Relative differential cross sections (DCS’s) have been measured for quasielastic scattering of positrons (positrons that have elastically scattered or produced rotational or vibrational excitations) from ${\mathrm{N}}_2,$ CO, ${\mathrm{O}}_2,$ ${\mathrm{N}}_2\mathrm{O},$ and ${\mathrm{CO}}_2$ in a crossed-beam experiment. The energy range extends from below the Ps formation threshold of each molecule to 100 eV (well above the Ps formation threshold) and the angular range extends from 30° to 120°. For positron scattering from ${\mathrm{N}}_2,$ CO, and ${\mathrm{O}}_2,$ the DCS’s exhibit structure (distinct minima and maxima) at low energies that tends to disappear as the energy is increased above the Ps formation threshold. In contrast, for ${\mathrm{N}}_2\mathrm{O}$ and ${\mathrm{CO}}_2$ we find at most only a very shallow minimum below the Ps formation threshold that also tends to disappear as the energy is increased. These observations along with prior positron-inert gas and ${\mathrm{CH}}_4\mathrm{}\mathrm{DCS}$ measurements suggest that the degree of spherical symmetry of the target may be correlated with the degree of prominence of the DCS structure for low energy positron scattering. Such distinctions between the diatomic and triatomic molecules have not been evident in prior corresponding electron DCS measurements.

In a recent experiment [Andersen et al., Phys. Rev. Lett. 79, 4770 (1997)], the ${\mathrm{H}}^{-}$ photodetachment cross section (PDCS) and resonances are measured to high precision. Detailed resonance profiles are obtained. To make a critical comparison with experiment, the ${\mathrm{H}}^{-}$ PDCS below the $n=2\mathrm{}$ and $n=3\mathrm{}$ thresholds is calculated with a saddle-point complex-rotation method. The theoretical PDCS are convoluted with a Gaussian profile using experimental resolution for full width at half maximum. The theoretical results are calculated with length, velocity, and acceleration gauges. They are compared with the precision data in the literature. Improved widths are obtained for some of the narrow resonances below the $n=3\mathrm{}$ threshold.

A model generalizing the isotropic-scattering-energy-corrected sudden (ECS) relaxation matrix to arbitrary rank spectra is developed on the basis of a theoretical expression proposed by A.P. Kouzov [Spectral Line Shapes, 14th International Conference on Spectral Line Shapes, State College, PA, 1998, edited by R. M. Herman, AIP Conf. No. 467 (AIP, Woodbury, NY, 1999), Vol. 10, p. 497] for the case of non-Markovian collisions. The constructed matrix obeys all known general relations obtained from first principles: both matrix and Ben-Reuven [Phys. Rev. 141, 34 (1966)] symmetries as well as the double-sided sum rules. In the framework of the present formalism the usual ad hocdetailed balance factor is replaced by a more physical one. Without any additional parameter with respect to the Markovian ECS, this model gives a reasonable description of the ${\mathrm{N}}_2$ anisotropic depolarized spectrum in the whole frequency range up to the far wing.

Ab initio calculations of ejected-electron momentum distributions are presented for proton on hydrogen collisions at impact energies ranging from 5 to 100 keV and an impact parameter of 1.2 a.u. At low energies the transverse momentum distributions show a double-peak structure similar to what is predicted by a two-state Sturmian theory [Macek and Ovchinnikov, Phys. Rev. Lett. 80, 2298 (1998)]. In contrast to that theory, however, we do not find the rapid oscillations in the transverse momentum distribution for impact energies between 5 and 15 keV. At higher collision energies the double-peak transverse momentum distribution coalesces into a single peak. For the energy range considered, the longitudinal momentum distribution shows a broad peak at half the projectile velocity except at the higher energies where the distribution moves toward the target nucleus.

The momentum distribution of fast binary-encounter electrons emitted from solid-state targets of various thicknesses is investigated both theoretically and experimentally. The production of electrons in binary-type collisions with fast, highly stripped ions is calculated in the electron-impact approximation. A transport theory is formulated which allows for energy loss as well as angular deflection of the electrons (within the separation of energy loss and angular scattering approximation) during penetration of the target. The broadening of the binary encounter peak with increasing target thickness observed experimentally in $13.6-\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}{\mathrm{Ar}}^{17+}$ and ${\mathrm{Ar}}^{18+}$ colliding with carbon foils is well described by theory.

Photoionization of alkali-metal clusters is investigated theoretically for different photon energy regimes. At low energies the photo cross section is characterized by the well-known plasmon peak resulting from collective electron dynamics. For high energies the ionization cross section exhibits an oscillatory behavior, which can be explained by single-particle effects. In this paper we use the random phase approximation (RPA) to calculate the photo cross section on an equal footing for both, the low- and the high-energy regime. Thereby, we can show that the cross sections for photoionization calculated in the collective RPA and in the single-particle picture indeed merge for high photon energies. Moreover, we demonstrate that the oscillatory behavior can already be identified at low photon energies where the cross section is not yet exponentially small. Hence, it should be possible to identify the oscillations experimentally.

