The charge-transfer rate coefficients for the reaction ${\mathrm{O}}^{2+}$(2${\mathit{p}}^2$ ${}_{}{}^3{}_{}{}^{}$P)+He, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and Co → products are measured at energies about 2.5 eV using a laser-induced-plasma ion source and ion storage. The rate coefficients are 1.12(0.15)×${10}^{\mathrm{-}12}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, 2.35(0.22)×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, 3.15(0.26)×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, and 3.40(0.29)×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, respectively. While our result for ${\mathrm{O}}^{2+}$ and ${\mathrm{H}}_2$ is consistent with the published value, the charge-transfer rate coefficient for ${\mathrm{O}}^{2+}$ and He disagrees with all the available theoretical and experimental values by as much as three orders of magnitude. No theoretical value is available for ${\mathrm{O}}^{2+}$+${\mathrm{N}}_2$. However, our measured value at 2×${10}^4$ K is different from that obtained by the drift tube method at 300 K by a factor of 2. No theoretical and experimental values are available for comparison for the charge-transfer reaction of ${\mathrm{O}}^{2+}$ and CO.

Ground-state properties of alkali-metal clusters are investigated within the framework of the jellium model from the standpoint of either the local-density approximation of the density-functional theory or the nonlocal Hartree-Fock theory. Accordingly, the optical response is calculated microscopically within the corresponding time-dependent theory. The present study deals with closed-shell systems with up to 200 delocalized electrons. A comparative investigation of various approximations is carried out. In spite of significantly different mean-field potentials, a consistent inclusion of polarization-type many-body effects leads to surprisingly similar theoretical predictions of the first moments of the oscillator strength distribution. This agreement is discussed in terms of sum-rule constraints. The remaining discrepancies with regard to the measured data originate from the jellium assumption which underlies our calculations.

We have measured the rate coefficients for the recombination of ${\mathrm{D}}^{+}$ ions with free electrons in the electron cooler of the storage ring CRYRING, as a function of the relative energy in the interval from 0 to ∼10 eV. Also measured is the rate coefficient for ${\mathrm{He}}^{+}$-e recombination at zero relative energy. The experimental data are reported at two different transverse temperatures of the electron beam, namely, 0.1 and 0.01 eV/k. The present data are compared with calculations for radiative recombination, which is expected to be the dominant recombination process for our experimental conditions. The calculations of the rate coefficients are performed using the nonrelativistic dipole approximation represented by the semiclassical Kramers approximation, corrected by the Gaunt factor. The measured rate coefficients are systematically higher than the theoretical predictions for radiative recombination, corrected for field-ionization effects in the dipole magnets of the ring. Other contributions to the recombination rate, i.e., collisional-radiative recombination and radiative stabilization, are discussed.

Electron capture cross sections from circular Rydberg states as a function of the angle cphi between the ion velocity and the angular momentum of the circular orbital have been reported recently by Hansen et al. [Phys. Rev. Lett. 71, 1522 (1993)]. We show that the observed cphi dependence can be explained in terms of the propensity rule that governs the dependence of electron capture cross sections on the magnetic quantum numbers of the initial excited states. We also carried out close-coupling calculations to show that electron capture from the circular H(3d,4f,5g) states by protons at the same scaled velocity has nearly the same cphi dependence.

Absolute measurements of the total single-electron detachment (SED) cross sections ${\mathrm{\sigma }}_{\mathrm{-}10}$ for 3–50-keV ${\mathrm{H}}^{\mathrm{-}}$ ions incident on helium atoms and for 5–50-keV ${\mathrm{H}}^{\mathrm{-}}$ ions incident on neon and argon atoms are reported in this paper. The present SED cross sections for helium and neon targets are of similar magnitude but have a different energy dependence. By contrast, the SED cross sections for argon targets are significantly larger than either of the other two target species reported in this paper. Comparisons are made with the previous measurements and calculations in the literature.

In this paper, we report measurements of the curvature present in the ${\mathrm{H}}^0$ and ${\mathrm{H}}^{+}$ scattered-beam growth curves for the ${\mathrm{H}}^{\mathrm{-}}$ + (He, Ne, Ar) collision systems. The main motivation for this study is to provide an experimental test of the scattered-beam growth-curve rate-equation solutions for tenuous target thicknesses at one level beyond that used to determine cross sections. In doing so, this study provides an indirect verification of the magnitudes of the dominant charge-changing cross sections for ${\mathrm{H}}^{\mathrm{-},0,+}$ projectiles incident on noble-gas targets. The measured quadratic coefficients of the growth curves are compared with the expected quadratic coefficients derived from the previously measured, charge-changing cross sections. These curvature measurements indicate that additional measurements of cross sections for charge-changing processes between hydrogen projectiles incident on neon and argon targets may be warranted.

An alternative theoretical description of axial electron channeling in the multi-GeV region has been developed. We solve a kinetic equation to evaluate an electron distribution function in axially oriented single crystals. Based on the single-string model, the required matrix elements for radiation and scattering by lattice vibrations are calculated employing solutions of the Dirac equation in cylindrical coordinates. Results obtained for 150-GeV electrons propagating along the 〈110〉 axis of germanium are in good agreement with experimental observations.

