Relative differential cross sections at forward and backward angles for excitation of the E ${}^3{\Sigma }_g^{+}$ state of ${\mathrm{N}}_2$ have been measured in the resonant, near threshold energy region. Measurements are performed by using a double trochoidal electron spectrometer and are calibrated on the absolute scale by simultaneous measurements of this process and vibrational excitation of the ground state of ${\mathrm{N}}_2$ via ${}^2{\Pi }_g$ resonance. The contributions at $0°$ and $180°$ are separated by electron beam modulation and time-of-flight detection of scattered electrons. Obtained results confirm directly theoretical predictions of the symmetry of angular distribution of scattered electrons from ${}^2{\Sigma }_u^{+}$ resonance.

The electron energy loss in a dispersive medium is obtained using macroscopic electrodynamics taking advantage of a static frame of reference. Relativistic corrections are described in terms of a dispersive Lorentz factor obtained by replacing the vacuum velocity c by the characteristic phase velocity $c/n,$ where n is the complex index of refraction. The angle-resolved energy-loss spectrum of a Drude conductor is analyzed in detail and it is shown that the low-energy peak due to Ohmic losses is enhanced compared to the classical approximation.

We introduce a global optimization procedure, the neural-assisted genetic algorithm (NAGA). It combines the power of an artificial neural network (ANN) with the versatility of the genetic algorithm. This method is suitable to solve optimization problems that depend on some kind of heuristics to limit the search space. If a reasonable amount of data is available, the ANN can “understand” the problem and provide the genetic algorithm with a selected population of elements that will speed up the search for the optimum solution. We tested the method in a search for the ground-state geometry of silicon clusters. We trained the ANN with information about the geometry and energetics of small silicon clusters. Next, the ANN learned how to restrict the configurational space for larger silicon clusters. For ${\mathrm{Si}}_{10}$ and ${\mathrm{Si}}_{20},$ we noticed that the NAGA is at least three times faster than the “pure” genetic algorithm. As the size of the cluster increases, it is expected that the gain in terms of time will increase as well.

A cooling process for optical dipole traps, rf-induced Sisyphus cooling in a circularly polarized far-off-resonance trap, is demonstrated. Cooling rates of up to $\approx 300\hspace{0.5em}\mu \mathrm{K}/\mathrm{s}$ are observed without significant atom loss. The final temperature achieved was (a somewhat disappointing) $17\hspace{0.5em}\mu \mathrm{K}.$ Experimental studies and theoretical analysis of this cooling process were carried out, which explain that the temperature limit is due to an equilibrium between the small fraction of total thermal energy that is removed per Sisyphus cycle and the multiple photon recoils that are required to complete each cycle. Light-induced collisional loss made it difficult to operate under conditions where a larger fraction of the energy would be removed. A method for storing atoms in an optical trap with only a single confined spin state is also demonstrated.

Consistency with relativistic causality narrows down dramatically the class of measurable observables. We argue that, by weakening the preparation role of ideal measurements, many of these observables become measurable. In particular, we show by applying entanglement assisted remote operations that all Hermitian observables of a $(2\mathrm{}\times 2)$-dimensional bipartite system are measurable.

The propagation dynamics of intense femtosecond laser pulses in argon have been investigated theoretically and the results are compared with experimental data. It was found that in the initial stage the pulse propagates with the focal point moving ahead of the original one. The central beam of the trailing part experiences defocusing owing to ionization by the leading part and then regains self-focusing provided by power from the outer part. On propagating further, a quasistable balance is established between self-focusing and defocusing due to ionization-induced nonlinearity and diffraction, causing the beam to propagate in a self-guided mode. Furthermore, it was shown that the front of the split pulse decays faster, while the trailing edge experiences self-focusing and self-defocusing until a self-guided propagation mode is achieved. Multiple pulse splitting and shortening as a result of the dynamics near the focal point were also observed.

The phenomenon of particle creation within an almost resonantly vibrating cavity with losses is investigated for the example of a massless scalar field at finite temperature. A leaky cavity is designed via the insertion of a dispersive mirror into a larger ideal cavity (the reservoir). In the case of parametric resonance the rotating wave approximation allows for the construction of an effective Hamiltonian. The number of produced particles is then calculated using response theory as well as a nonperturbative approach. In addition, we study the associated master equation and briefly discuss the effects of detuning. The exponential growth of the particle numbers and the strong enhancement at finite temperatures found earlier for ideal cavities turn out to be essentially preserved. The relevance of the results for experimental tests of quantum radiation via the dynamical Casimir effect is addressed. Furthermore, the generalization to the electromagnetic field is outlined.