We examine two-photon lasing in competition with the one-photon lasing into a second mode near resonance with a transition to an intermediate state. The model assumes pumping to the upper state of the two-photon transition. After a derivation of the relevant master equation for the atomic system, we consider the Maxwell-Bloch equation on the basis of which we study the steady state and the time-dependent dynamical behavior of the system. We show that two-photon lasing can be sustained in coexistence with one-photon lasing after the latter is saturated. The results are illustrated with an application to a specific atomic system (potassium) as an amplifying medium.

Angular resolved kinetic-energy distributions of fragments resulting from dissociation induced by intense, short, linearly polarized laser pulses are calculated using an accurate three-dimensional Fourier transform method in spherical coordinates. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field-polarization vector. But, unexpectedly, increasing the field strength may also produce less aligned fragments at the higher kinetic energies of the multiphoton above-threshold-dissociation spectrum. ${\mathrm{H}}_2^{+}$ photodissociating by interaction with an Nd:YAG laser at $\lambda =532\mathrm{}$ nm and for intensities of $10-50\hspace{0.5em}\mathrm{TW}/{\mathrm{cm}}^2$, is taken as an illustrative example, for which some angular resolved experimental spectra are available. A comprehensive interpretation is provided within the field-dressed Floquet picture by referring to two strong field mechanisms; namely, the potential barrier lowering (also responsible for bond softening), and the nonadiabatic transitions (also responsible for the vibrational trapping). Calculations are presented for three specific initial vibrational states leading to strongly anisotropic angular distributions that are discussed.

The characteristic short-time behavior of the survival probability of a decaying state, which underlies the quantum Zeno effect (QZE), is shown to follow quite simply from the Schrödinger equation even when the spectrum of the Hamiltonian has no finite lower bound, provided the mean energy of the initial state is finite. The failure of the Breit-Wigner form of energy distribution to predict QZE is traced to the nonexistence of a well-defined mean energy. On the basis of a suitably modified distribution, it is explicitly demonstrated that the existence of a finite lower bound to the energy distribution only affects the asymptotic decay law. For short times, the survival probability depends quadratically on time if the variance of the distribution is also finite, not otherwise. For a sufficiently well-behaved energy distribution, the asymptotic decay must be as $t^{{-}^n}$ if the spectrum has a finite lower bound, or faster than any inverse power of t if there is no lower bound.

A general formalism is derived for analytically representing the scattering properties of ultracold atoms subject to one or more resonant photoassociation lasers. The resulting formulas cleanly separate laser intensities and detunings from overlap integrals of molecular wave functions. As a consequence, laser-induced energy shifts and line broadenings are given in an explicit form. In the case of the initial photoassociation step, these quantities are given more simply in terms of the relevant molecular potential curves and wave functions near the Condon point.

Neutral photodissociation from ${\mathrm{N}}_2$ has been studied in the range 20–30 eV by observing dispersed fluorescence from excited N fragments. From comparisons with calculations using many-body perturbation theory it follows that these fragmentations follow from predissociations of ${\mathrm{N}}_2$ Rydberg series by a number of non-Rydberg doubly excited states.

In a recent paper, Bensky et al. [Phys. Rev. Lett. 81, 3112 (1998)] experimentally reattached an outgoing continuum electron to an ion using a pulsed electric field. Quantum and classical calculations are presented that illustrate the importance of various mechanisms for this reattachment process. The quantum calculations are based on a high-order impulse approximation. Both calculations support the idea that after an initial rapid dispersal, the bound part of the wave packet reforms after one Rydberg period; it continues to reform out to long times. A quantum calculation of the electron flux ejected from Ca Rydberg states in a static electric field suggests that classical calculations should not accurately describe pulsed-field recombination in the presence of a static field unless the atom is H or the fields are very weak; this observation agrees with experiment.

An experimental study of sideband laser cooling of trapped ions in miniature radio-frequency traps is presented. ${\mathrm{In}}^{+}$ ions are laser cooled by exciting the narrow intercombination line ${5s}^2{}^1{S}_0\to 5s5p{}^3{P}_1$, which shows resolved sidebands due to the vibrational motion of the ions in the trap. The influence of the micromotion at the frequency of the oscillating trapping field is investigated. Heating processes through coupling with the trapping field occur for certain regions of negative laser detuning. A method of bichromatic sideband cooling is introduced that combines efficient cooling from high temperatures with the possibility of reaching low mean vibrational quantum numbers $〈n〉<1\mathrm{}$ by using narrow optical transitions. This technique allows recording of high-resolution excitation spectra of the cooling transition in the Lamb-Dicke regime and thus determination of $〈n〉$. Single ions as well as two-ion Coulomb crystals have been cooled to temperatures below $100\hspace{0.5em}\mu \mathrm{K}$, and population of the ground state of motion to more than 50% over long time scales is demonstrated. These values are close to the lower limit for the detection of vibrational excitation with the present method.

