Absolute cross sections for $2s$-electron photoionization of atomic oxygen into the final ionic states ${\mathrm{O}}^{+}$ ${(2s}^{-1}{}^4{P}^e,{}^2{D}^e,{}^2{S}^e,{}^2{P}^e)$ and for the occupation of the ${\mathrm{O}}^{+}$ $\left(3s{}^{2,4}{P}^e\right)$ satellite states have been determined experimentally by photon-induced fluorescence spectroscopy. Comparison of the experimental results with theoretical predictions using multiconfiguration-interaction wavefunctions employing the ab initio $R$-matrix technique are made. Suitable harmony is seen only in the case of cross section ratios for ${2s}^{-1}$ removal or $3s$-satellite excitation.

Cross sections for direct electron-impact excitation from the ground level into the ${3p}^{5}5s$ and ${3p}^{5}3d$ levels are measured for incident electron energies from threshold to 200 eV. The optical cross sections for the ${3p}^{5}5\overrightarrow{s}{3p}^{5}4p$ and ${3p}^{5}3\overrightarrow{d}{3p}^{5}4p$ emissions yield the apparent excitation cross sections. The cascade cross sections are obtained by measuring the emission intensities for transitions into ${3p}^{5}5s$ and ${3p}^{5}3d$ from the higher levels, and are subtracted from the apparent excitation cross sections to give the direct excitation cross sections. Most of the emission lines are in the infrared region, requiring the use of Fourier-transform spectroscopic techniques. The general trends of the cross-section results are discussed. Excitation cross sections for the ${3p}^{5}4f$ levels are also reported. Because our experiments do not extend to the vacuum ultraviolet, the direct excitation cross sections for the levels optically connected to the ground state were not obtained.

We study the screening effects of the atomic electrons in the electromagnetic production of electron-positron pairs in relativistic nucleus-atom collisions for fixed target experiments. Our results are contrasted with those obtained in bare collisions, with particular attention given to its dependence on the beam energy and the target atom.

Cross sections for Auger emission following the creation of a K-shell vacancy in C have been measured in slow collisions of ${\mathrm{Ne}}^{q+}$ with ${\mathrm{CH}}_4(q=3\mathrm{}–9).$ Projectile energies ranging from 30 to 180 keV were investigated. For projectile charges lower than 7, the transfer of a K electron from C into the projectile is attributed to monoelectronic $2p\sigma -2p\pi$ transitions induced by rotational coupling at small internuclear distances. For higher charge states, the measured cross sections are found to be significantly larger at the lowest projectile energies than predicted cross sections for $2p\sigma -2p\pi$ transitions. It is shown that other mechanisms must be invoked, such as dielectronic excitation due to electron-electron interaction. Dielectronic excitation gives rise to a simultaneous transfer of inner- and outer-shell electrons of ${\mathrm{CH}}_4$ into a $2\mathcal{l}$ shell of Ne.

The ratio of double ionization to single ionization (DI to SI) as well as the ratio of ionization excitation to single ionization (IE to SI) in hydrogen molecules were studied by examining the effect of the projectile charge q on these processes. The DI to SI and IE to SI ratios were measured at a fixed velocity (corresponding to 1 MeV/amu) from $q=1\mathrm{}$ to 20. For a highly charged ${\mathrm{Cu}}^{20+}$ projectile, for example, the DI to SI and IE to SI ratios are 13% and 46%, respectively, which is a large increase from the ratios of 0.18% and 1.95%, respectively, for ${\mathrm{H}}^{+}$ projectiles. The DI to SI ratio initially increases much more rapidly with projectile charge than the IE to SI ratio. The measured rate of increase of both these ratios decreases for highly charged projectiles. These trends are in agreement with a simple model calculation based on the independent electron approximation.

We analyze the consequences of discretizing one of the two continua in three-body breakup to reduce it to a two-body close-coupling problem. We identify the origin of oscillations in the singly differential cross section in those “convergent close-coupling” calculations as lying only in the way the cross section is calculated from the wave function and not in the wave function itself. The anomalous “step-function” behavior of those calculations is derived from a stationary-phase argument. Calculations are presented on the Temkin-Poet model for electron-impact ionization of hydrogen, a breakup problem with exponential potentials, and an analytically solvable model. The anomalies associated with two-body close-coupling calculations are demonstrated using wave functions from complex exterior scaling calculations that otherwise give converged results without any anomalies.

