The room-temperature nitrogen fundamental isotropic Q-branch profile was both experimentally and theoretically studied in a large frequency interval of about $100\hspace{0.5em}{\mathrm{cm}}^{-1}$ never explored before, and for densities ranging from 1 to 150 amagat. As regards experiment, this frequency interval could be probed thanks to a highly sensitive spontaneous Raman technique previously developed in our institute. As regards theory, such a large interval of frequencies made it possible to rigorously check, for the scalar relaxation case, the validity of the recently proposed and successfully applied to nitrogen anisotropic spectra energy-corrected sudden (ECS) formalism [J.V. Buldyreva and L. Bonamy, Phys. Rev. A 60, 370 (1999)]. For all the densities probed, an excellent agreement between calculated and measured profiles was attested within a frequency interval of $20\hspace{0.5em}{\mathrm{cm}}^{-1}$ where the intensity decreases by more than three decimal orders from its peak value. The adequacy of the theory to experiment in such a wide frequency range confirmed the reliability of the used ECS modeling, especially for optical diagnostics purposes.

The relativistic theory of above-threshold ionization (ATI) of hydrogenlike atoms in ultrastrong radiation fields, taking into account the photoelectron-induced rescattering in the continuum spectrum, is developed. It is shown that the contribution of the latter in the multiphoton ionization probability even in the Born approximation by the Coulomb field is of the order of ATI probability in the scope of the Keldysh-Faisal-Reiss ansatz.

An atom interacting with a quantized electromagnetic field in a cavity with time-dependent parameters is considered. Variation of the cavity parameters results in nonstationary dynamics of the field which leads, in turn, to excitation of the atom, even if photons were initially absent in the cavity. We distinguish three mechanisms of such excitation: excitation due to absorption of real photons created by the dynamical Casimir effect, excitation due to absorption of virtual photons during the transient process, and excitation due to nonadiabatic parametric modulation of the atomic Lamb shift. The last mechanism has no relation to the dynamical Casimir effect and thus should be considered as a new vacuum QED effect. Normally all these three mechanisms give a contribution to the amplitude of the atom excitation and are accompanied by the creation of photons. Therefore the presence of an atom in the cavity alters the average number of created photons in comparison with the case of an empty non-stationary cavity. Our consideration is based mainly on a simple model of a two-level atom interacting with a single mode of quantized electromagnetic field. However, our results are qualitatively valid for more realistic models.

We analyze analytically and experimentally a cascade of parametric instabilities in an inhomogeneously broadened multimode laser under pump modulation. From the physical model we use a multiple-scale analysis to derive slow time and slow space amplitude equations, taking nonlocal coupling into account. Each of the two waves appearing at the primary instability acts as a pump for the parametric excitation of two new traveling waves with higher wave numbers. These predictions compare well with numerical simulations of the original laser equations and experimental results obtained with a modulated Yb-doped fiber laser.

A recent theoretical paper by Shi et al. [Phys. Rev. A 61, 064102 (2000)] proposes a scheme for entanglement swapping utilizing acousto-optic modulators without requiring a Bell-state measurement. In this Comment, we show that the proposal is flawed and no entanglement swapping can occur without measurement.

Experimental measurements and theoretical calculations of absolute cross sections are carried out for the electron-impact ionization of atomic ions in the Sm rare-earth-metal isonuclear sequence. Absolute cross sections for ${\mathrm{Sm}}^{q+}$ ions for $q=1\mathrm{}-12$ were measured using the electron-ion crossed-beam technique and calculated with the configuration-average distorted-wave method for the charge states $q=4\mathrm{}-12.$ The theory includes both direct ionization and excitation-autoionization contributions, and includes transitions from both ground and metastable levels. We present the cross sections in an electron-impact energy range from threshold up to 1000 eV for all Sm ions up to ${\mathrm{Sm}}^{12+}.$ These systems are extremally complex, but the configuration-average method, when combined with statistical averaging of the cross sections over the many levels, provides results that are in overall good agreement with the experimental measurements. The single-ionization cross sections are dominated by contributions from indirect mechanisms of excitation-autoionization in the low-energy region.

We show theoretically the existence of a metastable state and the possibility of decay to the ground state through macroscopic quantum tunneling in two-component Bose-Einstein condensates with repulsive interactions. Numerical analysis of the coupled Gross-Pitaevskii equations clarifies the metastable states whose configuration preserves or breaks the symmetry of the trapping potential, depending on the interspecies interaction and the particle number. We calculate the tunneling decay rate of the metastable state by using the collective coordinate method under the WKB approximation. Then the height of the energy barrier is estimated by the saddle point solution. It is found that macroscopic quantum tunneling is observable in a wide range of particle numbers. Macroscopic quantum coherence between two distinct states is discussed; this might give an additional coherent property of two-component Bose condensed systems. Thermal effects on the decay rate are estimated.

