The symmetric-eikonal model for single excitation of multielectronic targets is extended to include Hartree-Fock-Slater initial and final bound states. These wave functions are obtained by solving numerically the time-independent Schrödinger equation with a model potential. Total cross sections for excitation of $\mathrm{H}\mathrm{e}\left(1\mathrm{}{}^{1}S\right)$ and $\mathrm{H}\mathrm{e}\left(2\mathrm{}{}^{1}S\right)$ by proton impact are presented in comparison with available experimental data and other theoretical models. The model gives an accurate description at intermediate and high impact energies. Results are also presented for excitation of two-electron heavy ions and for alkaline targets.

Using monochromatized synchrotron radiation and the atomic-beam technique with evaporating thin wires, the vacuum-ultraviolet photoionization of Ta and Re in the region of the $5p$ and $4f$ excitation was studied. The photoion yield spectra are compared with calculations of the corresponding photoionization cross section in the region of the $5p$ and $4f$ resonances. In spite of the large number of transitions there is good qualitative agreement between the main features of the experimental and theoretical results.

An algebraic coupled-state calculation of positron-hydrogen scattering carried out with an enlarged eight-state (E8S) coupling scheme, that is composed of a sufficiently high number of short-ranged correlation terms and eight hydrogen and positronium states $\left[\right(1s,2s,2p,3\mathrm{}\overline{p}\left)\mathrm{H}-\right(1s,2s,2p,3\mathrm{}\overline{p}\left)\mathrm{Ps}\right],$ provides results of the phase shift and cross section, agreeing excellently with our accurate enlarged six pseudostate (E6PS) values in both $e^{+}-\mathrm{H}$ and $\mathrm{Ps}\mathrm{}\left(1s\right)-p$ entrance channels. Our results serve to assert that very accurate values of the phase shift and cross section in both entrance channels have been attained with our E6PS, E8PS, and E8S Harris-Nesbet calculations. The present E8S calculation supersedes our deficient modified enlarged six-state one done previously.

Electron-impact ionization cross sections have been measured for the hydrogenlike ions of molybdenum, dysprosium, gold, and bismuth at selected electron energies between 1.3 and 3.9 times threshold. The cross sections were obtained from x-ray measurements of the equilibrium ionization balance in an electron beam ion trap. The measured cross sections agree with recent relativistic distorted-wave calculations that include both the Moeller interaction and exchange.

A crossed-beam apparatus incorporating photoelectron spectroscopy has been used to determine the partial cross section for photodetaching an electron from the excited C${}^{-}{(}^{2}D)$ ion via the ${}^{3}Pks,d$ channels. The measurement, which was based on the saturation of the photodetachment process, was normalized to that of D${}^{-}$ taken under the same experimental conditions. At a photon energy of 2.076 eV, the cross section was found to be 13(2) Mb. Simultaneously, the beam fraction in the excited state in the incoming C${}^{-}$ ion beam was found to be 0.5(1).

The collision dynamics of He single ionization by $3.6{\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}\mathrm{}\mathrm{Se}}^{28+}$ impact was explored using the reaction microscope of the Gesellschaft für Schwerionenforschung, a high-resolution integrated multielectron recoil-ion momentum spectrometer. The complete three-particle final-state momentum distribution (nine Cartesian components $p_i$) was imaged with a resolution of $\Delta p_i\approx \pm 0.1\mathrm{a}.\mathrm{u}.\mathrm{}$ by measuring the three momentum components of the emitted electron and the recoiling target ion in coincidence. The projectile energy loss has been determined on a level of $\Delta E_p{/E}_p\approx {10}^{-7}$ and projectile scattering angles as small as $\Delta \vartheta \approx {10}^{-7}\mathrm{rad}$ became accessible. The experimental data which are compared with results of classical trajectory Monte Carlo calculations reveal an unprecedented insight into the details of the electron emission and the collision dynamics for ionization of helium by fast heavy-ion impact.

I recently reported [Phys. Rev. A 51, 2222 (1995)] on simulations of charge-exchange and excitation processes in collisions between a helium nucleus and a ground-state hydrogen atom that were performed in the multichannel perturbed-stationary-state framework without employing electron translation factors [Phys. Rev. A 51, 2199 (1995)]. A simulation with 45 adiabatic molecular orbitals reproduced the measured cross sections for electron capture and for the ensuing Lyman fluorescence lines of ${\mathrm{He}}^{+},$ up to the collisional ionization threshold $(\sim 9\mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u}).\mathrm{}$ In this study, the collision of ${\mathrm{He}}^{2+}$ on $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ is investigated further using a multichannel propagator defined over the former perturbed-stationary-state basis extended to include 35 $L^2$ ionization pseudostates of the linear combination of atomic orbitals (LCAO) type. The simulated total charge-transfer cross section is now in good agreement with experiment across the peak plateau and well into the falloff wing, where it begins to tail away at energies above 30 keV/amu because the LCAO ionization set is deficient. Concurrently, throughout the range where collisional ionization is important, state-selective cross sections are improved compared to our previous study. Approximately 25% of the calculated peak transfer cross section is associated with $n>~3{\mathrm{He}}^{+}$ levels. The multichannel propagator simulations imply that (i) decay cascades contribute almost one-third of the ensuing spontaneous He II Lyman-α fluorescence cross section [J. Phys. B 24, 4025 (1991)] that is accurately reproduced and (ii) capture-induced Balmer-α and Paschen-α emissions are considerably stronger than the values either measured in the aforementioned experiment or computed by translation-factor models.

