The low-temperature order-disorder transition for a ${\mathrm{Pd}}_7{\mathrm{Ni}}_6$ alloy cluster was considered using the isothermal-isobaric (NPT) ensemble. The ordered structure consists of a completely segregated arrangement of atoms and the disordered systems present a certain degree of mixing within the cluster. The transition was characterized by monitoring anomalies in the average value of the constant pressure heat capacity, $〈C_p〉,$ as a function of temperature. The maximum temperature in the $〈C_p〉$ versus T graph, $T_{\mathrm{mixing}},$ is used for estimating the equilibrium temperature at which the transition occurs, at a given pressure. It is observed that as the pressure increases, $T_{\mathrm{mixing}}$ decreases up to a value of 25 K, where the mixing transition becomes temperature-independent. The sampling difficulties presented in standard Monte Carlo simulations are circumvented by implementing the J-walking procedure to the NPT ensemble.

A basic theory is presented for the photoexcitation of a core state of a Rydberg atom in any type of static field; in this situation, the core state is excited by the photon while the Rydberg electron is essentially a spectator. This simple picture is made interesting through the interaction of the Rydberg electron with the core which can cause a change in the Rydberg electron’s state and can cause the Rydberg electron to autoionize. The method is computationally efficient and has been used for alkaline-earth atoms in a static electric field. An approximation to the formalism is presented that illustrates a mechanism controlling the isolated core excitation in a static electric field; this approximation may serve as a paradigm for extending the interpretation of isolated core spectra to other types of fields.

We report on the electron yield spectra of doubly excited magnesium atoms in electric fields in the region 50–1600 V/cm. One electron is excited to a stationary Rydberg state around $n=16,$ either a nonmixing state with low angular momentum or a Stark state. Subsequently the second isolated-core electron is excited with a narrow-band nanosecond laser pulse. The isolated-core excitation induces a shake-up which is measured as a function of excitation frequency. An initial low-angular-momentum state leads to a much broader shakeup than an initial Stark state, i.e., many more autoionization peaks are observed for an initial low-angular-momentum state than for an initial Stark state. Full quantum calculations introduced in the preceding paper [F. Robicheaux, preceding paper Phys. Rev. A 61, 033406 (2000)] show excellent agreement with the recorded spectra. We discuss the implications of this experiment for dielectronic recombination and compare the frequency spectra to previously reported time-dependent measurements on magnesium [J.B.M. Warntjes et al., Phys. Rev. Lett. 83, 512 (1999)].

The cross section for isolated-core excitation (ICE) of barium ${6p}_j{n}^{\prime }{k}^{\prime }$ autoionizing states from $6snk$ Stark levels has been measured as a function of applied electric-field strength. For the most part, states that are well mixed by the applied field have nearly Lorentzian line shapes indicating predominant ICE of a single autoionizing resonance with $n^{\prime }=n$ and $k^{\prime }=k.\mathrm{}$ Small non-Lorentzian shake-up features in the wings of the lines are examined in detail using high laser power to depletion broaden the main resonance and amplify the wings of the line shapes. While analogous shake-up features in zero-field ICE spectra are easily described theoretically, attempts to reproduce the spectra observed in a field generally fail to provide good agreement. Finally, significant differences in excitation of ${\mathrm{Ba}}^{+}\hspace{0.5em}{6p}_{1/2}$ rather than ${\mathrm{Ba}}^{+}\hspace{0.5em}{6p}_{3/2}$ cores are observed.

We investigate the combined effects of quantum statistics, resonant dipole-dipole atomic interactions, and reabsorption of radiation on the spontaneous emission of excited atoms in a confined Fermi sea. Even for dilute gases, reabsorption may be qualitatively important in determining the rate and angular distribution of emitted radiation. A temporal crossover behavior in the angular distribution is illustrated, in which the characteristic signatures of Fermi statistics are qualitatively changed by reabsorption within the gas.

We present a phenomenological-operator approach to describe energy dissipation in cavity QED phenomena. This approach, developed for an absolute-zero and a thermal environment, considerably simplifies the introduction of the inevitable errors due to the environmental degrees of freedom when describing processes involving dispersive atom-field interactions. The main result in the present work consists in furnishing a straightforward technique to estimate the fidelity resulting from dispersive atom-field interactions, precluding the necessity of performing the usually extensive ab initio calculations. Furthermore, we expect that the present work can help us account for dissipation in resonant atom-field interactions and even help us achieve a general phenomenological approach to estimate the effects of dissipation in whichever system. To illustrate the universal applicability of the present technique, we calculate the fidelity of a mesoscopic quantum superposition state engineered in a lossy cavity, considering also the excited-state spontaneous decay of the required atom. For the case of a stable atomic excited state, the fidelity computed here is in agreement with a recently announced exact calculation.