Calculations of the vacuum-polarization corrections to the low-lying energy levels of Li-like highly charged ions are presented. The calculations are carried out for extended nuclei in the range $Z=20\mathrm{}-100.$

Coherent population trapping (CPT) with losses towards off-resonant levels is analyzed on the basis of the coupled/noncoupled states. We derive that at a long interaction time the noncoupled state is depopulated at a rate that is decreasing for decreasing Raman detuning. This behavior produces a narrowing of the dark resonance with the interaction time. The asymptotic evolution of the atomic system is completely characterized by the effective width ${\Gamma }_{\mathrm{NC}}$ of the noncoupled state. The analytic expression for this width evidences a quadratic dependence on the Raman detuning. This dependence implies that the width of the dark resonance decreases asymptotically with the inverse of the square root of the interaction time. This narrowing law, independent of the interaction scheme, has a general validity for CPT in open systems. Our analysis points out the differences between CPT with losses and CPT in closed systems.

Various phase concepts may be treated as special cases of the maximum likelihood estimation. For example, the discrete Fourier estimation that actually coincides with the operational phase of Noh, Fougères, and Mandel is obtained for continuous Gaussian signals with phase modulated mean. Since signals in quantum theory are discrete, a prediction different from that given by the Gaussian hypothesis should be obtained as the best fit assuming a discrete Poissonian statistics of the signal. Although the Gaussian estimation gives a satisfactory approximation for fitting the phase distribution of almost any state, the optimal phase estimation offers in certain cases a measurably better performance. This has been demonstrated in a neutron-optical experiment.

Fractional Talbot images in the de Broglie near field of atoms diffracted by a dispersive light grating allow the measurement of the quantum state of an electromagnetic field in a cavity, in the optical domain. In particular, the photon-number statistics may be obtained, in principle, through a single experimental realization. It may be expressed as a Fourier transform of the atomic density on a plane situated at one-fourth of the Talbot distance from the light grating. The proposed measurement of the density matrix of a standing light wave using “Talbot fingerprints” should be feasible within the present state of the art.

Bose-Einstein condensates in a double-well trap, as well ${}^3\mathrm{H}\mathrm{e}-\mathrm{B}$ baths connected by micropores, have been shown to exhibit Josephson-like tunneling phenomena. Unlike the superconductor Josephson junction of phase difference $\phi$ that maps onto a rigid pendulum of energy $\cos \left(\phi \right)$, these systems map onto a momentum-shortened pendulum of energy $-\sqrt{1-p_{\phi }^2}\cos \left(\phi \right)$ and length $\sqrt{1-p_{\phi }^2}$, where $p_{\phi }$ is a population imbalance between the wells/baths. We study here the effect of damping on the four distinct modes of the nonrigid pendulum, characterized by distinct temporal mean values, $〈\phi 〉$ and $〈p_{\phi }〉$. Damping is shown to produce different decay trajectories to the final equilibrium $\phi {=0=p}_{\phi }$ state that are characteristic dynamic signatures of the initial oscillation modes. In particular, damping causes $\pi$-state oscillations with $〈\phi 〉=\pi$ to increase in amplitude and pass through phase-slip states, before equilibrating. Similar behavior has been seen in ${}^3\mathrm{H}\mathrm{e}-\mathrm{B}$ experiments.

Using the Kadanoff-Baym nonequilibrium Green’s-function formalism, we derive the self-consistent Hartree-Fock-Bogoliubov (HFB) collisionless kinetic equations and the associated equation of motion for the condensate wave function for a trapped Bose-condensed gas. Our work generalizes earlier work by Kane and Kadanoff for a uniform Bose gas. We include the off-diagonal (anomalous) pair correlations, and thus we have to introduce an off-diagonal distribution function in addition to the normal (diagonal) distribution function. This results in two coupled kinetic equations. If the off-diagonal distribution function can be neglected as a higher-order contribution, we obtain the semiclassical kinetic equation recently used by Zaremba, Griffin, and Nikuni (based on the simpler Popov approximation). We discuss the static local equilibrium solution of our coupled HFB kinetic equations within the semiclassical approximation. We also verify that a solution is the rigid in-phase oscillation of the equilibrium condensate and noncondensate density profiles, oscillating with the trap frequency.