A spectral line-shape model accounting for the speed dependence of the relaxation parameters has been proposed for the collision regime [D. Robert et al., Phys. Rev. A 47, R 771 (1993)]. In this model, the speed class exchanges are governed by a memory parameter playing a crucial role. It has been exhaustively used to accurately describe the observed inhomogeneous features of ${\mathrm{H}}_2$ vibration line profiles in various mixtures with respect to concentration and temperature. Molecular dynamics calculations of the characteristic memory parameter are performed in order to test this model from first principles. The resulting data are in remarkable agreement with those deduced from experiments through the model.

The diatomics-in-molecules (DIM) method for the construction of polyatomic potential-energy functions from the potential energies of atomic and diatomic fragments is generalized to obtain a description of potential-energy functions of both quasistationary and bound states of polyatomic anions. The formulation is based on the combination of the DIM method with the projection-operator approach of scattering theory. The proposed theory allows the construction of diabatic discrete states, electron-molecule scattering continua, and discrete-continuum coupling elements from the corresponding data of the fragments. The polyatomic projection-operator description obtained in this way provides the basis for a rigorous treatment of the nuclear dynamics in short-lived electron-molecule collision complexes and ion-molecule collisions in terms of energy-dependent, complex, and nonlocal effective potentials. More approximate local complex potential-energy surfaces of quasistationary states of polyatomic anions also can be obtained with the generalized DIM method via the determination of the poles of the multichannel electron-molecule scattering matrix in the fixed-nuclei limit. Although the focus of the present work is on anions, the proposed theory is also applicable to quasistationary states of neutral and positively charged polyatomic systems including clusters. To illustrate the concepts, the generalized DIM method is applied to obtain the potential-energy functions of the ground and first excited states of the ${\mathrm{H}}_3^{-}$ anion, making use of information that is available for the bound and resonance states of ${\mathrm{H}}^{-}$ and ${\mathrm{H}}_2^{-}.$

The scattering of orthopositronium from He, Ne, Ar, ${\mathrm{Li}}^{+},$ ${\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ is investigated with the fixed-core stochastic variational method. The scattering length for $\mathrm{Ps}-\mathrm{He}$ scattering was ${1.57a}_0,$ a value consistent with the threshold cross sections derived from three positron lifetime experiments. The scattering lengths for the $\mathrm{P}\mathrm{s}-\mathrm{N}\mathrm{e}$ and $\mathrm{P}\mathrm{s}-\mathrm{A}\mathrm{r}$ systems were ${1.55a}_0$ and ${1.79a}_0,$ respectively. That there was only a 15% variation among the scattering lengths is compatible with the experiment of Coleman et al. [J. Phys. B 27, 981 (1994)], who found the low-energy cross sections for He, Ne, and Ar to be about the same size. The scattering lengths for Ps scattering from ${\mathrm{Li}}^{+},$ ${\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ were ${12.9a}_0,$ ${28.5a}_0,$ and $-{1.9a}_0.$ The relatively small magnitude of the ${\mathrm{P}\mathrm{s}-\mathrm{K}}^{+}$ scattering length strongly supports previous suggestions that the ${\mathrm{P}\mathrm{s}-\mathrm{K}}^{+}$ system does not support a bound state. The annihilation parameter ${}^1{Z}_{\mathrm{eff}}$ has also been computed as part of the analysis. The present values of ${}^1{Z}_{\mathrm{eff}}$ are about 2.5–3 times smaller than the accepted experimental values since short-range electron-positron correlations were not taken into consideration when the annihilation matrix element was evaluated.

The dependence of multiple ionization cross section on the orientation of the molecular axis in collisions of fast ions with diatomic and linear triatomic molecules is studied theoretically. The ionization process is described within the statistical energy deposition model. The energy transferred in the collision is calculated within the unitary-convolution approximation by Schiwietz and Grande. The impact-velocity dependence of the orientation effect as well as its dependence on the molecular properties are studied.