Recently a striking structure has been observed experimentally in the ionization of helium in symmetric out-of-plane geometry. We will show that this structure, a very sharp dip, is well represented in a distorted-wave Born approximation calculation and that it is the result of a strong interference effect between incident and final channel distorted waves. We have located similar structures in other targets, in particular $\mathrm{Li}\mathrm{}\left(2s\right),\hspace{0.5em}\mathrm{Ar}\mathrm{}\left(2s\right)\mathrm{}$, and $\mathrm{Ne}\mathrm{}\left(2s\right)\mathrm{}$. No such structure was found for hydrogenic targets. For $\mathrm{Ar}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{Ne}\mathrm{}\left(2p\right)\mathrm{}$ targets we predict that such interference effects will be masked by the different behavior of the magnetic sublevels.

By means of a crossed-beam technique, we have conducted measurements of absolute differential cross sections for the vibrational excitation of the electronic ground state of ${\mathrm{N}}_2$ by electron impact. The ${}^2{\Pi }_g$ shape resonance region was treated, with the impact energies being those of the resonance’s first four elastic scattering peaks—approximately 1.9, 2.1, 2.4, and 2.6 eV. The scattering angles covered were from 12$°$ through 156$°$, in 12$°$ increments. Absolute integrated-excitation cross sections were computed from the differential cross sections. Our results show the expected strong $D$-wave character, and are compared with the recent results of others, both experimental and theoretical.

The symmetric noncoplanar $\mathrm{}(e,2e)\mathrm{}$ cross sections (momentum profiles) of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ for the transitions to the $2p{\sigma }_u$ and $2s{\sigma }_g$ excited ion states have been measured, relative to the ground-state ion transition, using a high-sensitivity multichannel momentum-dispersive electron momentum spectrometer (EMS) at an impact energy of 1200 eV. Newly calculated plane-wave impulse approximation (PWIA) cross sections, based on a full-configuration-interaction ${\mathrm{H}}_2$ wave function, are compared to the experimental results. These calculations are in better agreement with the experimental results for the transition to the $2s{\sigma }_g$ ion state than the earlier theoretical work of Liu and Smith, Jr. [Phys. Rev. A 31, 3003 (1985)] which has been found to be in error. Nevertheless, significant discrepancies between the relative experimental and theoretical cross sections are observed for the transitions to excited ion states, suggesting a failure of the PWIA description of these processes. The results are supported by additional measurements obtained using an energy-dispersive EMS spectrometer.

In this work we present a theoretical treatment of electron ejection from a clean metallic semi-infinite solid with an ideal orthorhombic Bravais lattice following the impact of moderately fast charged particles with respect to the Fermi momentum of the initially bound electron. For an aluminum semi-infinite solid target multiple differential cross sections have been evaluated using a jellium-type wave function of the undisturbed surface in the initial state. Image-charge final-state electron-surface interactions have been included as well as the scattering of the projectile from the multicenter bulk potential. From a kinematic analysis of the process various ionization mechanisms are inferred and confirmed by the present dynamical model.

The Bogoliubov method for the excitation spectrum of a Bose-condensed gas is generalized to apply to a gas with an exact large number $N$ of particles. This generalization yields a description of the Schrödinger picture field operators as the product of an annihilation operator $A$ for the total number of particles and the sum of a “condensate wave function” $\xi \left(\mathbf{x}\right)$ and a phonon field operator $\chi \left(\mathbf{x}\right)$ in the form $\psi \left(\mathbf{x}\right)\approx A\{\xi \left(\mathbf{x}\right)+\chi \left(\mathbf{x}\right)/\sqrt{N}\}$ when the field operator acts on the $N$ particle subspace. It is then possible to expand the Hamiltonian in decreasing powers of $\sqrt{N}$, and thus obtain solutions for eigenvalues and eigenstates as an asymptotic expansion of the same kind. It is also possible to compute all matrix elements of field operators between states of different $N$. The excitation spectrum can be obtained by essentially the same method as Bogoliubov only if $\xi \left(\mathbf{x}\right)$ is a solution of the time-independent Gross-Pitaevskii equation for $N$ particles and any chemical potential $\mu$, which yields a valid and stable solution of the Gross-Pitaevskii equation. The treatment within a subspace of fixed $N$ is identical in form to that usually used, but the interpretation of the operators is slightly different. A time-dependent generalization is then made, yielding an asymptotic expansion in decreasing powers of $\sqrt{N}$ for the equations of motion. In this expansion the condensate wave function has the time-dependent form $\xi (\mathbf{x},t)$, and the condition for the validity of the expansion is that $\xi (\mathbf{x},t)$ satisfies the time-dependent Gross-Pitaevskii equation $\partial \xi /\partial t=-({ħ}^2/2m){\nabla }^2\xi +V\xi +Nu|\xi {|}^2\xi$. The physics is then described in a kind of interaction picture, called the condensate picture, in which the phonon operator can be expressed as $\chi (\mathbf{x},t)={\sum }_k{\xi }_k(\mathbf{x},t){\alpha }_k,$ where the operators ${\alpha }_k$ are time-independent annihilation operators, and the state vector has a time evolution described by a Schrödinger equation in which the Hamiltonian is a time-dependent quadratic form in the phonon creation and annihilation operators, whose coefficients are explicitly determined in terms of the time-dependent condensate wave function $\xi (\mathbf{x},t)$.