We give in this paper a general method by which to determine the transverse spatial distribution of quantum fluctuations in a light beam as it propagates through a nonlinear medium. We apply this method to the particular case of spatial solitons that propagate without distortion in ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ or ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ materials. We find, for example, that in the ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ case the central part of the soliton has lower intensity quantum fluctuations than its outer part, and that in most cases strong correlations develop among the field quadrature components measured in different areas of the transverse plane.

Our calculations show that nonadiabatic effects dominate phase-matched high-order harmonic generation with few cycle laser pulses. The nonadiabatic behavior is a direct consequence of the large ionization rate realized with few-cycle laser pulses, which introduces a nonlinear growth of the harmonic phase with propagation distance. The deviation from linear growth has two striking consequences. On the one hand the nonadiabatic effect drastically reduces the efficiency of conventional phase-matching mechanisms, such as quasi-phase-matching, and on the other hand, it is responsible for a novel phase-matching mechanism, termed nonadiabatic self-phase-matching that is predicted to make high-order harmonic generation in the x-ray regime possible.

We have theoretically explored the behavior of a coherently pumped $J^{″}=\overrightarrow{1}J=\overrightarrow{0}{J}^{\prime }=1\mathrm{}$ isotropic cavity laser and compared it with the behavior of other related laser systems involving atomic or molecular levels with angular quantum numbers $0$ and $1.$ It is shown that at low and moderate pumping strengths it behaves very differently from the $J^{″}=\overrightarrow{0}J=\overrightarrow{1}{J}^{\prime }=0\mathrm{}$ laser exhibiting no pump-induced gain anisotropy and allowing for linearly polarized (LP) solutions with arbitrary azimuth and circularly polarized (CP) solutions, depending on the values of the molecular relaxation rates. Above the instability threshold, a variety of dynamic regimes involving the polarization degree of freedom can be found, including LP states with rotating azimuth (as in incoherently pumped $J=\overrightarrow{0}{J}^{\prime }=1\mathrm{}$ or $J=\overrightarrow{1}{J}^{\prime }=0\mathrm{}$ lasers), antiphase dynamics, and full polarization chaos.

Values for the average electron density $〈\rho 〉$ and the electron-electron coalescence density or intracule density at the origin $I\left(0\right)\mathrm{}$ have been calculated for a number of atoms and molecules using several ab initio methodologies. $〈\rho 〉$ can be taken as a measure of local one-electron density concentration, and shows little dependence on the theoretical level employed. In contrast, $I\left(0\right)\mathrm{}$ has found to be highly dependent on the level of calculation. In particular, for a given ab initio method, the difference $〈\rho 〉/4\mathrm{}-I\left(0\right)\mathrm{}$ can be related to the degree of Coulomb electron correlation introduced by that method in an atomic or molecular system.

We show that Sisyphus cooling in a one-dimensional gray optical lattice can be improved by using a bichromatic standing wave that increases the efficiency of cooling as compared to typical lattices. Using the time of flight technique to determine the velocity distribution of all the atoms, we calculate the efficiency by measuring the fraction of all atoms that are cooled by the lattice. We show that the efficiency depends on the relative displacement between the two standing waves formed by the two colors. The optimal configuration of the bichromatic light field yields a 40% improvement in efficiency compared to typical optical lattices, while maintaining the same atomic temperature.

We have investigated the multiple ion yields of Kr, up to ${\mathrm{Kr}}^{4+}$ as a function of the field intensity, that are measured using a linearly polarized short-pulsed Ti:sapphire laser of $\lambda =800\mathrm{}\mathrm{nm}$ in the range of up to mid-${10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2,$ within an S-matrix model for nonsequential multiple ionization. Experimental data and theoretical predictions are in good agreement and the influence of different nonsequential channels on the measured ion yields are analyzed.

We prove some properties of fidelity (transition probability) and concurrence, the latter defined by a straightforward extension of Wootters’ notation. Choose a conjugation and consider the dependence of fidelity or of concurrence on conjugated pairs of density operator. These functions turn out to be concave or convex roofs. Optimal decompositions are constructed. Some applications to two and tripartite systems illustrate the general theorems.