The interaction radii at which holes are created in the L shell of light projectiles and in the M shell of germanium during ion–surface-atom collisions have been determined from the thresholds for doubly charged ion production. In contrast to the data with a silicon target, the interaction radius at which electron promotion occurs, does not decrease with projectile atomic number ${\mathit{Z}}_1$, as it should if promotion occurs at a definite level crossing. In addition, the yields of doubly charged ions do not follow the ${\mathit{Z}}_1$ dependence predicted by the Fano-Lichten electron-promotion model [Phys. Rev. Lett. 14, 627 (1965)].

We present a numerical technique to solve the time-independent nonlinear Schrödinger equation with an external potential. We apply it to the case of a dilute Bose-condensed assembly of trapped neutral atoms where the potential varies on the same scale as the condensate. This situation should soon be accessible to experimental observation.

The description of the wave-packet time-resolved dynamics in a two-channel molecular system driven by a cw laser field is considered within the time-independent Floquet representation. It is shown that, at high field intensity, the wave-packet motions are governed solely by the pair of adiabatic dressed potential-energy surfaces (PES’s) associated with a single Brillouin zone. The same expressions of the wave-packet motions in terms of the adiabatic PES’s are obtained within a short-time approximation, thereby furnishing a new numerical algorithm for the wave-packet propagation in a laser-driven two-channel system at any intensity. Numerical tests of this algorithm are presented. The numerical results establish unambiguously the adiabaticity of nuclear motions at high field intensities.

We describe techniques for creating long-lived magneto-optical and magnetostatic traps for neutral atoms. These traps exist in a sealed cryogenic environment with a temperature near 4 K, where the background gas pressure can be extremely low. To date we have observed cesium magneto-optical traps with background-limited lifetimes in excess of 1 h, and magnetostatic traps with lifetimes of nearly 10 min. From these observations we use the known He-Cs van der Waals collision cross section to infer typical background gas pressures in our apparatus below 4×${10}^{\mathrm{-}12}$ Torr. With hardware improvements we expect this pressure can be made much lower, extending the magnetostatic-trap lifetimes one to two orders of magnitude. Furthermore, with a cryogenic system one can use superconducting magnets and SQUID detectors to trap and to nondestructively sense spin-polarized atoms. With superconducting microstructures one can achieve very large magnetic-field gradients and curvatures, as high as ∼${10}^6$ G/cm and ∼${10}^9$ G/${\mathrm{cm}}^2$, respectively, for use in magnetic and magneto-optical traps.

A laser cooling scheme for atoms with an effective three-level Λ configuration is presented. The cooling force is obtained by using two standing light waves, each interacting only with one of the two atomic transitions and both detuned red with respect to the transition. For a given laser power, the force exceeds the largest force on a two-level atom by a factor of 2 or more and the atomic velocity capture range is at the same time significantly increased. The spatially averaged force as well as the spatially averaged momentum diffusion coefficient, as a function of the atomic velocity, is calculated. The semiclassical steady-state momentum distributions are presented for some specific cases and an experimental realization of the cooling scheme is proposed for the case of metastable helium.

We report the production of an extremely cold lithium plasma by optical-field-induced ionization. Such a cold plasma was realized by irradiating a 0.5-ps KrF laser focused into a singly ionized lithium plasma at an intensity of ${10}^{17}$ W ${\mathrm{cm}}^{\mathrm{-}2}$. An average electron temperature of less than 1 eV was estimated by measuring the intensity ratio of the Lyman series line emission in a hydrogenlike lithium plasma. The time history of the Lyman-α laser (13.5 nm) in a lithium plasma strongly ensures the production of an extremely cold plasma.

The magnetic-field component of a very intense laser field interacting with an atom cannot normally be neglected for intensities that lead to relativistic velocities of the electrons. Here we investigate stabilization and harmonic generation in this relativistic regime from a three-dimensional hydrogen atom modeled as a classical system with a distribution of initial conditions derived from a Monte Carlo average. Particular emphasis is placed on the problems of ionization in the direction of propagation of the applied laser field, which will be shown to arise from the inclusion of the magnetic-field component of the laser field. In the harmonic spectra, Doppler shifting occurs for observation directions orthogonal to the direction of laser-field propagation. Retardation effects show up in the harmonic spectra in the forward direction and inhibit the magnetic-field effects of the free-electron contribution of the forward-direction spectrum. In general, few harmonics are observed in our single-atom treatment because of the magnetically induced three-dimensional motion of the electron for intensities approaching the relativistic regime, and because of high ionization probabilities, i.e., also the breakdown of stabilization, for strongly relativistic laser intensities.