Photoion yields from ${\mathrm{Ne}}^{+}$ to ${\mathrm{Ne}}^{2+}$ and from ${\mathrm{Ne}}^{2+}$ to ${\mathrm{Ne}}^{3+}$ were measured near the K-shell $\mathrm{}(1s-2p)\mathrm{}$ autoionizing resonance region in the 841–858-eV and 850–863-eV photon energy range, respectively. Measurements were performed using a photon-ion merged-beam apparatus with an electron cyclotron resonance ion source. Multiconfiguration Dirac-Fock calculations were performed to interpret the experimental results. The calculations show fairly good agreement with the experimental results. Isonuclear series for the photoionization of ${\mathrm{Ne}}^{+},$ ${\mathrm{Ne}}^{2+},$ and ${\mathrm{Ne}}^{3+}$ are discussed.

We calculate the ionization-exitation and double ionization cross sections of the valence ${2s}^2$ shell of beryllium. Our model combines a multiconfiguration Hartree-Fock expansion of the beryllium atom ground state and a momentum space close-coupling expansion of the final ionized state. A near-complete set of negative and positive energy pseudostates is employed to represent various singly and doubly ionized channels. The role of the frozen ${1s}^2$ core is elucidated by comparing the beryllium single and double photoionization cross sections with those of the “hollow” helium ${2s}^2$ atom in which the $2s$ orbital is made orthogonal to the vacant $1s$ orbital. The angular correlation in the two-electron continuum is studied by calculating the triply differential cross section of Be at equal energy sharing between the photoelectrons.

We have calculated the total cross section for proton-impact ionization of atomic hydrogen for the intermediate energy range 25–100 keV. We evaluate error bars of 5% on our theoretical values. The present results disagree with the experiment of Shah and Gilbody [J. Phys. B 14, 2361 (1981)] and Shah et al. [J. Phys. B 20, 2481 (1987)], differing by 20% at the peak of the ionization cross section. On the other hand, our results show better agreement with two recent theoretical calculations, one performed by close coupling and the other by integrating the time-dependent Schrödinger equation on a three-dimensional lattice. The present results also compare favorably with theoretical and experimental determinations of capture and excitation cross sections. We describe the two-center momentum space discretization method, used here, in detail, explaining how the cross sections and corresponding error bars are determined.

We present measurements of spin-relaxation of Rb atoms in $\mathrm{He}\mathrm{}(98\%)\mathrm{}-{\mathrm{N}}_2\mathrm{}(1\%)\mathrm{}-\mathrm{Xe}\mathrm{}(1\%)\mathrm{}$ mixtures similar to those used in magnetic resonance imaging polarizers. The pressure dependence allows us to separate out contributions from Rb-Xe van der Waals molecules and binary collisions. For the first time, we observe the predicted increase in the molecular contribution at high pressure. Our data suggest that the deduced molecular breakup rate has a strong temperature dependence.

The ionization of alkali-metal atoms by ultrashort laser pulses, which perform less than two optical cycles, is investigated by means of a theoretical method based on Coulomb-Volkov (CV) states. This approach was shown to provide reliable predictions for the ionization of hydrogen atoms both at small and high laser intensities. In the present paper, it is extended to the ejection of electrons from the valence shell of alkali-metal atoms. CV predictions are shown to be in good agreement with the results given by a full numerical treatment of the time-dependent Schrödinger equation. Compared to hydrogen targets, the presence of the core results in a much more extended domain of application of the CV approach.

We consider a system composed of a cavity field, a counterpropagating pump laser field, and a two-level Bose condensate travelling along a direction normal to the optical fields. We reduce the system, in the limit of first-order scattering approximation, to a four-wave parametric amplifier involving three momentum modes and an effective two-photon cavity mode. We treat this wave mixing as a multimode “laser” operation, and discuss the mode-locking condition under which these “laser” modes can be synchronized to operate in a continuous wave fashion. We derive the threshold lasing condition and discuss it in analogy with the Raman-anti-Stokes process in nonlinear optics. We develop a matrix formalism that enables the longitudinal spatial degree of freedom to be integrated analytically, leading to an efficient numerical algorithm for determining the steady states and their stabilities. Numerical examples will be provided and discussed. In particular, instability analysis seems to suggest Bose-Einstein condensate holds a great promise for nonlinear instability studies because of its unprecedented long interaction time with photons.

We analyze a mean-field model of an atomic Bose-Einstein condensate parametrically coupled to a molecular condensate via a Raman photoassociation process. We show that an interplay of nonlinear interspecies and intraspecies interactions leads to the formation of mutually trapped states of a hybrid condensate, which are spatially localized and dynamically stable even without a trap.

