Nonperturbative formal scattering theory is applied to multiphoton ionization of atoms in an intense, two-color laser field. The two beams of laser light are the fundamental field and its second-order harmonic with equal intensity and adjustable phase difference. We study the photoelectron yield from krypton and xenon gases as the phase difference varies. Our results show a qualitative agreement with the experimental data given by Schumacher and Bucksbaum [Phys. Rev. A 54, 4271 (1996)]. We interpret the phase-difference effect in above-threshold-ionization spectra as a quantum interference between different channels characterized by the numbers of transferred photons in the process of ionization in the field followed by the photoelectron exit process. Our method is not specific for tunneling ionization and has no direct relation to the Keldysh parameter γ.

We investigate the physics of a single trapped electron interacting with a radiation field without the dipole approximation. This gives physical insights in the so-called geonium theory.

We analyze the expansion process of a thermal trapped gas taking place during the detection of Bose-Einstein condensation through an expansion of the condensate. By extending our Monte Carlo method to study this phenomenon, we demonstrate that the velocity distributions of the thermal trapped atoms are anisotropic due to anisotropic explosion of atoms. The radial and axial components of translational temperature are different from the trap temperature and have opposite evolution tendencies with the gas expansion.

The analysis of electron potentials in multicharged molecules and small clusters allows one to determine which of these systems may be ionized in a strong laser field by the quasiresonance mechanism. The presence of moderately transparent interionic potential barriers (for electron tunneling) is necessary for the quasiresonance electron energy enhancement and, consequently, for ionization [Zuo and Bandrauk, Phys. Rev. A 52, R2511 (1995)]. In multicharged systems, which spatially expand by Coulomb explosion, the interionic barriers increase with time. The simulation of electron motion in such systems demonstrates the presence of a different kind of charge resonance enhanced ionization mechanism whose efficiency depends on the dynamics of the increase of the interionic barriers. This dynamic charge resonance enhanced ionization mechanism is of classical origin and its efficiency is higher than that of the static (frozen geometry) mechanism.

Singly and multiply charged ion yield curves are reported for resonant and nonresonant two-photon-ionization processes for a collection of 31 metal atoms. The atoms are created by sputtering from a solid target using an Ar-ion gun. Single and multiple ionization of these atoms is performed using linearly polarized 248.6-nm (KrF) laser pulses with a full width at half maximum duration of 500 fs, employing intensities between $\sim {10}^9$ and $\sim {10}^{12}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ A four-grid high-resolution reflecting time-of-flight spectrometer is used for ion detection. This advanced spectrometer has a well-defined and small source volume, enabling absolute measurements of ionization probabilities and saturation intensities. Because our measurements are not affected by the increase of the interaction volume for increasing intensities, we can discriminate between resonant and nonresonant multiphoton ionization processes without varying the laser pulse duration. For many metals, the intensity dependence of the ion yield can be accurately reproduced by rate calculations based on a resonantly enhanced two-photon-ionization scheme. As a result, we can determine absolute values of the one-photon cross sections in the resonant processes and these are compared to theoretical values we calculated. For the nonresonant processes, we give generalized multiphoton-ionization cross sections and compare these to a scaling law of Lambropoulos [J. Opt. Soc. Am. B 4, 821 (1987)].

The morphological features are explored of dissociative ionization (DI) of tetra-, tri-, and di-chloromethane molecules induced by intense electric fields of picosecond and attosecond duration. Picosecond-long fields are generated using linearly polarized light pulses (532 nm wavelength, ${10}^{13}\hspace{0.5em}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2}$ intensity) from a neodymium-doped yttrium aluminum garnet laser whereas attosecond-long fields are obtained by means of a fast $\left(\sim 50\hspace{0.5em}\mathrm{MeV}\right)$ beam of ${\mathrm{Si}}^{3+}$ ions from a tandem accelerator. The temporal and directional properties of the intense fields that the chloromethanes are immersed in influence the DI pattern. For all three chloromethanes, the DI patterns measured in light-induced and in ion-induced fields differ from each other, and from those obtained using 70-eV electrons. The ion-impact results are conspicuous by the copious quantities of highly charged fragments and large fluxes of ${\mathrm{H}}^{+}$ ions, belying recently expressed expectations that short-duration field-molecule interactions would result in little or no fragmentation.

We analyze some aspects of the internal and translational dynamics of a two-level atom interacting with a resonant standing wave of an ideal cavity. We show that the cavity vacuum field can split the incoming wave packet of the excited two-level atom into two parts, whose scalar product in the Hilbert space determines the behavior of the Rabi oscillations. The state of the whole system is derived and allows us to study the correlations between the internal and the translational atomic dynamics. We find that these correlations become negligible when the two parts are sufficiently away from each other in the space.

