The $e^{-}+{\mathrm{Sc}}^{3+}$ recombination cross section exhibits a strong interference between the radiative and dielectronic recombination pathways at low energies. The resonances associated with the ${3p}^5{3d}^{22}F$ term in Sc${}^{2+}$ are found to have unusually large autoionization widths. Their strong interference with the background radiative recombination results in quite distinct asymmetric resonance profiles. Current ion storage ring experiments should be able to verify the strength of the interference effects.

The spectral dependence of the angular distributions of photoelectrons produced by the single-photon detachment of ${\mathrm{Al}}^{\mathrm{-}},$ ${\mathrm{Si}}^{\mathrm{-}},$ and ${\mathrm{P}}^{\mathrm{-}}$ ions has been measured at five discrete photon wavelengths ranging from 457.9 to 647.1 nm (2.71–1.92 eV) using a crossed laser-ion beam apparatus. Values of the asymmetry parameter have been determined by fits to the photoelectron yield as a function of the angle between the laser polarization vector and the linear momentum vector of the collected photoelectrons. The experimental results for ${\mathrm{Al}}^{\mathrm{-}}$ are compared with a recent theoretical calculation [C. N. Liu and A. F. Starace, Bull. Am. Phys. Soc. 42, 1026 (1997)] and are found to be in good agreement.

We investigated the $b^1{\Sigma }_g^{+}–X^3{\Sigma }_g^{-}$ (0,0) band of the ${}^{16}{\mathrm{O}}^{18}\mathrm{O},$ ${}^{16}{\mathrm{O}}^{17}\mathrm{O},$ ${}^{18}{\mathrm{O}}_2,$ ${}^{17}{\mathrm{O}}^{18}\mathrm{O},$ and ${}^{17}{\mathrm{O}}_2$ isotopomers of oxygen. The weak magnetic dipole transitions around 760 nm were observed using cavity-ring-down absorption spectroscopy. The positions of over 340 lines are presented together with (re-) analyses of the rotational constants. We discuss the importance of these data in view of tests of the symmetrization postulate in ${}^{16}{\mathrm{O}}_2$ and present a sensitivity scale of 13 orders of magnitude that could be practical for future test experiments.

The derivation of the dispersion coefficients for various symmetries of the lower $\mathrm{nP}$-$\mathrm{nP}$ asymptote of the homonuclear alkali-metal dimers is presented. Using the perturbation theory the dispersion coefficients $C_5$ are obtained in the first order and the $C_6$ and $C_8$ coefficients are obtained in the second order in terms of double sums involving atomic radial matrix elements. The sums are separated in individual contributions of each atom by using a complex integral representation. The atomic matrix elements involving Green’s functions are evaluated using the Dalgarno-Lewis method. Numerical results for the $C_5$, $C_6$, and $C_8$ dispersion coefficients for Li, Na, K, Rb, and Cs dimers are presented.

Low-energy electrons emitted during the interaction of ${\mathrm{N}}^{6+}$ ions with a Au surface were measured. The projectile energy was varied in a wide range from 90 eV to 60 keV. A single-stage spectrometer was used to achieve reliable electron measurements at energies as low as a few eV. The experimental data are compared with simulations based on the classical over-the-barrier model. For slow projectiles the observed low-energy electrons are attributed to autoionizing transitions in high Rydberg states of the projectile. The autoionization electrons provide a signature for the first deexcitation steps of hollow atoms formed above the surface.

The electronic energy loss for low-velocity protons channeled in the $〈$100$〉$ direction of single-crystal Au and Al is calculated. The proton trajectories are determined by solving the equation of motion. In the proton dynamics two forces are included, a repulsive term arising from the nuclei and core electrons and a friction force, depending on proton velocity, arising from the valence electrons. The repulsive force on the proton is evaluated using a superposition of conservative potentials. The friction coefficient is evaluated by using the local density of valence electrons, and a quantum-mechanical transport cross-section approach with a self-consistent model based on Friedel’s sum rule. The results allow us to describe the nonlinear behavior of energy loss with ion velocity observed experimentally in Au, as well as the linear behavior observed in Al.

Energy loss in the MeV range of simple clusters impinging on thin carbon targets has been measured using a time-of-flight method. Stopping-power ratios defined as the ratio of the stopping power of the cluster to the sum of the stopping powers of the constituent atoms moving at the same velocity were investigated. Stopping- power ratios close to unity were observed for ${\mathrm{O}}_2$ and ${\mathrm{B}}_3$ clusters, while deenhancement effect is observed in the stopping-power ratios of ${\mathrm{C}}_3$ and ${\mathrm{C}}_4.$ The experimental results are compared both with an existing theoretical model, which takes into account the spatial correlation of the fragments, and with a simple united-atom model, which also includes the charge state evolution of the fragment ions inside the target.