A simplified relativistic configuration interaction method is used to calculate the dielectronic recombination rate coefficients to the excited states of He from ${\mathrm{He}}^{+}.$ In this method, the infinite resonant doubly excited states involving high Rydberg states are treated conveniently in a unified manner by interpolation. The dielectronic recombination processes for $\Delta N=1\mathrm{}$ and $\Delta N=2\mathrm{}$ transitions are included in our calculations, and the cross sections are in agreement with the experimental measurements. The rate coefficients to the excited states are fitted to an analytical formula and the n dependences of the fitting parameters are discussed.

To determine the lifetimes of Efimov resonances, the problem of resonance scattering of a light particle on a pair of identical particles has been considered. An analytic expression has been obtained for resonance widths in the limit of forces of zero radius and low binding energies in pairs. Calculations are compared with the numerical solution of the Faddeev integral equations in a wide region of masses of the light particle. It is shown that the widths of underthreshold resonances in the scattering amplitude obtained from the integral equations with the Yamaguchi potential are well described by the analytic expression, which allows this expression to be used in a mass region inaccessible for numerical calculations.

Excitation and charge-transfer cross sections for collisions of protons with lithium are calculated by direct solution of the time-dependent Schrödinger equation on a three-dimensional Cartesian lattice. For 5–15-keV incident energies the ${1s}^2$ core of lithium may be treated effectively using a pseudopotential. For a proton incident on lithium, projections of the time-evolved wave function onto the lattice states of lithium yield excitation cross sections for the $\mathrm{Li}\mathrm{}\left(2s\right)\mathrm{}\to \mathrm{Li}\mathrm{}(2p,3l)\mathrm{}$ transitions. For lithium incident on a proton, projections of the time-evolved wave function onto the lattice states of hydrogen yield charge-transfer cross sections for the $\mathrm{Li}\mathrm{}\left(2s\right)\mathrm{}\to \mathrm{H}\mathrm{}(2l,3l)\mathrm{}$ transitions. The $\mathrm{Li}\mathrm{}\left(2s\right)\mathrm{}\to \mathrm{Li}\mathrm{}\left(2p\right)\mathrm{}$ excitation and $\mathrm{Li}\mathrm{}\left(2s\right)\mathrm{}\to \mathrm{H}\mathrm{}(2s,2p)\mathrm{}$ charge-transfer cross sections are found to be in good agreement with the crossed-beams experimental measurements of Aumayr et al. [J. Phys. B 17, 4185 (1984); 17, 4201 (1984); 18, 2493 (1985)].

The relative dissociative recombination rate coefficients for specific vibrational states of ${\mathrm{HD}}^{+}$ have been measured. The method is based on using merged electron and molecular ion beams in a heavy-ion storage ring together with molecular fragment imaging techniques which allow us to probe the vibrational-state population of the stored beam as a function of time as well as the final state of the dissociation. The initial vibrational distribution of the stored ion beam (from a Penning ion source) is found to be in good agreement with a Franck-Condon model of electron impact ionization, apart from slightly larger experimental populations found for low vibrational states; its time evolution in the storage ring reflects the predicted vibrational level lifetimes. Dissociative recombination measurements were performed with the electron and ion beams at matched velocities (corresponding to average collision energies of about 10 meV), and at several well-defined collision energies in the range of 3–11 eV. The obtained vibrational-state specific recombination rate coefficients are compared with theoretical calculations and show that, although an overall agreement exists between experiment and theory, large discrepancies occur for certain vibrational states at low electron energy.