A variational technique is applied to solve the time-dependent nonlinear Schrödinger equation (Gross-Pitaevskii equation) with the goal to model the dynamics of dilute ultracold atom clouds in the Bose-Einstein condensed phase. We derive analytical predictions for the collapse, equilibrium widths, and evolution laws of the condensate parameters and find them to be in very good agreement with our numerical simulations of the nonlinear Schrödinger equation. It is found that not only the number of particles, but also both the initial width of the condensate and the effect of different perturbations to the condensate may play a crucial role in the collapse dynamics. The results are applicable when the shape of the condensate is sufficiently simple.

Light-induced dipole-dipole interaction in a coherent atomic-field results in an effective nonlinearity for atoms. This nonlinearity can induce self-trapping and self-focusing of a coherent atomic beam undergoing propagation through a traveling-wave laser beam; we show how such a scheme could be realized and evaluate the critical density required for atomic self-trapping and self-focusing. An analogy to optical self-trapping and self-focusing is discussed.

We consider the Bose-Einstein condensation of atoms in a trap where the density of particles is so high that the low-density approach of Gross [J. Math. Phys. (N.Y.) 4, 195 (1963)] and Pitaevskii $($V. L. Ginsburg and L. P. Pitaevskii, Zh. Éksp. Teor. Fiz. 34, 1240 (1958) [Sov. Phys. JETP 7, 1858 (1958)]$)$ will not be applicable. For this purpose we use the slave boson representation which is valid for hard-core bosons at any density. This description leads to the same results as the Gross-Pitaevskii approach in the low-density limit, but for higher densities, it predicts the depletion of the condensate in the regions where the density of the atomic cloud is high.

We describe the creation of circular states of hydrogen by adiabatic transfer of a Rydberg state in crossed electric and magnetic fields, and also by adiabatic passage in a rotating microwave field. The latter method permits rapid switching between the two circular states of a given $n$ manifold. The two methods are demonstrated experimentally, and results are presented of an analysis of the field ionization properties of the circular states. An application for the circular states is illustrated by millimeter-wave resonance in hydrogen of the $n=29\mathrm{}\to n=30\mathrm{}$ transition.

We propose an experiment to observe the quantum Zeno effect in atomic Bragg scattering. The quantum Zeno effect is the inhibition of the free evolution of a system when that system is subject to a decohering measurement process. This effect is usually studied theoretically in an idealized two-level system, though this system has been difficult to realize in practice. The coherent population exchange between the forward and Bragg beams provides an ideal two-level system, though as there is no practical scheme for continuously monitoring the motion of an atom we include decoherence by adding noise to the laser. Using the method of stochastic Hamiltonians we show that the noise suppresses tunneling in the two-state system and, in the large noise limit, the system can freeze in its initial state.

The tools of adiabatic Floquet theory are developed to study the control of population transfer by intense chirped laser pulses. We give a formulation of the adiabatic evolution for systems with a slowly varying frequency, which exhibits an effective frequency carrying relevant physical information. These tools are applied to dissociation. The path leading to dissociation corresponds to a single instantaneous Floquet state evolving adiabatically. We obtain $96\%$ dissociation of the HF molecule by a chirped ultrashort infrared laser pulse of duration $2.3$ ps and intensity ${10}^{13}$ W/cm${}^2$.

The paper examines the effect of irreversible dissipation from the intermediate state on the efficiency of population transfer by partially overlapping delayed pulses in three-state systems. Several general approximations to the final-state population for both the intuitive and counterintuitive pulse sequences are derived. They show that the loss of transfer efficiency is much stronger for the intuitive pulse sequence, as then the intermediate state is significantly populated during the transfer. For the counterintuitive sequence, the damping of the final-state population is found to be exponential for small decay rates and polynomial for large ones; moreover, the range of decay rates, over which the transfer efficiency remains high, is proportional to the squared pulse area. The paper also presents an analytically solvable model, involving smooth delayed pulses, as well as numerical results and analytic approximations for Gaussian pulses.

We demonstrate the existence of velocity-tuned (Doppleron) resonances in two ${\sigma }^{+}$- and ${\sigma }^{-}$-polarized standing waves, between the two ground-state sublevels of a $\Lambda$-type system. We derive the resonance conditions, and the properties of these resonances. We show that fast population transfer between these states may take place, while excited atomic states are only marginally populated. We derive an interesting dependence of the transfer time on the relative spatial phase of the two standing waves.