Various correlated photoionization processes in helium are calculated by direct solution of the time-dependent Schrödinger equation. An inhomogeneous set of time-dependent close-coupled partial differential equations are partitioned on a numerical lattice for easy implementation on massively parallel computers. Projection of the time evolved wave function onto lattice eigenstates for ${\mathrm{He}}^{+}$ yields cross sections for photoionization with excitation and double photoionization. The computational results are compared with recent experimental measurements.

Electron-impact double-ionization cross sections differential in the angles of the two ejected electrons have been measured at impact energies of 422 and 1052 eV. The energies of the ejected electrons were fixed at 100 eV each. The cross sections are very different at the two incident energies. At 1052 eV the ejected electrons are preferentially found in the forward direction with respect to the incident beam. At 422 eV they are found in the forward and backward directions with approximately equal probability. The 422-eV cross sections are largest when the incident-electron and ejected-electron momentum vectors lie in a common plane. The observations are discussed in the context of several models for double ionization.

We report measurements of the absolute differential cross sections for charge transfer scattering of 0.5-, 0.85-, 1.5-, 2.8-, and 5-keV ${\mathrm{O}}^{+}$ by ${\mathrm{N}}_2$ at scattering angles between 0.04° and 3.1° in the laboratory frame. Cross sections for both ${\mathrm{O}}^{+}{(}^{4}S)$ ground-state and ${\mathrm{O}}^{+}{(}^2{D,}^{2}P)$ metastable projectiles are presented. The ground-state cross section is much smaller than that for the metastable state and the influence of the electronic state of the projectile on the angular distribution of the scattered neutrals is also very significant. The estimated total cross sections are compared with previous measurements.

We have extended the classical over-barrier model to simulate the neutralization dynamics of highly charged ions interacting under grazing incidence with conducting and insulating surfaces. Our calculations are based on simple model rates for resonant and Auger transitions. We include effects caused by the dielectric response of the target and, for insulators, localized surface charges. Characteristic deviations regarding the charge-transfer processes from conducting and insulating targets to the ion are discussed. We find good agreement with previously published experimental data for the image energy gain of a variety of highly charged ions impinging on Au, Al, LiF, and KI crystals.

Substantial fractions of fast atoms or ions are converted to negative ions during grazing scattering from a clean and flat monocrystalline surface of alkali-metal halides. We interpret the experimental data by a model of local electron capture from the halogen sites of the crystal in binary-type collision events. Due to the band gap of the insulator, the probability for subsequent electron loss is low, resulting in large fractions of negative ions that survive from the collisional formation.

We use the spherical shell model and the ball model for the structure of randomly oriented large clusters to study the vicinage effects on the cluster self-energy and stopping power. Collective excitations of the electron gas due to a swift cluster in the solid are described by the linear-response formalism. Wake effects in the cluster self-energy, in conjunction with the cluster size and the interparticle exclusion distance within the cluster, generate regions where the cluster structure may be globally stable against the Coulomb explosion, leaving ways to a partial cluster fragmentation in solids. This conclusion contrasts the results obtained by using Yukawa-type interionic potentials for the cluster self-energy, where the neglect of the wake effects leads to the symmetric Coulomb explosion of a fast C${}_{60}$ cluster.

We have experimentally and theoretically studied the high-intensity $(>\mathrm{}{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}),$ femtosecond photoionization of inertially confined noble-gas clusters. We have examined the energies of electrons and ions ejected during these interactions and found that particles with substantial kinetic energy are generated. Electrons with energies up to 3 keV and ions with energies of up to 1 MeV have been observed. These experimental observations are well explained by a theoretical model of the cluster as a small plasma sphere that explodes following rapid electron collisional heating by the intense laser pulse.

We describe a measurement of the long-range van der Waals interaction $V=-C_6{/r}^6$ between free neutral sodium atoms and clusters $({\mathrm{Na}}_n,1\mathrm{}<~n<~20)$ and ${\mathrm{C}}_{60}$ fullerenes. The $C_6$ coefficients were derived from absolute integral cross sections for low-energy scattering of a supersonic beam of metal particles by ${\mathrm{C}}_{60}$ vapor. Their values are extremely high, ranging from $\sim {10}^4\mathrm{a}.\mathrm{u}.\mathrm{}$ up to more than ${10}^5\mathrm{a}.\mathrm{u}.,$ corresponding to center-of-mass scattering cross sections of up to $\sim 6000{\mathrm{\AA }}^2$ (which exceeds the hard-sphere dimensions of the particles by a factor of 20). These experimental values of $C_6$ are in excellent quantitative agreement with the predictions of the London theory of dispersion forces based on atom and cluster spectral properties. The strength of the long-range potential is due to high cluster polarizabilities.

For a single ion trapped in a harmonic potential under localization conditions beyond the Lamb-Dicke region, a motional wave packet can be coherently split by irradiating the trapped ion with appropriate counterpropagating traveling-wave laser pulses tuned to the second-sideband Raman resonance. Quantum interference between the split wave packets gives rise to interference fringes in the spatial probability distribution when the wave packets spatially overlap at a certain time during a periodic oscillation. An ion-trap interferometer is proposed on the basis of the wave-packet splitter in combination with quantum tomographic determination of the spatial interference fringes.

