permanent electric dipole moment of an atom.