We find that hexagonal structures forming in semiconductor resonators can range from coherent patterns to arrangements of loosely bound spatial solitons, which can be individually switched. Such incoherent arrangements are stabilized by gradient forces, as evidenced by the stability of hexagonal structures with single- or multiple-soliton defects. We interpret the experimental observations by numerical simulations based on a model for a large aperture semiconductor microresonator.

We show that the spatial entanglement of two twin images obtained by parametric down conversion is complete, i.e., concerns both amplitude and phase. By considering a homodyne detection scheme, which allows comparison of field quadrature components of the two images pixel by pixel, Einstein-Podolsky-Rosen correlations are shown to exist between symmetrical pixels of the two images. The best possible correlation is obtained by adjusting the phase profile of the local oscillator in the amplification area. The results for quadrature components hold even in the absence of any input image, i.e., for pure parametric fluorescence. In this case, they are not related to intensity and phase fluctuations.

We utilize a two-photon polarization spectroscopy technique to observe Ramsey fringes that appear due to light scattering by coherently prepared rubidium atoms. We examine the effect in a vapor cell for spatially and temporally separated laser fields. We present a theoretical interpretation of the experimental results.

Recently, there has been considerable interest in atoms confined to optical cavities or embedded within photonic band-gap materials, where the spectrum of vacuum fluctuations is modified. As a first step in extending traditional semiclassical methods into this regime, the present paper describes a semiclassical, density-matrix methodology for treating radiative processes in a low-Q cavity. This semiclassical approach is tested by comparing its predictions to the optical cavity experiments of Heinzen and Feld [Phys. Rev. Lett. 59, 2623 (1987)] and by calculating transient-nutation decay in free space induced by a broadband laser. The methodology is then used to examine transient nutation in a low-Q optical cavity, where it is found that the detuning between the atomic and cavity resonances has a significant effect on the transient nutation’s decay rate.

We consider hexagonal pattern forming in the cross section of an optical beam produced by a Kerr cavity, and we study the quantum correlations characterizing this structure. By using arguments related to the symmetry broken by the pattern formation, we identify a complete scenario of six-mode entanglement. Five independent phase quadrature combinations connecting the hexagonal modes are shown to exhibit sub-shot-noise fluctuations. By means of a nonlinear quantum calculation technique, quantum correlations among the mode photon numbers are demonstrated and calculated.

The radiative lifetime of the ${3s}^2{3p}^4$ ${}^1{S}_0$ level in ${\mathrm{Ar}}^{2+}$ ions has been measured by time-resolved observation of the magnetic dipole $\mathrm{}\left(M1\mathrm{}\right)\mathrm{}$ transition to the ${\mathrm{ns}}^2{\mathrm{np}}^4$ ${}^3{P}_1$ level, on ions circulating in a heavy-ion storage ring. The result of $145\pm 5\hspace{0.5em}\mathrm{ms}$ compares well with various theoretical predictions, but is significantly more accurate and more precise than earlier measurements.

The Poynting vector, energy density, and energy velocity of light pulses propagating in an anomalous dispersion medium with two closely spaced gain lines are calculated. The results show that a negative energy density in the medium propagates along the opposite direction of incidence with a velocity similar to the negative group velocity $v_g$ while Poynting vector is in the same direction of incidence. In other words, one might say that a positive energy density in the medium would propagate along the positive direction with a speed $|v_g|.$ We further point out that neither energy velocity nor group velocity is a good concept to describe the propagation process of light pulse inside the medium in the Wang, Kuzmich, and Dogaiu [Nature (London) 406, 277 (2000)] experiment owing to the strong accumulation and dissipation effects.

We use a stable, 5 dB, amplitude squeezed source for a quantum nondemolition (QND) experiment. The performance of our QND system is enhanced by an electro-optic feedforward loop which improves the signal transfer efficiency. At best, we measure a total signal transfer of 1.81 and conditional variance of 0.55.

The dynamics of a quantum system undergoing frequent measurements (quantum Zeno effect) is investigated. Using asymptotic analysis, the system is found to evolve unitarily in a proper subspace of the total Hilbert space. For measurements represented by spatial projections, the generator of the “Zeno dynamics” is the Hamiltonian with Dirichlet boundary conditions.