We present a general analysis for the interaction of a probe-laser radiation with a coherently prepared molecular Raman medium. We describe a general formalism that includes dispersive effects, such as group velocity and group-velocity dispersion (GVD). When dispersion is negligible, the analysis is especially simple and insightful. We show that molecular oscillations result in a modulated instantaneous susceptibility of the medium. The effect of the time-varying susceptibility on a probe-laser pulse is twofold: the output frequency becomes modulated because of the time-varying phase velocity, and the pulse shape becomes deformed because of the time-varying group velocity. We identify two mechanisms for pulse compression: (1) Frequency chirping with subsequent pulse compression by normal linear GVD (possibly in the same medium) and (2) Compression due to the time-varying group velocity. We analyze various aspects of pulse compression in the coherent Raman medium and derive conservation relations for this process. When we consider a probe-laser pulse that is much shorter than the molecular oscillation period, we observe frequency chirping, compression, or stretching of this pulse, depending on its relative timing with respect to the molecular oscillations. Based on our analysis, we propose a method for selective compression or frequency conversion of single ultrashort pulses.

The dependence of the higher-order harmonic generation signal produced by two time-delayed laser pulses on the delay between the two pulses has been studied with respect to the spatial laser-intensity distribution. The calculation is restricted to the laser-intensity dependence of the atomic response without taking into account coherence and/or propagation effects. It is found that the rapid modulations which the harmonic signal is expected to show in such a higher-order autocorrelation trace due to its nonmonotonic dependence on the laser intensity are totally eliminated after spatial integration over the laser-intensity distribution is performed. This study shows that the attosecond structures observed in experimental temporal traces of a superposition of higher harmonics produced by two delayed laser pulses cannot be attributed to laser-intensity-based artifacts, as recently suggested by Tempea et al. [Phys. Rev. Lett. 87, 109401 (2001)].

Using a concise approach with undetermined coefficients, instead of the conventional diagonalization method, we obtain rigorously the energies and analytical wave functions of the ground state and excited states of a polariton model. The results indicate that our method is not only equivalent to the conventional one, but also has its own advantage. We also study several interesting properties of the polariton ground state.

In 1968, Shapiro published the suggestion that one could search for an electron dipole moment (EDM) by applying a strong electric field to a substance that has an unpaired electron spin; at low temperature, the EDM interaction would lead to a net sample magnetization that can be detected with a superconducting quantum interference device (SQUID) magnetometer. One experimental EDM search based on this technique was published, and for a number of reasons including high sample conductivity, high operating temperature, and limited SQUID technology, the result was not particularly sensitive compared to other experiments in the late 1970s. Advances in SQUID and conventional magnetometery led us to reconsider this type of experiment, which can be extended to searches and tests other than EDMs (e.g., test of Lorentz invariance). In addition, the complementary measurement of an EDM-induced sample electric polarization due to application of a magnetic field to a paramagnetic sample might be effective using modern ultrasensitive charge measurement techniques. A possible paramagnetic material is Gd-substituted yttrium iron garnet which has very low conductivity and a net enhancement (atomic enhancement times crystal screening) of order unity. Use of a reasonable volume (hundreds of ${\mathrm{cm}}^3)$ sample of this material at 50 mK and 10 kV/cm might yield an EDM sensitivity of ${10}^{-32}\hspace{0.5em}\mathrm{e}\mathrm{}\mathrm{cm}$ or better, a factor of ${10}^5$ improvement over current experimental limits.

Six resonance transitions of the antiprotonic helium atom in helium gas at densities of $3\times {10}^{20}–3\times {10}^{21}\hspace{0.5em}{\mathrm{cm}}^{-3}$ were studied at the antiproton decelerator (AD) of CERN. The decay rates of the daughter states of these transitions were determined either from the time distributions of the resonance spikes or from the widths of the resonance lines. Whereas most of the observed decay rates agree with theoretical calculations of Auger rates, two states, $\mathrm{}(n,l)=(37,33)\mathrm{}$ and (32,31), were found to have decay rates two orders of magnitude larger than predicted by these calculations. The effect of coupling with near-lying electron-excited states is considered to be the reason for the anomaly of the (37, 33) state, as pointed out by Kartavtsev et al. [Phys. Rev. A $61,$ 062507 (2000)].

We have investigated inelastic electron scattering from the laser-excited $\left(\dots 6s6p{}^1{P}_1\right)$ state of barium at 20 eV incident electron energy and scattering angles of 5.5°, 9°, 13°, and 16°. By using circularly polarized laser light and measuring the circular polarization dependence of inelastic energy loss features associated with various excitations, we have determined the transferred orbital angular momentum parameter. $L_{\mathrm{perp}}^{+}$ for the time-inverse related deexcitations $\overrightarrow{X}\left(\dots 6s6p{}^1{P}_1\right),$ where $X=\left(\dots 6s7p{}^1{P}_1\right),$ $\left(\dots 6s6d{}^1{D}_2\right),$ $\left(\dots 6s7s{}^1{S}_0\right),$ $\left(\dots {5d}^2{}^3{P}_2\right)$ and the unresolved states $(\dots {5d}^2{}^1{D}_2,\dots 6p5d{}^1{D}_2).\mathrm{}$ Our structure calculations show some of these states to be essentially pure, two-electron states with relatively strong singlet-triplet mixing. Scattering calculations were carried out in the convergent close-coupling formalism and, in general, good agreement with experimental data is shown. A detailed analysis of the $\overrightarrow{S}P$ and $\overrightarrow{D}P$ deexcitations in comparison to previously reported results for $\overrightarrow{S}P$ and $\overrightarrow{D}P$ excitations [Y. Li and P. W. Zetner, Phys. Rev. A 49, 950 (1994); P. V. Johnson, B. Eves, P. W. Zetner, D. Fursa, and I. Bray, Phys. Rev. A 59, 439 (1999)] shows a selection rule of $\Delta m=+1\left(-1\right)$ to hold for excitation (deexcitation) in small-angle scattering.