The stopping powers for slow H and He projectiles in different solids are calculated using density-functional theory with local density approximation for exchange correlations in the scattering potential and the concept of local plasma density approximation for excitation channels. The calculated results for the stopping power ratio $R=(1/4)\mathrm{}\left[\right(-{dE/dx)}_{\mathrm{He}}/\left(-{dE/dx)}_{\mathrm{H}}\right]$ in some solids are in agreement with recent experimental predictions.

We have extended the recently developed dynamical classical over-barrier model for charge transfer in soft ion-cluster collisions in order to simulate the projectile kinetic energy gain of 3.3$q$-keV Ar${}^{q+}$ ions in large-impact-parameter collisions with neutral C${}_{60}$ targets. Our semiclassical theory allows for the direct calculation of the energy defect $Q$ and of the projectile kinetic-energy gain in two different ways: either as difference of electronic binding energies before and after the collision or by integration of the dynamically varying force between the collision partners along the trajectory. A comparison between the two ways provides an intrinsic test of the model calculations. A comparison with recent experimental data shows good agreement in the main features of the projectile energy gain spectra and facilitates their interpretation in terms of the number of transferred electrons and projectile shells into which electrons are captured.

We have performed projectile-by-projectile measurements of backward and forward secondary-electron emission of thin carbon foils under impact of MeV ${\mathrm{H}}^0$ projectiles. The emitted electrons were detected in coincidence with the protons or the neutrals emerging from the target. We have used a very thin target for which we know that, at energies above 2 MeV, the emergent neutrals are essentially transmitted, i.e., they have kept their electron throughout the target. In these conditions the emission yields measured in coincidence with emergent neutrals are found lower than for protons of the same velocity, but the reduction factor is not the same for backward and for forward emission. We show that this can be explained by the screening of the proton charge by the electron during the ${\mathrm{H}}^0$-target interaction. We have observed other effects related to forward electron emission: if the ${\mathrm{H}}^0$ projectile emerging from the foils results from an electron-capture event taking place close to the exit surface, the forward emission is enhanced (at energies above 500 keV) by the contribution of Auger electrons resulting from the rearrangement of the carbon atoms ionized in the capture events. For ${\mathrm{H}}^0$ projectiles ionized in the target we have used the statistics of the number of forward-emitted electrons to deduce the probability for an incident electron to be transmitted through a very thin target and to produce cascade electrons.

We describe how two vibrational degrees of freedom of a single trapped ion can be coupled through the action of suitably chosen laser excitation. We concentrate on a two-dimensional ion trap with dissimilar vibrational frequencies in the $x$ and $y$ directions of motion and derive from first principles a variety of quantized two-mode couplings, concentrating on a linear coupling that takes excitations from one mode to another. We demonstrate how this can result in a state rotation, in which it is possible to transfer the motional state of the ion from, say, the $x$ direction to the $y$ direction without prior knowledge of that motional state.

Coherent and incoherent neutron-matter interaction is studied inside a recently introduced approach to subdynamics of a macrosystem. The equation describing the interaction is of the Lindblad type and, using the Fermi pseudopotential, we show that the commutator term is an optical potential leading to well-known relations in neutron optics. The other terms, usually ignored in optical descriptions and linked to the dynamic structure function of the medium, give an incoherent contribution to the dynamics, which keeps diffuse scattering and attenuation of the coherent beam into account, thus warranting fulfillment of the optical theorem. The relevance of this analysis to experiments in neutron interferometry is briefly discussed.

We have recently derived two-fluid hydrodynamic equations for a trapped weakly interacting Bose gas. In this paper we use these equations to discuss first and second sound in a uniform Bose gas. These results are shown to agree with the predictions of the usual two-fluid equations of Landau when the thermodynamic functions are evaluated for a weakly interacting gas. In a uniform gas, second sound mainly corresponds to an oscillation of the superfluid (the condensate) and is the low-frequency continuation of the Bogoliubov-Goldstone symmetry-breaking mode.

The possibility of creating a Bose condensate of trapped atoms in a non-ground-state is suggested. Such a nonequilibrium Bose condensate can be formed if one first obtains the conventional Bose condensate in the ground state and then transfers the condensed atoms to a non-ground-state by means of a resonance pumping. The properties of ground and non-ground-states are compared and plausible applications of such nonequilibrium condensates are discussed.

