Trap-loss rates due to hyperfine–structure-changing collisions involving cold Na and K atoms in a mixed species magneto-optical trap have been measured. The investigations were performed in the low intensity regime, where hyperfine–structure-changing collision are believed to be the dominant mechanism for atomic losses. Comparison between homonuclear (Na-Na) and heteronuclear (Na-K) collision rate coefficients reveals a higher loss rate in the case of heteronuclear collisions. Results obtained in the regime of low trap laser intensity are compared with existent loss rates for several alkali metals, showing themselves to be in good agreement with those expected.

Electron-impact ionization of hydrogen in the presence of a strong laser field is studied in the framework of the time-dependent Hartree-Fock theory. The probability distribution of the impact electron is further approximated by a plane circular disk, moving on a classical path along the polarization direction of the laser field. The remaining time-dependent Schrödinger equation for the hydrogen atom is solved numerically by finite-difference methods for different impact parameters, electron energies, and laser intensities. The integrated cross section for field-free impact ionization is found to agree remarkably well with previous results in the intermediate and high energy range (60–600 eV). Field ionization and collisional ionization can be simultaneously calculated with this model.

We have trapped and sympathetically cooled the molecular ions ${\mathrm{HCO}}^{+}$ and ${\mathrm{N}}_2{\mathrm{H}}^{+}$ in a Penning trap. Through their Coulomb interaction with laser-cooled ${\mathrm{Mg}}^{+}$ ions the molecules were sympathetically cooled to cryogenic temperatures. We identify the molecules through a measurement of their characteristic mass-dependent breathing mode frequencies. From a measurement of the temperature of the ${\mathrm{Mg}}^{+}$ ions we estimate that the final temperature of the sympathetically cooled molecules is 4 K.

To explore the insights of recent experiments on the excitation of the NO molecule by chirped subpicosecond IR pulses, a corresponding theoretical study is presented. The agreements between experimental and theoretical results show the importance of the full-dimensional quantum-mechanical calculation.

The multiphoton ionization and photoelectron spectra of magnesium were studied at laser intensities of up to $6\times {10}^{13}{\mathrm{}\mathrm{W}\mathrm{}\mathrm{cm}}^{-2}$ using 150-fs laser pulses of a wavelength of 400 nm. The results indicated that a variety of different ionization mechanisms played a role in both types of spectra. A theoretical model describing the processes is presented and the routes to ionization are identified. The work demonstrates the significance of the two-electron nature of the atom in interpreting the experimental results.

We report on the observation of a high-energy “shoulder” in the kinetic-energy distribution of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ ions formed in the interaction of ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ with ultrashort light pulses of an intensity higher than approximately ${10}^{15}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ It extends up to a kinetic energy of $\approx 8\hspace{0.5em}\mathrm{eV}.$ The “shoulder” ions are formed by Coulomb explosion after photoionization of the respective molecular ion. The shape of the “shoulder” indicates that photoionization of ${\mathrm{H}}_2^{+}$ in the vibrational ground state $v=0\mathrm{}$ and of ${\mathrm{D}}_2^{+}$ in $v=0,1\mathrm{}$ is the mechanism for atomic ion formation near the “shoulder” cutoff.

We have carried out a precise measurement of the wavelength dependence of the two-step photoionization cross section of triatomic hydrogen via the $3s{}^2{A}_1^{\prime }$ Rydberg state at energies beyond the ionization limits that belong to the first symmetric-stretch excited vibrational state $\{{1,0}^0\}$ of the ion core. In this energy region, rotational and vibrational channel couplings due to the interaction between the outer p electron and the ${\mathrm{H}}_3^{+}$ core are expected. We find strong coupling between the two p series with total angular momentum $N=2\mathrm{}$ which converge to the rotational levels $N^{+}=1\mathrm{}$ and 3 of the underlying ${\mathrm{H}}_3^{+}\{{1,0}^0\}$ core. Surprisingly, regular perturbations are also detected in the symmetric-stretch-excited $N=1\mathrm{}$ p series. Rydberg levels of the ${\mathrm{ndE}}^{\prime }$ $\{{1,0}^0\}$ series are identified as the perturbing channel. This implies a change in the orbital angular momentum of the Rydberg electron during the short-range interaction. The experimental spectra are analyzed in two-channel quantum-defect models. Eigenchannel quantum defects, coupling constants, and transition moment ratios are determined and compared to the results of recent ab initio calculations.