Partial and total photoionization cross sections of the K shell of the ${\mathrm{N}}_2$ molecule are calculated using the generalization of the random-phase approximation that has been applied earlier to the valence shells of ${\mathrm{N}}_2$ [Phys. Rev. A 61, 032704 (2000)]. At zero order the relaxed core Hartree-Fock approximation is used. It is demonstrated that due to strong intershell correlations the $\sigma *$ shape resonance reveals itself not only in the $1{\sigma }_g\to \epsilon {\sigma }_u$ channel as it takes place in all single-particle approximations, but also in the $1{\sigma }_u\to \epsilon {\sigma }_g$ channel. The influence of vibrational motion on the cross sections is taken into account. Good agreement with the most recent experimental data for different partial cross sections is achieved.

Absolute cross sections have been determined for the dissociative excitation and electron capture followed by ion-pair formation $({\mathrm{He}}^{+}+{\mathrm{He}}^{-})$ for ${\mathrm{He}}_2^{+}$ collisions with He in the energy range of 4–10 keV. The ${\mathrm{He}}^{+}$ absolute cross section that is uniquely due to the dissociative excitation was deduced, taking advantage of the measurements of the ${\mathrm{He}}^{-}$ cross section. The center-of-mass energy for the fragment ions was measured. The available potential-energy curves show that the vibrationally excited ground state may be responsible for the fragment ions. The collisional deactivation of ${\mathrm{He}}_2^{+}$ ions using high source pressure showed no effect on the measured cross section. Complete theoretical high-lying repulsive potential-energy curves for ${\mathrm{He}}_2^{+}$ ions are currently not available, therefore it is not possible to quantitatively establish the final state of the ions.

We show that in dilute boson-fermion mixtures with fermions in two internal states, even when the bare fermion-fermion interaction is repulsive, the exchange of density fluctuations of the Bose condensate may lead to an effective fermion-fermion attraction, and thus to a Cooper instability in the s-wave channel. We give a simple condition to know when this is going to happen, and an analytic expression for the associated $T_c$ in the most important limit where the phonon branch of the Bogoliubov excitation spectrum of the bosons is important. We find a $T_c$ of the same order as for a pure Fermi gas with bare attraction.

Using the exact N-particle ground state wave function for a one-dimensional gas of hard-core bosons in a harmonic trap we develop an algorithm to compute the reduced single-particle density matrix and corresponding momentum distribution. Accurate numerical results are presented for up to $N=8\mathrm{}$ particles, and the momentum distributions are compared to a recent analytic approximation.

We investigate strategies for estimating a depolarizing channel for a finite dimensional system. Our analysis addresses the double optimization problem of selecting the best input probe state and the measurement strategy that minimizes the Bayes cost of a quadratic function. In the qubit case, we derive the Bayes optimal strategy for any finite number of input probe particles when bipartite entanglement can be formed in the probe particles.

We present a detailed study of the $0_g^{-}{(P}_{3/2})$ pure long-range electronic state of the ${}^{87}{\mathrm{Rb}}_2$ molecule. The high-resolution spectral data provided by photoassociative spectroscopy of ${}^{87}{\mathrm{Rb}}_2$ is analyzed by using the generalized simulated annealing method. The dipole matrix element of the $5p$ ${}^{87}\mathrm{Rb}$ atomic state and other effective asymptotic parameters for the $0_g^{-}{(P}_{3/2})$ long-range state are determined and a complete analytical description of the potential well is obtained. We extract a radiative lifetime of the ${5P}_{3/2}$ and ${5P}_{1/2}$ states equal to 26.25(8) ns and 27.75(8) ns, respectively, with a precision (0.3%) comparable to atomic physics experiment.

We study a rotating two-component Bose-Einstein condensate in which an optically induced Josephson coupling allows for population transfer between the two species. In a regime where separation of species is favored, the ground state of the rotating system displays domain walls with velocity fields normal to them. Such a configuration looks like a vortex split into two halves, with atoms circulating around the vortex and changing their internal state as they cross the domain wall.