We investigate the behavior of large amplitude oscillations of a trapped Bose-condensed gas of alkali-metal atoms at zero temperature, by solving the equations of hydrodynamics for collective modes. Due to the atom-atom interaction, the equations of motion are nonlinear and give rise to significant frequency shift and mode coupling. We provide analytic expressions for the frequency shift, pointing out the crucial role played by the anisotropy of the confining potential. For special values of the anisotropy parameter the mode coupling is particularly strong and the frequency shift becomes large, revealing a peculiar behavior of the Bose-condensed gas. Consequences on the theory of collapse and revival of collective excitations are also discussed.

We report on a study of the superfluid state of spin-polarized atomic ${}^6$Li confined in a magnetic trap. Density profiles of this degenerate Fermi gas and the spatial distribution of the BCS order parameter are calculated in the local-density approximation. The critical temperature is determined as a function of the number of particles in the trap. Furthermore, we consider the mechanical stability of an interacting two-component Fermi gas, in the case of both attractive and repulsive interatomic interactions. For spin-polarized ${}^6$Li we also calculate the decay rate of the gas and show that within the mechanically stable regime of phase space, the lifetime is long enough to perform experiments on the gas below and above the critical temperature if a bias magnetic field of about 5 T is applied. Moreover, we propose that a measurement of the decay rate of the system might signal the presence of the superfluid state.

The dynamics of quasiparticles in repulsive Bose condensates in a harmonic trap is studied in the classical limit. In isotropic traps the classical motion is integrable and separable in spherical coordinates. In anisotropic traps the classical dynamics is found, in general, to be nonintegrable. For quasiparticle energies $E$ much smaller than the chemical potential $\mu$, besides the conserved quasiparticle energy, we identify two additional nearly conserved phase-space functions. These render the dynamics inside the condensate (collective dynamics) integrable asymptotically for $E/\mu \to 0$. However, there coexists at the same energy a dynamics confined to the surface of the condensate, which is governed by a classical Hartree-Fock Hamiltonian. We find that also this dynamics becomes integrable for $E/\mu \to 0$ because of the appearance of an adiabatic invariant. For $E/\mu$ of order 1 a large portion of the phase-space supports chaotic motion, both for the Bogoliubov Hamiltonian and its Hartree-Fock approximant. To exemplify this we exhibit Poincaré surface of sections for harmonic traps with the cylindrical symmetry and anisotropy found in TOP traps. For $E/\mu \gg 1$ the dynamics is again governed by the Hartree-Fock Hamiltonian. In the case with cylindrical symmetry it becomes quasiintegrable because the remaining small chaotic components in phase space are tightly confined by tori.

We have identified low-energy structures of silicon clusters with 9 to 14 atoms using a nonorthogonal tight-binding method (TB) based on density-functional theory (DF). We have further investigated the resulting structures with an accurate all-electron first-principles technique. The results for cohesive energies, cluster geometries, and highest occupied to lowest unoccupied molecular orbital (HOMO-LUMO) gaps show an overall good agreement between DF-TB and self-consistent-field (SCF) DF theory. For Si${}_9$ and Si${}_{14},$ we have found equilibrium structures, whereas for Si${}_{11},$ Si${}_{12},$ and Si${}_{13},$ we present clusters with energies close to that of the corresponding ground-state structure recently proposed in the literature. The bonding scheme of clusters in this size range is different from the bulk tetrahedral symmetry. The most stable structures, characterized by low energies and large HOMO-LUMO gaps, have similar common subunits. To aid in their experimental identification, we have computed the full vibrational spectra of the structures, along with the Raman activities, IR intensities, and static polarizabilities, using SCF-DF theory within the local-density approximation (LDA). This method has already been successfully applied to the determination of Raman and IR spectra of silicon clusters with 3–8, 10, 13, 20, and 21 atoms.

A numerical method for the evaluation of the Born–Floquet amplitude for laser-assisted scattering is proposed for the case when a large basis set is required to achieve convergence. The method is applied to analyze the elastic scattering of fast electrons by the $\mathrm{H}\mathrm{}\left(2s\right)\mathrm{}$ state in a low-intensity laser field of varying frequency and to study the resonant scattering with increasing laser intensity. While the behavior of an atom in a resonant field of low intensity is determined by virtual transitions between resonant levels, at high intensity a great number of nonresonant virtual transitions may significantly influence laser-assisted processes. As a consequence, the attenuation of resonant effects could appear, as well as the “local” stabilization of the atom against ionization.

An approximation in quantum theory of electron inelastic scattering on an arbitrary static potential in the field of a strong electromagnetic wave is developed. The obtained generalized eikonal wave function of the electron enables us to leave the framework of an ordinary eikonal approximation in stimulated bremsstrahlung, which is not applicable at large distances.