We have used two sets of opposite circularly polarized traveling wave pulses separated in time to establish, and subsequently rephase, a spatially modulated coherence between the magnetic sublevels of the $F=3\mathrm{}$ ground state in Doppler broadened ${}^{85}\mathrm{Rb}$ vapor. The rephasing results in a magnetic grating echo whose duration is an accurate measure of the velocity distribution. We demonstrate that the time scale of the experiment is determined solely by the transit time of atoms across the laser beam. The shortening of the lifetime due to a perturber (Ar) allows us to infer the Rb-Ar velocity changing collision cross section.

When an atom interacts with a phase-fluctuating field of fairly arbitrary spectral character, the Fourier spectrum of atomic population variations manifests a “bright line” at the atomic system’s Rabi frequency. This bright line is termed a Rabi resonance. Here, we generalize our previous studies of this phenomenon by considering the characteristics of the Rabi resonance when it is excited by an off-resonant stochastic field. We find both experimentally and theoretically that (i) the Rabi resonance occurs at the Rabi nutational frequency, $\Omega =\sqrt{{\Delta }^2+{\omega }_1^2},$ where Δ is the detuning and ${\omega }_1$ is the Rabi frequency, (ii) that the strength of the Rabi resonance is maximized when |Δ| equals ${\omega }_1,$ and (iii) that the strength of the Rabi resonance is an asymmetric function of detuning.

Bound-state energies of the helium atom in magnetic fields up to $2.35\times {10}^9\hspace{0.5em}\mathrm{G}$ are calculated. At ${10}^9\hspace{0.5em}\mathrm{G}$ accuracies range between $\lesssim {10}^{-9}\mathrm{a}.\mathrm{u}.\mathrm{}$ for the ground state and $<5\mathrm{}\times {10}^{-4}\mathrm{a}.\mathrm{u}.\mathrm{}$ for the majority of states evolving from the field-free $1s3l$ states, which matches the resolution of astronomical spectra. Correlation effects contribute $\lesssim {10}^{-2}\hspace{0.5em}\mathrm{a}.\mathrm{u}.\mathrm{}$ to the energy shifts by the field. The energies are variational upper bounds obtained with Hylleraas-like explicitly correlated functions.

We analyze the influence of classical phase-space structures on the ionization behavior of Rydberg atoms in parallel electric and magnetic fields. Classically, ionization above the Stark saddle-point energy is suppressed for moderate magnetic fields compared to zero magnetic field. The location of a stable periodic orbit in the surface of section serves as a criterion to understand this trend. For strong magnetic fields the effect is reversed, and ionization is enhanced. This fact is related to the onset of global chaos in the Hamiltonian system. We provide an analytic criterion for the field values at which this crossover takes place.

We experimentally study the use of two-dimensional magneto-optical trapping (2D-MOT) for the generation of slow beams of cold atoms out of a vapor cell. A particularly high flux of $9\times {10}^9$ rubidium atoms/s at a mean velocity of 8 m/s is obtained using a combination of magneto-optical trapping in two dimensions and Doppler cooling in the third dimension $({2\mathrm{D}}^{+}-\mathrm{MOT}).\mathrm{}$ The resulting width of the velocity distribution is 3.3 m/s [full width at half maximum (FWHM)] with a beam divergence of 43 mrad (FWHM). We investigate the total flux as a function of vapor cell pressure and determine the velocity distribution of our slow atom sources. For comparison, we also realized a low-velocity intense source (LVIS), first reported by Lu et al. [Phys. Rev. Lett. 77, 3331 (1996)]. We find that the ${2\mathrm{D}}^{+}-\mathrm{MOT}$ yields a significantly higher flux than the LVIS, even when used with an order of magnitude less laser power.

The complex processes leading to the collisional population of ultra-long-lived Rydberg states with very high angular momentum can be explained surprisingly well using classical mechanics. In this paper, we explain the reason behind this striking agreement between classical theory and experiment by showing that the classical and quantum dynamics of Rydberg electrons in weak, slowly varying external fields agree beyond the mandates of Ehrenfest’s theorem. In particular, we show that the expectation values of angular momentum and Runge-Lenz vectors in hydrogenic eigenstates obey exactly the same perturbative equations of motion as the time averages of the corresponding classical variables. By time averaging the quantum dynamics over a Kepler period, we extend this special quantum-classical equivalence to Rydberg wave packets relatively well localized in energy. Finally, the perturbative equations hold well also for external fields beyond the Inglis-Teller limit, and in the case of elliptic states, which yield the appropriate quasiclassical initial conditions, the matching with classical mechanics is complete.