The quantum-kinetic energy of a finite number of trapped fermionic atoms provides a restoring force for shear motion due to a distortion of the momentum distribution. In analogy to the twist mode of nuclear physics, it is proposed that counter rotating the upper and lower hemisphere of a spherical atomic cloud yields a finite-frequency mode closely related to transverse zero sound waves in bulk Fermi liquids.

Using a single circular Rydberg atom, we have prepared two modes of a superconducting cavity in a maximally entangled state. The two modes share a single photon. The entanglement is revealed by a second atom probing the correlations between the two modes after a delay. This experiment opens interesting perspectives for quantum information manipulation and fundamental tests of quantum theory.

We show that a cooling scheme and an appropriate quantum nonstationary strategy can be used to improve the signal to noise ratio for the optomechanical detection of weak impulsive forces.

A different type of optical resonance has been observed in Doppler-free saturated absorption spectroscopy when the pump beam has elliptical polarization slightly deviating from the ${\sigma }^{+}$ and the probe has ${\sigma }^{-}$ polarization. An interpretation in terms of competition between two trap states and a dressed-atom model is presented. The observed extreme sensitivity of the resonance frequency to the external magnetic field is attributed to the “scissors effect” of the dressed states’ crossing point.

We describe a purely algebraic method for finding the best separable approximation to a mixed state of a composite $2\times 2$ quantum system, consisting of a decomposition of the state into a linear combination of a mixed separable part and a pure entangled one. We prove that, in a generic case, the weight of the pure part in the decomposition equals the concurrence of the state.

We derive expressions for third-order matrix elements of one- and two-particle operators within the framework of relativistic many-body perturbation theory and use these formulas to evaluate specific-mass isotope shifts and isotope field shifts in atoms and ions with one valence electron. Third-order corrections to isotope-shift constants for $3s,$ $3p,$ and $3d$ levels in sodium and sodiumlike ions with $Z=12\mathrm{}–18$ and $Z=26\mathrm{}$ are given. Applications to $4s,$ $4p,$ $3d,$ and $4d$ levels in K and $4s,$ $4p,$ and $3d$ levels in ${\mathrm{Ca}}^{+}$ are also given. Comparisons are made with previous calculations and with experiment. Including third-order corrections improves the agreement between theoretical and experimental values of the specific-mass isotope-shift constants for the cases considered.

The fraction of metastable ions in ${\mathrm{B}}^{3+}{(1s}^2{}^{1}S,1s2s{}^{3}S)$ beams produced in collisions with thin-foil and gas targets has been measured as a function of the incident energy in the range of 0.85–9 MeV. This was done by comparing the electron yield of doubly excited states formed in the collision of ${\mathrm{B}}^{3+}$ with hydrogen and helium targets. Significant differences were observed in the energy dependence of the metastable fraction between production in foil and gas targets. It was shown that the production of $1s2s{}^{3}S$ metastable ions in a foil yields a constant fraction over the investigated energy range, unlike the fraction of metastable $1s2s{}^{3}S$ ions produced in collisions with gas targets, which strongly depends on the incident beam energy. A theoretical study of the processes contributing to the formation of the ${}^{3}S$ metastable ions in collisions with foil and gas has been made. K-vacancy production in the ion-beam stripping process has been identified as a dominant mechanism and used to explain the observed difference in the energy dependence of the metastable fraction between production in collisions with gas and foil targets. A model is proposed for the calculation of the metastable fraction for He-like beams.

Collision cross sections and rate constants between two ground-state oxygen molecules are investigated theoretically at translational energies below $\sim 1$ K and in zero magnetic field. We present calculations for elastic and spin-changing inelastic collision rates for different isotopic combinations of oxygen atoms as a prelude to understanding their collisional stability in ultracold magnetic traps. A numerical analysis has been made in the framework of a rigid-rotor model that accounts fully for the singlet, triplet, and quintet potential-energy surfaces in this system. The results offer insights into the effectiveness of evaporative cooling and the properties of molecular Bose-Einstein condensates, as well as estimates of collisional lifetimes in magnetic traps. Specifically, ${}^{17}{\mathrm{O}}_2$ looks like a good candidate for ultracold studies, while ${}^{16}{\mathrm{O}}_2$ is unlikely to survive evaporative cooling. Since ${}^{17}{\mathrm{O}}_2$ is representative of a wide class of molecules that are paramagnetic in their ground state we conclude that many molecules can be successfully magnetically trapped at ultralow temperatures.