The dependence on laser intensity of recently discovered plateau structures in laser-assisted x-ray–atom scattering, both with and without a static electric field present, is analyzed. Using the “three-step” model and the strong-field approximation we demonstrate a connection between laser-assisted, x-ray–atom scattering and high-order harmonic generation: For high laser-field intensities without a static field present, both processes have plateaus whose energies extend to the cutoff value ${3.17U}_p,$ where $U_p$ is the ponderomotive potential energy. For x-ray–atom scattering in the presence of a static electric field we show that at high laser-field intensities two plateaus appear: One is our recently predicted high-energy plateau for the same process [Phys. Rev. Lett. 81, 5097 (1998)], while the other, low-energy plateau, has a differential cross section six orders of magnitude larger. The energy positions and relative magnitudes of these new plateaus are explained using semiclassical arguments.

Stabilization of autoionizing states of barium by laser-induced, stimulated emission of light is demonstrated. Relative to purely flourescent stabilization, the data clearly show an enhancement of the stabilization process for laser pulses short compared to the flourescent lifetime of the autoionizing states. Shakeup spectra in which the principal quantum number of both electrons changes during the stimulated emission process are also clearly demonstrated.

We model the response of an atomic cluster to an intense laser pulse by numerically solving the time-dependent Schrödinger equation for a simplified one-dimensional system. We address the questions of the generation of high-order harmonics of the laser frequency and of the ionization dynamics of the atoms within the cluster for very short laser pulses.

A single-particle kinetic theory of the magneto-optical trap (MOT) for cold atoms is developed for a multilevel dipole interaction scheme. The theory analyzes the one-dimensional motion of $\mathrm{}(3+5)\mathrm{}$-level atoms in an inhomogeneous magnetic field and in the field of two counterpropagating circularly polarized laser waves. It is shown that in multilevel interaction schemes two-photon and generally even-order multiphoton processes are responsible for the formation of the narrow velocity and spatial structures in the radiation pressure force. The structures are located at zero velocity and near the origin of the trap. It is found that in the case of a $\mathrm{}(3+5)\mathrm{}$-level atom, single- and two-photon processes produce the two-component structure of the MOT potential well. Near the bottom of a conventional spatially broad potential well caused by one-photon processes, there is a much narrower well caused by two-photon processes. The double-structure MOT potential well is shown to be responsible for the two-component spatial distribution of the trapped atomic cloud. It is proposed that tight confinement of atoms near the narrow bottom of the MOT potential well can be used for the creation of a pointlike source of cold atoms for various applications in atom optics.

We calculate the excitation frequencies of low-lying modes of a trapped Bose-condensed gas at nonzero temperatures. In our calculation we include the dynamics of the noncondensed cloud, and find agreement with experimental results if we assume that in the experiment both the in-phase and out-of-phase monopole modes are excited simultaneously. In order to explore whether this is indeed the correct explanation, we also calculate how strongly the modes couple to a perturbation of the external trapping potential.

We propose a laser field configuration that acts as a quasiperiodic atom optical diffraction grating. Exact analytical results for the atomic center-of-mass wave function in the Fresnel region after the grating reveal a quasiperiodic density pattern, a semiclassical focusing effect, and a quasiperiodic self-imaging of the atomic wave function analogous to a Talbot effect. The high-contrast quasiperiodic pattern that forms at the focal plane survives both transverse and longitudinal velocity averaging. The quasi-Talbot effect should be observable using atoms from Bose-Einstein condensates.

The second-order coherence of photons scattered from a trapped Bose-Einstein condensate is found to be enhanced for the scattering angles that are either the same or symmetrical with respect to the direction of laser propagation. The enhancement occurs periodically with respect to the time interval between photons, and becomes less and less pronounced with increasing the strength of the interaction between atoms.