We present a study of the production of metastable negative ions by cesium sputter sources. We report measurements of relatively intense (∼0.5 nA) beams of ${\mathrm{N}}_2^{\mathrm{-}}$ and ${\mathrm{CO}}^{\mathrm{-}},$ two ions with extremely low (<-2.3 eV) electron affinities, and show that they form metastable negative ions with comparatively long lifetimes. Surprisingly, the current of ${\mathrm{N}}_2^{\mathrm{-}}$ is comparable in intensity to ${\mathrm{N}}_3^{\mathrm{-}},$ a molecular negative ion with a binding energy of 2.8 eV. The existence of metastable ${\mathrm{N}}_2^{\mathrm{-}}$ has a twofold significance. First, ${\mathrm{N}}_2^{\mathrm{-}}$ may play an important role in atmospheric chemistry. Second, ${\mathrm{N}}_2^{\mathrm{-}}$ is an excellent test of calculations on many-body systems involving electron-correlation effects. We verified the identification of the ions discussed in this paper using the techniques of accelerator mass spectrometry and Coulomb-explosion imaging. Using an in-beam decay technique, we determined lower bounds for the lifetimes of the most stable states of ${\mathrm{N}}_2^{\mathrm{-}},$ ${\mathrm{CO}}^{\mathrm{-}},$ and ${\mathrm{CO}}_2^{\mathrm{-}}:$ $180\pm 30,$ $140\pm 25,$ and $170\pm 30\mu \mathrm{s},$ respectively. We also report intensities of ${\mathrm{Be}}^{\mathrm{-}}$ and beryllium cluster negative ions, ${\mathrm{Be}}_n^{-},$ for $n=2\mathrm{}–6.$

We report on a high-resolution Auger spectroscopy study of the interaction of ${\mathrm{N}}^{6+}$ and ${\mathrm{O}}^{7+}$ ions with a clean and a LiF-covered Au(111) target. The electron spectra from collisions on Au(111) and LiF-covered Au(111) are distinctly different. The ones resulting from the interaction with Au(111) covered with one monolayer of LiF resemble spectra taken on bulk LiF, which, in contrast to LiF-covered Au(111), is an insulator. On the Au(111) surface and for scattering geometries preventing projectile penetration below the first atomic layer, a more efficient L-shell filling for ${\mathrm{O}}^{7+}$ than for ${\mathrm{N}}^{6+}$ ions is observed. Surprisingly enough, for a single monolayer of LiF on Au the total K Auger intensity of nitrogen is nearly 30$\%$ larger as compared to the clean Au(111) target, while for oxygen no enhancement is found. These findings demonstrate that, for specific projectile-target systems (e.g., N-LiF), an efficient mechanism that fills the projectile L shell is active well before the projectiles reach the surface, so that $\mathrm{KLL}$ electrons are emitted at an early stage of the interaction.

The time-dependent local-density approximation is applied to the optical response of the silver clusters, ${\mathrm{Ag}}_2,$ ${\mathrm{Ag}}_3,$ ${\mathrm{Ag}}_8,$ and ${\mathrm{Ag}}_9^{+}.$ The calculation includes all the electrons beyond the closed-shell ${\mathrm{Ag}}^{11+}$ ionic core, thus including explicitly the filled d shell in the response. The excitation energy of the strong surface plasmon near 4 eV agrees well with experiment. The theoretical transition strength is quenched by a factor of 4 with respect to the pure s-electron sum rule in ${\mathrm{Ag}}_8$ due to the d electrons. Also, in this system the theoretical integrated strength function is remarkably well described by the Mie theory using the bulk dielectric constant. However, the strength is somewhat less than reported experimentally.

We present a nonlinear decoherence model which models decoherence effects caused by various decohering sources in a quantum system through a nonlinear coupling between the system and its environment, and apply it to investigating decoherence in nonclassical motional states of a single trapped ion. We obtain an exactly analytic solution of the model and find very good agreement with experimental results for the population decay rate of a single trapped ion observed in the NIST experiments by Meekhof and co-workers [D. M. Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)].