A Fokker-Planck equation is derived for the energy distribution of atoms in a three-dimensional harmonic trap with fluctuations in the spring constant and the equilibrium position. Using this model, we predict trap lifetimes based on the measurable noise spectra of the fluctuations. The energy distributions evolve into a single eigenmode where the apparent temperature of the distribution remains constant while the population decays as a consequence of the energy input. The method of analysis and the corresponding results are applicable to any optical, magnetic, or ion trap that is approximately harmonic, and offer useful insights into both noise-induced and optical heating processes.

Theoretical simulations of processes leading to the complete Coulomb explosion of ${\mathrm{H}}_2$ under a 150-fs, 800-nm Ti:sapphire laser pulse are made using two approaches. The first approach consists of purely nuclear, multichannel wave-packet calculations while the second approach involves electronic-nuclear, three-body wave-packet propagations on one-dimensional Coulomb-type potentials. The calculations that give proton kinetic energy spectra in qualitative agreement with the experimental results show that the high-energy, broadband found in these spectra results from the Coulomb explosion of ${\mathrm{H}}_2^{+}$ following the enhanced ionization of the ion at a large internuclear separation, while the structures found at lower energies arise from the laser-induced dissociation of the molecular ion through the Floquet one- and two-photon pathways by the bond-softening mechanism. The experimental observations pertaining to the dependence of these structures on the field intensity indicate that the onset of the molecular dissociation of ${\mathrm{H}}_2^{+}$ occur at mid-pulse, i.e., at the end of the rise time of the laser pulse. The reported simulations illustrate the importance of coherently describing the initial phase of the dynamics, more precisely, the preparation of ${\mathrm{H}}_2^{+}$ by the laser-induced first ionization of ${\mathrm{H}}_2.$

We report measurements of high-order harmonic spectra obtained with a 800-nm 150-fs laser pulse with a time-varying degree of ellipticity. The modulation of the polarization in time is achieved by using birefringent optics and self-phase modulation in a glass plate. We can create one or two temporal gates of a few femtoseconds width, during which the polarization is linear and harmonic emission is efficient. The harmonic spectra observed experimentally demonstrate that harmonics generated with linear polarization are frequency chirped. The values measured experimentally are consistent with theoretical predictions based on the strong field approximation.

Evolution of atomic ionization into the strong-field limit offers the opportunity to study the fundamentals of atom-laser interaction. In this study, we report on high precision measurements of the ion and electron distributions from laser-excited helium and neon atoms which reflect the changing continuum dynamics as the ionization process evolves into the pure tunneling regime. The experiments present evidence of both single- and two-electron ionization. These data provide a direct quantitative test of various theories of strong-field ionization. We show that a relatively simple semiclassical model which includes a description of a field-driven electron elastically rescattering from an accurate ion core potential reproduces the measured electron distributions for both atoms. However, using this model to calculate $e-2e$ inelastic rescattering yields cross sections which are incompatible with the measured two-electron ionization.

We demonstrate the possibility of three-dimensional cooling of neutral atoms by illuminating them with two counterpropagating laser beams of mutually orthogonal linear polarization, where one of the lasers is a speckle field, i.e., a highly disordered but stationary coherent light field. This configuration gives rise to atom cooling in the transverse plane via a Sisyphus cooling mechanism similar to the one known in standard two-dimensional optical lattices formed by several plane laser waves. However, striking differences occur in the spatial diffusion coefficients as well as in local properties of the trapped atoms.

Raman cooling of non-zero-spin atoms in the presence of gravitational and external magnetic fields is investigated. The magnetic field is adjusted so as to compensate for the gravitational force acting on ground-state atoms. A dark state is created and supported in momentum space with additional velocity-selective two-photon transitions. The minimum allowed temperature is found to be determined only by the width of velocity selection, and therefore can be much less than the gravitational limit. A complete set of analytical formulas describing cooling of a dilute atomic sample is derived. They serve as the basis for numerical simulations which are carried out in the one-dimensional case.

The dynamics of Rydberg states of atomic hydrogen illuminated by resonant elliptically polarized microwaves is investigated both semiclassically and quantum mechanically in a simplified two-dimensional model of an atom. Semiclassical predictions for quasienergies of the system are found to be in very good agreement with exact quantum data enabling a classification of possible types of motion and their dynamics with the change of the ellipticity of the microwaves. Particular attention is paid to the dynamics of the nonspreading wave packet states, which are found to exist for an arbitrary microwave polarization.