The Schrödinger equation is solved numerically using the method of exterior complex scaling for several models of the breakup of an atom by electron impact. Using the accurate wave functions thereby obtained for these model problems, several well-known integral expressions for quantum-mechanical breakup amplitudes are tested. It is shown that some formally correct integral expressions for the breakup amplitudes can yield numerically unstable or poorly convergent results. Calculations are presented for a case with simple exponential potentials and a case in which a metastable state of the target, analogous to an autoionizing state, can decay into the breakup channel. For cases involving only short-range (non-Coulomb) interactions, alternative expressions can be found that are stable in calculations of practical scale.

Using the Coulomb explosion imaging method, the change of the relative population for the first six vibrational states of ${\mathrm{H}}_2^{+}$ during the interaction with low-kinetic-energy electrons has been measured. A model based on rate coefficients for dissociative recombination and superelastic collision processes is developed to explain the time dependence of the relative vibrational populations. Using this model, we demonstrate that superelastic collisions with rate coefficients of $\left(1\mathrm{}-4\right)\times {10}^{-6}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ (about an order of magnitude higher than available theoretical predictions) can explain the observed electron-induced vibrational deexcitation of ${\mathrm{H}}_2^{+}.$

High-order harmonic generation by a hydrogen molecular ion in a strong laser field is theoretically considered within the frames of a two-level model. Franck-Condon factors for ionization of a neutral hydrogen molecule are derived taking into account the laser field. Nuclear dynamics of a hydrogen molecular ion in a laser pulse is investigated within the frames of classical mechanics. Harmonic spectrum contains a lot of noninteger harmonics of laser frequency. This is explained by the effective Rabi transitions between the even and odd electronic levels. The extended plateau and the sharp cutoff in the harmonic spectrum has been found. The height of the plateau and the cutoff increase with laser intensity.

Atoms in doubly excited states above the first ionization limit can decay via autoionization in which an electron is emitted leaving an ion, or by photoemission which leaves the atom in a singly excited state. In this paper, it is demonstrated that interaction between the atoms and a laser pulse that is short compared to the autoionization lifetime can lead to large enhancement of the photoemission process by stimulating the atoms to emit a photon. Since the resultant singly excited atoms do not autoionize, this process can be viewed as an enhancement of the stabilization of the doubly excited atoms against autoionization. A simple theoretical model is outlined that shows good agreement with the experimental results.

We consider the theoretical description of two parallel and adjacent lasers in a monolitic crystal. Starting from first principles, a semiclassical theory is developed. A system of nonlinear differential equations for both lasers field and inversion is obtained. In the weak-coupling limit, we show that the coupling between the laser fields is linear. Moreover, the coupling strength is measured by a complex coefficient: (i) the real part is related to the overlap of fields with a weight given by the pump beam profile, while (ii) the imaginary part is associated with the refractive index structure created inside the crystal. The parameters are defined explicitly in terms of pump, index, and “cavity eigenmodes” structures. The resulting system of two coupled nonlinear oscillators is analyzed. The effect of this coupling on the laser’s dynamical behaviors, like stable synchronization and beating phenomena, is studied.

We analyze the entangling capabilities of unitary transformations U acting on a bipartite ${(d}_1\times d_2)$-dimensional quantum system. To this aim we introduce an entangling power measure $e\left(U\right)\mathrm{}$ given by the mean linear entropy produced acting with U on a given distribution of pure product states. This measure admits a natural interpretation in terms of quantum operations. For a uniform distribution explicit analytical results are obtained using group-theoretic arguments. The behavior of the features of $e\left(U\right)\mathrm{}$ as the subsystem dimensions $d_1$ and $d_2$ are varied is studied both analytically and numerically. The two-qubit case $d_1{=d}_2=2\mathrm{}$ is argued to be peculiar.

We have trapped cesium atoms over many minutes in the focus of a ${\mathrm{CO}}_2$-laser beam employing an extremely simple laser system. Collisional properties of the unpolarized atoms in their electronic ground state are investigated. Inelastic binary collisions changing the hyperfine state lead to trap loss, which is quantitatively analyzed. Elastic collisions result in evaporative cooling of the trapped gas from 25 $\mu \mathrm{K}$ to 10 $\mu \mathrm{K}$ over a time scale of about 150 s.