We have developed a simplified relativistic configuration-interaction method to calculate the dielectronic recombination cross sections and rates. In this method, the infinite resonant doubly excited states can be treated conveniently in the frame of quantum defect theory. As a stringent test, we calculated the dielectronic recombination cross sections for hydrogenlike helium. The results for the KLL and KLM resonances are in good agreement with the experimental measurement.

A theory of coherent control of above-threshold ionization (ATI) of hydrogen using light fields composed of a fundamental frequency and its third harmonic is developed. Phase control and intensity control over integral and differential quantities is demonstrated. Modulation of the first ATI kinetic-energy peak height by more than 70%, variation in the ratio of the first two peaks of up to a factor of 4, and separation in the alignment of the first two ATI peaks by as much as ∼50°, as a function of the relative phase between the two fields, are demonstrated. Intensity control, in which both partial and total integral ATI rates attain a local maximum at a specific combination of light intensities, is predicted to occur.

The dynamic Kingdon trap recently proposed by Blümel [Phys. Rev. A 51, R30 (1995)] is a storage device for charged particles. The equation of motion for one trapped ion, the so-called Kingdon equation r¨+gammaṙ+(1-2ηcos2t)${\mathit{r}}^{\mathrm{-}}$1=0, allows stable limit cycle solutions for a wide range of parameters. In this Brief Report we calculate the periodic orbit of a trapped ion approximately and by using a singular perturbation analysis, and discuss its main properties.

We predict and confirm numerically the existence of shape-invariant Rydberg wave packets in the hydrogen atom in a strong linearly polarized electromagnetic field. They correspond to similar wave packets recently discussed for the same quantum system, but when the electromagnetic field was circularly polarized. The trajectory of the packets is elliptical within the present approximation and the parameters of the ellipse are found. For every electric-field strength there is a twin pair of states.

We have solved the time-dependent Schrödinger equation for a hydrogen atom, initially in its ground state, subject to a strong, ultrashort laser pulse of high frequency, ω=2.0 a.u. We compare and interpret our results in terms of the time-independent Floquet eigenvalues. Even for an ultrashort pulse of 3 cycles half-width, the Floquet adiabatic picture is a very good approximation. We discuss the physical reasons and also the visible deviations from the single-resonance-state approximation.

A two-level atom interacting with a single-mode squeezed vacuum cavity field does not manifest collapses and revivals of the oscillations [Milburn, Opt. Acta 31, 671 (1984)]. Here we show that the dynamic response of the inversion of an assemblage of A two-level atoms to such a field (Dicke model) may be qualitatively different. The dynamic properties of the model with an initial squeezed vacuum field differ from those for coherent or thermal fields, which have been stressed in the title.

In a recent article [D. Suter, Phys. Rev. A 51, 45 (1995)] Suter has claimed to present an optical implementation of the quantum time-translation machine that ‘‘shows all the features that the general concept predicts and also allows, besides the quantum-mechanical, a classical description.’’ It is argued that the experiment proposed and performed by Suter does not have the features of the quantum time-translation machine and that the latter has no classical analog.

Inverse spectral theory provides a theorem on the shifting of a single level, leaving all others unchanged, in a quantum-mechanical system over a finite range in one dimension. We show that the mechanism can be used to remove a pair of states from the spectrum by shifting the energy of one state onto that of a neighboring state. We extend the procedure to potentials defined on the whole axis possessing a purely discrete or a partly discrete and partly continuous spectrum.

The spectrum of the hydrogen atom perturbed by the generalized van der Waals interaction ΔV(x,y,z)=γ(${\mathit{x}}^2$+${\mathit{y}}^2$+${\mathrm{\beta }}^2$${\mathit{z}}^2$) is analyzed with a nonperturbative method of global approximation on a subspace grid. Considerable deviation from the perturbation formulas and violation of the analytic spectrum, which the system exhibits in special cases of the dynamical symmetry β=1/2, 1, and 2, have already been found for the field strengths H=√8γ∼${10}^{\mathrm{-}3}$ a.u. ∼${10}^6$ G, in the region γ${\mathit{n}}^4$≪1 (n∼10).

We have implemented a completely analytical linear combination of atomic orbitals-Xα local density-functional method with exact energy gradients. The superiority of the analytical method over the corresponding grid-based technique in terms of speed, smooth energy surfaces, and accurate gradients is demonstrated in calculations for the umbrella inversion mode of ammonia. The equilibrium geometry and inversion barrier height are also shown to compare well to experimental data.