The laser-induced multifragmentation of ${\mathrm{C}}_2$${\mathrm{H}}_2$ into protons and multicharged carbon ions is shown to be a direct instantaneous explosion of the molecule. The evolution of the overall nuclear structure is studied through ion-ion correlation peak shapes. The ratios of the maxima of the kinetic-energy release distributions to the Coulomb repulsion energies calculated at the equilibrium internuclear distances are measured to be 45% for the protons and 53% for the ${\mathrm{C}}^{\mathrm{Z}+}$ ions for all the detected ${\mathrm{H}}^{+}$+${\mathrm{C}}^{\mathrm{Z}\prime +}$+${\mathrm{C}}^{\mathrm{Z}+}$+${\mathrm{H}}^{+}$ fragmentation channels. The time scale for electronic polarization and stripping compared with the intramolecular electronic and nuclear time evolutions does not allow using a frozen molecular ion structure for the description of the explosion, thus explaining in part the observed fragmentation pattern. During the laser-induced alignment and subsequent stabilization of the molecular frame around the laser polarization direction, the carbon-carbon axis undergoes small damped oscillations that remain larger than the corresponding oscillations of the hydrogen-hydrogen axis. This difference comes from the lower moment of inertia of the hydrogen atoms compared with that of the carbon atoms in the molecule. However, the subsequent deviation from the initial linear structure remains small and is observed when the molecular ion is not completely aligned along the laser electric field.

This paper reports on experimental investigations of absorption, dispersion, and amplitude profiles of the Autler-Townes doublet in a ssV-shaped three-level system where the probe field intensities varied from weak to strong. The experiments were carried out on the ground-state hyperfine transitions of the nitrogen-vacancy color center in diamond using the Raman heterodyne technique, a sensitive optically detected magnetic resonance technique. A strong pump field is on resonance with the ${\mathit{I}}_{\mathit{z}}$=‖0〉⇆‖1〉 transition at 4.7 MHz and the resultant Autler-Townes profiles are probed with a separate field scanning through the ${\mathit{I}}_{\mathit{z}}$=‖0〉⇆‖-1〉 transition at 5.4 MHz. As the probe power increases the absorptive profile of the Autler-Townes doublet becomes saturated and power broadened and the two components overlap constructively. In contrast, the inner parts of the two dispersive doublet components interfere destructively due to a π phase difference. The amplitude profile has an anomalous line shape at high probe intensity due to the different saturation behaviors of the absorptive and dispersive components. The experimental profiles are in good agreement with theoretical profiles calculated by solving the equation of motion of the density matrix under the steady-state limit.

Inelastic collision rates between two very cold ground-state atoms can be strongly decreased by using an optical shielding laser to repel the colliding atoms at very large internuclear separation so that they never get close to each other. This can be achieved by using a sufficiently intense laser to excite a repulsive state of the quasimolecule formed by the two colliding atoms. We study a simple two-state model and compare the predictions of fully quantum-mechanical close coupling and time-dependent wave packet methods with semiclassical approximations based on the concept of a Landau-Zener curve crossing of field-dressed states. The effect of spontaneous emission in the excited state is treated by the Monte Carlo wave-function method. Calculations are given for a model sodium system at 250 μK. Strong optical shielding, without significant heating due to the quasimolecular excitation, can be achieved at relatively modest laser power. We find that Landau-Zener models give near quantitative agreement with the fully quantum calculations in spite of the extremely low temperature.

We present the results of a numerical simulation showing the harmonic spectra produced by the interaction of ultrashort laser pulses with helium, neon, and their ions. We calculate the harmonic spectrum for intensities close to and above saturation (${10}^{14}$-{m W/cm${\mathrm{\}}}^2$ to ${10}^{17}$-{m W/cm${\mathrm{\}}}^2$) using wavelengths in the region 248 nm to 527 nm. We find that, in order to observe harmonic production from ions, it is best to use a relatively short-wavelength laser, i.e., KrF. We also discuss the effects of the pulse shape on the harmonic generation, including a blueshifting of the single-atom response. Our results are mostly based on a numerical integration of the one-dimensional Schrödinger equation, but we include a comparison with results produced by a three-dimensional model for critical cases. We also include a discussion of various methods of calculating the harmonic spectrum in the strongly ionizing regime.

Recent experiments by Mohideen et al. [Phys. Rev. Lett. 71, 509 (1993)] demonstrated that more hot electrons result from high-intensity above-threshold ionization of atoms than had been predicted by existing models. In this paper we explain most of the discrepancy between the theory and experimental data on the basis of a quasistatic tunneling independent-electron (QTIE) model, formulated using a simple set of rate equations with quasistatic expressions for ionization probabilities. Because of the efficiency of the QTIE approach, one can easily perform calculations of strong-field ionization for many-electron atoms. We demonstrate the properties of tunneling ionization in applications to He, Ne, Ar, Kr, and Xe.

Light-induced ‘‘bound’’ states of atoms originate from the movement of poles of the multichannel scattering matrix on the Riemann energy surface. The appearance of additional ‘‘bound’’ states of the negative hydrogen ion, recently predicted in the high-frequency theory, is related to the motion of resonance poles that correspond to autoionizing states in the absence of the field. Various pole trajectories leading to light-induced states are described for photodetachment from a one-dimensional square potential well. Certain light-induced states in atomic hydrogen are discussed in relation to the spectrum of the dressed Coulomb potential.