A computational scheme, based on the theory of the continuum-bound transitions of Bell and Seaton [J. Phys. B 18, 1589 (1985)] and the close-coupling R-matrix approach, has been developed to treat dielectronic recombination (DR) in high-lying resonance-energy regions. This scheme and our presented numerical method to compute DR in low-lying resonance-energy regions [Phys. Rev. A 62, 022706 (2000)] have been applied together to elucidate the experimental spectra of the DR of ${\mathrm{O}}^{6+}$ ions in the metastable $1s2s{}^{3}S$ and $1s2s{}^{1}S$ states. For comparison, a perturbative theoretical calculation of DR for ${\mathrm{O}}^{6+}$ has also been accompanied. The reasonable representation of the general dielectronic spectral shape is yielded by both our close-coupling and perturbative calculations. However, both the methods do not reproduce the experimental double-peak structure at $\sim 6$–8 eV. This shows that the further investigation on DR of this kind of ions is required both experimentally and theoretically.

The rate of $\mathrm{dd}\mu$ muonic-molecule resonant formation in $d\mu$ atom collisions with a condensed deuterium target is expressed in terms of a single-particle response function. In particular, $\mathrm{dd}\mu$ formation in solid deuterium at low pressures is considered. Numerical calculations of the rate in the case of fcc polycrystalline deuterium at 3 K have been performed using the isotropic Debye model of a solid. It is shown that the energy-dependent $\mathrm{dd}\mu$ formation rates in the solid differ strongly from those obtained for ${\mathrm{D}}_2$ gaseous targets, even at high $d\mu$ kinetic energies. Monte Carlo neutron spectra from $\mathrm{dd}$ fusion in $\mathrm{dd}\mu$ molecules have been obtained for solid targets with different concentrations of orthodeuterium and paradeuterium. The recent experimental results performed in low-pressure solid targets (statistical mixture of ortho-${\mathrm{D}}_2$ and para-${\mathrm{D}}_2)$ are explained by the presence of strong recoil-less resonance peaks in the vicinity of 2 meV and very slow deceleration of $d\mu$ atoms below 10 meV. Good agreement between the calculated and experimental spectra is achieved when a broadening of ${\mathrm{D}}_2$ rotational and vibrational levels in solid deuterium is taken into account. It has been shown that resonant $\mathrm{dd}\mu$ formation with simultaneous phonon creation in the solid gives only about 10% contribution to the fusion neutron yield. The neutron time spectra calculated for pure ortho-${\mathrm{D}}_2$ and para-${\mathrm{D}}_2$ targets are very similar. A practically constant value of the mean $\mathrm{dd}\mu$ formation rate, observed for different experimental conditions, is ascribed to the fact that all the recent measurements have been performed at temperatures $T\lesssim 19\hspace{0.5em}\mathrm{K},$ much lower than the target Debye temperature ${\Theta }_{\mathrm{D}}\approx 110\hspace{0.5em}\mathrm{K}.$ In result, the formation rate, obtained in the limit $T/{\Theta }_{\mathrm{D}}\ll 1,$ depends weakly on the temperature.

We present measured energy-loss spectra of nitrogen ions, which are scattered off a $(1\mathrm{}\times 2)$ missing row reconstructed Pt(110) single-crystal surface. The primary energy is varied from below 1 keV up to above 1 MeV, i.e., ${0.04v}_0<v<\mathrm{}{2v}_0$ with $v_0$ the Bohr velocity. We use grazing angles low enough in order to have surface channeling at all energies. Experimental results are compared with theoretical energy-loss values obtained from trajectory and stopping power calculations of charged particles scattered under grazing incidence conditions from metallic surfaces. The stopping power is calculated using the scattering theory formalism. Different trajectory classes are found by the calculations and assigned to different contributions in the energy-loss spectra. Regarding the simplicity of the presented model the agreement with the experiment is good.

The improvements in the characterization of II–VI-compound-based solar cells and the recent experimental characterization of small clusters and nanoparticles make the study of small II–VI clusters very interesting. In this work, the ground states of small ${\mathrm{Zn}}_i{X}_i$ clusters are studied, $X=\mathrm{S}\mathrm{e},\mathrm{T}\mathrm{e},$ $i=1\mathrm{}–9.$ Ringlike structures have been found to be the global minima for clusters as large as $i=5,$ and three-dimensional spheroid structures for larger ones, $i=6\mathrm{}-9.$ This trend has been ascribed to the stability of obtuse $X-\mathrm{Zn}-X$ angles in the first case, and to the stability gained from higher coordination in the second case. The three-dimensional structures may be envisioned as being built from ${\mathrm{Zn}}_2{X}_2$ and ${\mathrm{Zn}}_3{X}_3$ rings, as it was the case for ${\mathrm{Zn}}_i{\mathrm{S}}_i$ and ${\mathrm{Zn}}_i{\mathrm{O}}_i$ three-dimensional structures. Calculated natural charges are larger as cluster size increases, showing a tendency towards bulk charges.