This paper discusses a generalization of stimulated Raman adiabatic passage (STIRAP) in which the single intermediate state is replaced by N intermediate states. Each of these states is connected to the initial state ${\psi }_i$ with a coupling proportional to the pump pulse and to the final state ${\psi }_f$ with a coupling proportional to the Stokes pulse, thus forming a parallel multi-Λ system. It is shown that the dark (trapped) state exists only when the ratio between each pump coupling and the respective Stokes coupling is the same for all intermediate states. We derive the conditions for existence of a more general adiabatic-transfer state that includes transient contributions from the intermediate states but still transfers the population from state ${\psi }_i$ to state ${\psi }_f$ in the adiabatic limit. We present various numerical examples for success and failure of multi-Λ STIRAP, which illustrate the analytic predictions. Our results suggest that in the general case of arbitrary couplings, it is most appropriate to tune the pump and Stokes lasers either just below or just above all intermediate states.

The transition state is fundamental to modern theories of reaction dynamics: essentially, the transition state is a structure in phase space that all reactive trajectories must cross. While transition-state theory (TST) has been used mainly in chemical physics, it is possible to apply the theory to considerable advantage in any collision problem that involves some form of reaction. Of special interest are systems in which chaotic scattering or half-scattering occurs such as the ionization of Rydberg atoms in external fields. In this paper the ionization dynamics of a hydrogen atom in crossed electric and magnetic fields are shown to possess a transition state: We compute the periodic orbit dividing surface (PODS) which is found not to be a dividing surface when projected into configuration space. Although the possibility of a PODS occurring in phase space rather than configuration space has been recognized before, to our knowledge this is the first actual example: its origin is traced directly to the presence of velocity-dependent terms in the Hamiltonian. Our findings establish TST as the method of choice for understanding ionization of Rydberg atoms in the presence of velocity-dependent forces. To demonstrate this TST is used to (i) uncover a multiple-scattering mechanism for ionization and (ii) compute ionization rates. In the process we also develop a method of computing surfaces of section that uses periodic orbits to define the surface, and examine the fractal nature of the dynamics.

We analyze the laser cooling of trapped bosonic gases beyond the Lamb-Dicke limit. We use a quantum master equation formalism to study the cooling dynamics, and show that dark-state cooling methods similar to those previously proposed for a single trapped atom allow for the condensation of a collection of bosons into a single state of the trap, either the ground state or an excited state. Using Monte Carlo simulations we analyze the condensation dynamics for different dimensions, and two different cooling schemes; we also demonstrate the appearance of multistability and hysteresis phenomena.

We have studied both theoretically and experimentally the optical pumping of Cs atoms trapped in (body-centered-cubic and hexagonal-close-packed) crystalline ${}^4\mathrm{He}$ matrices. The theoretical approach is based on rate equations for which time-dependent and asymptotic solutions are obtained in the case of depopulation and repopulation pumping. Comparison with experiments show that repopulation pumping, i.e., a process in which spin polarization in the excited state is not destroyed, is the dominant pumping mechanism in both crystalline phases.

A slow atom source, which does not rely on lasers, has been developed and characterized. The device, acting as an atomic low-pass velocity filter, utilizes permanent magnets to passively select the slow atoms present in a thermal atomic beam. Slow atoms are guided along a curved, conduction-limited tube by an octupole magnetic field, while fast atoms, unable to follow the curved trajectory, strike the tube wall and are removed from the beam. The performance of the device is demonstrated by loading a magneto-optical trap. Approximately $2\times {10}^8$ lithium atoms are loaded with a rate of $\sim 6\times {10}^6$ atoms/s, while maintaining a background gas pressure of $\sim {10}^{-11}$ torr. This loading technique provides an exceptionally simple, economical, and robust alternative to laser cooling methods.

A method of slowing, accelerating, cooling, and bunching molecules and neutral atoms using time-varying electric-field gradients is demonstrated with cesium atoms in a fountain. The effects are measured and found to be in agreement with our calculations. Time-varying electric-field-gradient slowing and cooling is applicable to atoms that have large dipole polarizabilities, including atoms that are not amenable to laser slowing and cooling, to Rydberg atoms, and to molecules, especially polar molecules with large electric dipole moments. The possible applications of this method include slowing and cooling thermal beams of atoms and molecules, launching cold atoms from a trap into a fountain, and measuring atomic dipole polarizabilities.