We study the conditions under which atomic condensates can be used as holographic recording media and then suggest a scheme which allows us to reconstruct the original object with an atomic reading beam. The recording method relies on the observation that the shape of a condensate in the Thomas-Fermi limit closely follows the form of the confining potential. Holographic information can thus be stored as density modulations. The reading scheme proceeds in analogy to optical Fresnel sideband holography with an atomic beam replacing the reading laser.

We combine the ideas of dressed Bose-Einstein condensates, where an intracavity optical field allows one to design coupled, multicomponent condensates, and of dark states of quantum systems, to generate a full quantum entanglement between two matter waves and two optical waves. While the matter waves are macroscopically populated, the two optical modes share a single photon. As such, this system offers a way to influence the behavior of a macroscopic quantum system via a microscopic “knob.”

Cryogenically cooled microelectromagnet mirrors were used to reflect a cloud of free-falling laser-cooled ${}^{85}\mathrm{Rb}$ atoms at normal incidence. The mirrors consisted of microfabricated current-carrying Au wires in a periodic serpentine pattern on a sapphire substrate. The fluorescence from the atomic cloud was imaged after it had bounced off a mirror. The transverse width of the cloud reached a local minimum at an optimal current corresponding to minimum mirror roughness. A distinct increase in roughness was found for mirror configurations with an even versus odd number of lines. These observations confirm theoretical predictions.

The theory of laser oscillation with an intracavity sinusoidal modulation of the optical frequency is revisited and analyzed in the framework of general principles governing the properties of time-dependent periodic systems. It is shown that the two traditional and complementary descriptions of frequency modulation (FM) laser oscillation and pulsed FM mode-locking [S.E. Harris and O.P. McDuff, IEEE J. Quantum Electron. $\mathbf{QE}-1,$ 245 (1965); D.J. Kuizenga and A.E. Siegman, ibid. QE-6, 694 (1970)] can be unified by means of a more general approach based on a Floquet analysis of the laser equations in presence of a periodic phase perturbation. Starting from a spatially extended model of intracavity laser frequency modulation for a homogeneously broadened two-level ring laser, the relevant Floquet modes and corresponding Floquet exponents governing the stability properties of the nonlasing state are derived as solutions of a nonlinear eigenvalue problem. Resonance phenomena, which occur when the modulation frequency is made close to an integer multiple of the cavity axial mode separation, explain the onset of FM laser oscillation and the transition to the pulsed FM mode locking closer to the synchronous modulation. In particular, the transition from FM laser oscillation to the pulsed FM mode locking is shown to be sharp and due to a crossing of the threshold curves of two distinct Floquet modes. The role of cavity dispersion on the transition is also investigated.

We present a feasible scheme for reconstructing the quantum state of a field prepared inside a lossy cavity. Quantum coherences are normally destroyed by dissipation, but we show that at zero temperature we are able to retrieve enough information about the initial state, making possible to recover its Wigner function as well as other quasiprobabilities. We provide a numerical simulation of a Schrödinger cat-state reconstruction.

We consider a class of states in an ensemble of two-level atoms: a mesoscopic superposition of two distinct atomic coherent states, which can be regarded as atomic analogs of the states usually called Schrödinger-cat states in quantum optics. According to the relation of the constituents, we define polar and nonpolar cat states. The properties of these are investigated by the aid of the spherical Wigner function. We show that nonpolar cat states generally exhibit squeezing, the measure of which depends on the separation of the components of the cat, and also on the number of the constituent atoms. By solving the master equation for the polar cat state embedded in an external environment, we determine the characteristic times of decoherence and dissipation, and also the characteristic time of a new parameter, the nonclassicality of the state. This latter one is introduced by the help of the Wigner function, which is used also to visualize the process. The dependence of the characteristic times on the number of atoms of the cat and on the temperature of the environment shows that the decoherence of polar cat states is surprisingly slow.