Absolute values for the generalized multiphoton-ionization cross sections and saturation intensities of rare gases are reported for 500-fs, 248.6-nm pulses. For He, Ar, and Kr these values are experimentally determined from the intensity dependence of the ponderomotive shift for intensities comparable to and above the saturation intensity. The measured experimental cross sections [${\sigma }_{\mathrm{}\left(5\right)\mathrm{}}^{\mathrm{He}}{=(0.9\mathrm{}}_{+9\mathrm{}}^{-0.4})\times {10}^{-152}{\mathrm{cm}}^{10}{\mathrm{}\mathrm{s}}^4,$ ${\sigma }_{\mathrm{}\left(4\right)\mathrm{}}^{\mathrm{Ar}}{=(1.3\mathrm{}}_{+1\mathrm{}}^{-0.5})\times {10}^{-116}{\mathrm{cm}}^8{\mathrm{}\mathrm{s}}^3,$ ${\sigma }_{\mathrm{}\left(3\right)\mathrm{}}^{\mathrm{Kr}}{=(1.2\mathrm{}}_{+1.7\mathrm{}}^{-0.6})\times {10}^{-82}{\mathrm{cm}}^6{\mathrm{}\mathrm{s}}^2,$ ${\sigma }_{\mathrm{}\left(3\right)\mathrm{}}^{\mathrm{Xe}}{=(50\mathrm{}}_{-45}^{+350\mathrm{}})\times {10}^{-82}{\mathrm{cm}}^6{\mathrm{}\mathrm{s}}^2$] are found to be in good agreement with theoretical values [${\sigma }_{\mathrm{}\left(5\right)\mathrm{}}^{\mathrm{He}}=4.4\mathrm{}\times {10}^{-152}{\mathrm{cm}}^{10}{\mathrm{}\mathrm{s}}^4$ (ab initio), ${\sigma }_{\mathrm{}\left(4\right)\mathrm{}}^{\mathrm{Ar}}=1.1\mathrm{}\times {10}^{-115}{\mathrm{cm}}^8{\mathrm{}\mathrm{s}}^3$ (scaling), ${\sigma }_{\mathrm{}\left(3\right)\mathrm{}}^{\mathrm{Kr}}=0.7\mathrm{}\times {10}^{-82}{\mathrm{cm}}^6{\mathrm{}\mathrm{s}}^2$ (scaling), ${\sigma }_{\mathrm{}\left(3\right)\mathrm{}}^{\mathrm{Xe}}=5\mathrm{}\times {10}^{-82}{\mathrm{cm}}^6{\mathrm{}\mathrm{s}}^2$ multichannel quantum defect theory]. For Ne it is found that a single-rate description is not valid, and therefore the present model for the analysis of the experimental results cannot be applied. The reported Xe experimental cross sections are based on the conventional method of measuring the ion yield vs laser intensity with its concomitant reduced accuracy.

The effects of zero-point energy, in comparison with those of excitation energy, are investigated by studying the behavior of two coupled harmonic oscillators for four types of coupling: rotating-wave coupling, counterrotating wave coupling, dipole-dipole (or dipole-field) coupling, and elastic-attraction coupling. It is found that in all but the rotating-wave coupling case, which has only slowly varying terms in the interaction Hamiltonian, the initiation of coupling between ground-state oscillators is followed by an oscillating energy increase in both oscillators at the expense of the coupling energy. In the same three cases, the ground state of the coupled system has a lower zero-point energy than the energy of the state in which the individual oscillators are in their ground states, thus leading to the presence of a van der Waals attraction. This result motivates the conclusion that only the rapidly varying terms in the interaction Hamiltonian are responsible for the attraction. The energies of the individual oscillators while the coupled system is in its ground state are calculated. In the van der Waals cases, these energies turn out to be greater than the zero-point energies of the individual oscillators. A physical explanation for the increase is offered. The feasibility of the utilization or absorption of zero-point energy is discussed.

In sub-Doppler laser cooling of atoms, the lowest temperatures are known to occur at low (but not too low) excitation, and the temperature increases roughly linearly with laser intensity. However, under conditions for which the lowest temperatures are obtained, the small velocity capture range and low optical pumping rate limit the number of atoms that can be collected into the cold sample. In this study, we present measurements for laser cooling of Rb and metastable He atoms for two counterpropagating laser beams with orthogonal linear polarization (lin⊥lin) over a wide range of saturation parameters and laser detunings, together with results of semiclassical (Fokker-Planck) and quantum density matrix calculations. We find that at higher laser intensity, a larger number of atoms can be collected into a final velocity distribution that is significantly narrower than that given by a linear extrapolation vs the square root of laser intensity. Two cooling mechanisms are at work: the sub-Doppler Sisyphus mechanism and also Doppler cooling. Under certain conditions that we discuss, Doppler cooling aids Sisyphus cooling by collecting high-velocity atoms beyond the Sisyphus capture range. ${\mathrm{He}}^{*}$ presents a rather unusual situation in that the minimum average kinetic energy ${(E}_{\mathrm{av}})$ for purely Sisyphus cooling is comparable to the minimum value of $E_{\mathrm{av}}$ with Doppler cooling alone.