It has been suggested recently by Chen et al. [Phys. Rev. A 49, 3472 (1994)] that the measured long-lived ${\mathrm{NeAr}}^{2+}$ formed in fast ${\mathrm{NeAr}}^{+}$ + Ar charge-stripping collisions is mostly in its v=12 vibrational state bound to the electronic ground state, and that this molecular ion decays by tunneling through the potential barrier. Such a decay rate is expected to depend strongly on the reduced mass of the molecular ion leading to large isotopic effects. We have measured the mean lifetimes of the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}_{}^{40}{}_{}{}^{}$${\mathrm{Ar}}^{2+}$ and ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}_{}^{40}{}_{}{}^{}$${\mathrm{Ar}}^{2+}$ isotopes in order to see this isotopic effect. Surprisingly, the mean lifetimes of both isotopes are similar to each other. Thus, it is suggested that the observed ${\mathrm{NeAr}}^{2+}$ molecular ions do not decay via a tunneling mechanism, which would indicate that they reside in the metastable electronic ground state. Rather, electronic transitions from bound or metastable excited states into other repulsive states are the origin for the experimentally observed decay. Qualitative estimates for the shapes and ordering of these states in the electronic spectrum of ${\mathrm{NeAr}}^{2+}$ are given.

We study the ground-state structural properties of a hydrogen molecule in superstrong magnetic fields (B≊${10}^{12}$ G) using quantum Monte Carlo (fixed-phase and variational) approaches. We determine that the ground state (spin-triplet) belongs to the sector of total (z-component) angular momentum M=-1 ${(}^3$${\mathrm{\Pi }}_0$), meaning that paramagnetic contributions to the total energy cannot be neglected. This non-time-reversal invariant ground state has a strong interatomic interaction, suggesting that a hydrogen gas under the same physical conditions has a tendency to form strong bonded molecules.

The ${\mathrm{Ar}}^{2+}$(3${\mathit{s}}^2$3${\mathit{p}}^3$nl) satellite states between 73 and 85 eV have been observed directly by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The present study extends the previous observations of these states by optical emission spectroscopies to the region close to the ${\mathrm{Ar}}^{3+}$ ionization. Different features have been discerned in the TPESCO spectrum. A preliminary assignment of these features using the Rydberg formula and values of the quantum defect that are deduced from lower-lying states is proposed.

The cross sections for single and double photoionization of helium at energies from the threshold of double ionization to 280 eV are calculated by combining the previously developed hyperspherical close-coupling method with a discretization procedure for the continuum spectrum of ${\mathrm{He}}^{+}$. No pseudoresonances due to the discretization are found. Both the length and acceleration forms of the dipole operator are used, and the results hardly depend on these different gauges. The results are compared with previous theories and experiments.

We present detailed calculations of a new scheme for the adiabatic transfer of population in molecular vibrational ladders and for controlling chemical reactions: Raman chirped adiabatic passage. The scheme makes use of far-off-resonant Raman transitions with one chirped and one monochromatic laser pulse. We show that this method can be used to climb vibrational ladders in molecular systems and, with sufficiently large chirping, can vibrationally dissociate diatomic molecules.

We demonstrate a method to directly observe the dynamics of motion of cold atoms in an optical standing wave using a recoil-induced resonance. By detecting a transient response of this resonance, we observed the interaction between an atomic population grating and a periodic optical potential. Using this method, we investigated the lifetime of the atomic grating under various conditions in the optical potential. As a result, we obtained the appropriate conditions for the long lifetime and observed the narrowest linewidth in the recoil-induced resonance corresponding to the atomic temperature of 4.4±0.3 μK for ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ atoms.

We report the observation of three-dimensional cooling of cesium atoms using a four-beam optical molasses tuned to the blue side of the ${\mathit{F}}_{\mathit{g}}$=3→${\mathit{F}}_{\mathit{e}}$=2 hyperfine component of the 6${\mathit{S}}_{1/2}$→6${\mathit{P}}_{3/2}$ transition. The cooling mechanism is a Sisyphus effect involving atomic states that are nearly uncoupled from the light field. Starting from a magneto-optical trap at 70 μK, the cooling time was on the order of 1 ms with almost 100% capture efficiency and temperature smaller than 5 μK measured. We also present a probe transmission spectrum showing that almost all the atoms are found in the nearly dark states.

A two-photon optical imaging experiment was performed based on the quantum nature of the signal and idler photon pairs produced in spontaneous parametric down-conversion. An aperture placed in front of a fixed detector is illuminated by the signal beam through a convex lens. A sharp magnified image of the aperture is found in the coincidence counting rate when a mobile detector is scanned in the transverse plane of the idler beam at a specific distance in relation to the lens.