We discuss two double-resonance scenarios within the context of laser-assisted electron-atom scattering in which double poles of the $S$ matrix occur. When the energy of the incident electron is chosen to coincide with an autoionizing resonance and the frequency of the laser field resonantly couples this autoionizing state to a quasibound state (i.e., a state that is bound in the absence of the laser) or another autoionizing resonance, double poles occur in the $S$ matrix for particular laser frequencies and intensities. We demonstrate the effect of the double poles on the resonant contribution to the cross section as well as on the time delay matrix.

Following the ideas of L. Tribe et al. [Phys. Rev. A 54, 5447 (1996)], we analyze the effects of resonant atomic tunneling, and also the resonant overbarrier reflection, on the evolution of an atom through an arrangement conformed by two Gaussian laser beams. We demonstrate that the transmission near the tunneling region, instead of just one resonant peak as Tribe et al. predicted, has a Fabry-Pérot-like structure. The position of the resonances in the tunneling region can be analytically found using a Wentzel-Kramers-Brillouin approximation.

We have used an atomic fountain frequency standard to perform a new measurement of the frequency shift induced by a static electric field $E$ on the clock transition of the cesium $6{}^2{S}_{1/2}$ hyperfine splitting. The result in units of ${\mathrm{H}\mathrm{z}/(\mathrm{V}/\mathrm{m})\mathrm{}}^2$ is $\delta {\nu }_0=-2.271\left(4\right)\mathrm{}\times {10}^{-10}{E}^2.$ The accuracy of this measurement represents a tenfold improvement over previous values. This result is of importance for testing ab initio theoretical calculations of the cesium atomic structure as well as semiempirical models used in parity violation experiments. It will also allow a better evaluation of the frequency shift induced by the blackbody radiation in cesium frequency standards.

Transitions from bound atomic Rydberg Stark states in a static electric field to autoionizing Rydberg states above the electric-field-induced ionization threshold are studied using a broadband, tunable free-electron laser (photon energy 160–1400 ${\mathrm{cm}}^{-1}$, pulse duration $\sim 1$ ps) and compared with multichannel quantum defect theory calculations. An atomic streak camera is used to record the time-resolved electron emission transients of the autoionizing atoms. For Stark states located on the downfield side of the potential, the far-infrared ionization spectrum is found to be smooth and the electron emission prompt $(<\mathrm{}2\mathrm{}$ ps), whereas for Stark states located on the upfield side, the far-infrared spectrum has sharp resonances, and the lifetime of the quasicontinuum states is considerably longer. The electron-emission transients from optical ionization of ground-state atoms are compared to transients from far-infrared ionization of Rydberg atoms, showing that the angular motion of the wave packet is responsible for the ionization dynamics for both cases, but different coherent superpositions of angular momentum states are excited depending on the initial state. Finally, we discuss the feasability of using Rydberg atoms as an ultrafast far-infrared detector, starting from a downfield state, or as a wavelength-selective detector, starting from an upfield state.

The approximate time-dependent radial probability distribution of a ${4s}_{1/2}\mathrm{nd}$ wave packet in Ca has been measured using time-resolved excitation of the ${4s}_{1/2}$ core electron to the ${4p}_{3/2}$ state. In principle, this method can be very effective for monitoring the evolution of radial Rydberg wave packets in multielectron atoms.

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields. The theory is based on the extension of the time-dependent Kohn-Sham formalism. The time-dependent exchange-correlation potential with proper short- and long-range behavior is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction correction (SIC) term. The resulting TDOEP-SIC equations are structurally similar to the time-dependent Hartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation potential. We also introduce a generalized pseudospectral time-propagation method, allowing optimal spatial grid discretization, for accurate and efficient numerical solution of the TDOEP-SIC equations. The theory is applied to the study of the role of dynamical electron correlation on the multiple high-order harmonic generation (HHG) processes of He atoms in intense laser fields. We also perform a detailed study of the mechanisms responsible for the production of the higher harmonics in He atoms observed in a recent experiment that cannot be explained by the single-active-electron model. We found that both the dynamical electron correlation and the He${}^{+}$ ion are important to the generation of the observed higher harmonics. The present TDDFT is thus capable of providing a unified and self-consistent dynamical picture of the HHG processes.

We show that the efficiency of population transfer between two discrete states via a common continuum can be considerably improved by suppressing the incoherent ionization with a third laser pulse using a Fano-type resonance.

A relativistic treatment of strong-field photoionization based on the phase-space-averaging method is presented. A procedure is described for preparing a relativistic microcanonical ensemble of the atomic ground state and for calculating the final compensated energies for relativistic electrons. The method is applied to over-the-barrier ionization of hydrogen with ultrashort laser pulses. Ionization probabilities and electron energy spectra have been obtained for laser pulse amplitudes near and well above the atomic unit. The results are in excellent agreement with previous experiments under similar conditions. In sufficiently strong fields, saturation of ionization is shown to limit the attainable electron energies.