Cross sections for electron excitation out of the 2${\mathrm{}}^3$S metastable level of He into the 2${\mathrm{}}^3$P, 3${\mathrm{}}^3$S, 3${\mathrm{}}^3$P, 3${\mathrm{}}^3$D, 4${\mathrm{}}^3$S, 4${\mathrm{}}^3$P, 4${\mathrm{}}^3$D, 5${\mathrm{}}^3$S, and 5${\mathrm{}}^3$D levels have been obtained for energies up to 18 eV. We have observed a broad excitation function for the 2${\mathrm{}}^3$P level with a peak apparent cross section of 1.2×${10}^{\mathrm{-}14}$${\mathrm{ncm}}^2$. For the n=3, 4, and 5 levels, the excitation functions show a pattern of sharp peaks for excitation into the n${\mathrm{}}^3$S levels, slightly less sharp peaks for excitation into the n${\mathrm{}}^3$P levels, and relatively broad peaks for excitation into the n${\mathrm{}}^3$D levels. Absolute cross sections have been obtained for all the above mentioned levels using a laser-induced fluorescence technique and the results agree well with experimental values reported by Lagus et al. [Phys. Rev. A 53, 1505 (1996)] The cross sections for the 2${\mathrm{}}^3$S→n${\mathrm{}}^3$P excitations which correspond to dipole-allowed optical transitions are smaller than the corresponding 2${\mathrm{}}^3$S→n${\mathrm{}}^3$S and 2${\mathrm{}}^3$S→n${\mathrm{}}^3$D excitation cross sections, in contrast to the trends observed for excitations out of the ground level. This reversal behavior is discussed in terms of the dipole matrix element sum rule. Our cross-section data are compared with those of the alkali-metal atoms.

A systematic treatment is presented of near-threshold electron-atom scattering based on a variational principle. Results obtained here extend the modified effective-range theory derived some time ago (in which long-range polarization forces are taken into account) by providing a prescription, based on a minimum principle, for calculating the effective-range parameters. With the aid of perturbation-theory methods introduced recently for obtaining asymptotic solutions of the wave equation for scattering by a superposition of long-range power-law potentials [see M. J. Cavagnero, Phys. Rev. A 50, 2841 (1994)], higher-order corrections to the modified effective-range expansion in powers of the polarizability parameters are readily obtained. Explicit examples are provided. The theory is developed for both single-channel and multichannel scattering.

The Bethe theory for the energy loss of swift charged particles colliding with atomic targets is extended to treat explicitly the case when the projectile has bound electrons. The theory is characterized by taking into account the excitation and ionizations (but not charge exchange) of both projectile and target. As a consequence of the Coulombic character of the interaction between projectile and target, the electronic stopping cross section ${\mathrm{S}}_{\mathrm{e}}$ is split into contributions arising from the electronic structure of the projectile and of the target. In the lower part of the velocity region where this theory is applicable, the electronic structure of the projectile makes an important contribution, of the order 10–20%, to ${\mathrm{S}}_{\mathrm{e}}$. At high projectile velocities, however, this contribution decreases, leaving only the standard Bethe term. The number of electrons bound to the projectile, ${\mathrm{N}}_1$, as a function of the velocity of the projectile is calculated using the adiabatic Bohr criterion in conjunction with the Thomas-Fermi model of the atom. We obtain an analytic expression for the total stopping cross section using the Bethe approximation, and we compare the results of calculations of He, Li, and B ions on C and Al targets with experiment.

The interaction between large ion clusters or very intense ion beams with fusion plasma is studied using the dielectric function formalism with appropriate quantum corrections. The contributions from individual and collective modes to the energy loss are calculated. The general properties of the interference effects are characterized in terms of the relevant parameters, and simple scaling laws are obtained. In particular, the conditions for a maximum enhancement in the energy deposition are derived. The study provides a unified view and a general formulation of collective effects in the energy loss for low and high velocities of the beam particles.

The long-range interactions of two atoms, of an atom and a dielectric wall, of an atom and a perfectly conducting wall, and of an atom between two perfectly conducting walls are calculated, including the effects of retardation, for Li using dynamic polarizabilities determined from highly correlated, variationally determined wave functions.

We have investigated numerically the energies, intensities, and polarizations of channeling radiation (CR) photons emitted by relativistic electrons traversing Si and Ni crystals in the transition region from axial to planar channeling. Our work is based on the two-dimensional continuum model commonly used to describe axial channeling in crystals; it is a detailed numerical study of these CR properties embracing this entire region. More specifically, we calculated these properties of CR from 4-MeV electrons in Si as they underwent the transition from axial channeling along the 〈110〉 direction to planar channeling by the (100) planes, and in Ni in the transition from axial channeling along this direction to planar channeling by the (001) planes. The two-dimensional model correctly reproduces the experimentally observed planar CR photon energies in both cases, and also correctly predicts that the planar CR is plane polarized. In the transition region, our calculations predict the occurrence of complicated irregular oscillations of the photon energies corresponding to the strongest radiative transitions, and predict even larger oscillations of the intensities of such transitions. This oscillatory behavior could possibly be detected experimentally.

The position in angular space of the Bloch correction to the perturbative electronic stopping power formula of Bethe is investigated. It is demonstrated that this nonperturbative correction which originates in close collisions between the penetrating ion and target electrons appears at small scattering angles and that its position depends on the (adiabatic) cutoff at large distances. In the limit of infinite range the correction moves to infinitely small angles but remains finite and both the exact and the perturbative cross sections approach the Rutherford value at all finite angles. The picture is consistent with the lack of a Bloch-type correction in the energy-loss straggling.

A quantum kinetic master equation (QKME) for bosonic atoms is formulated. It is a quantum stochastic equation for the kinetics of a dilute quantum Bose gas, and describes the behavior and formation of Bose condensation. The key assumption in deriving the QKME is a Markov approximation for the atomic collision terms. In the present paper the basic structure of the theory is developed, and approximations are stated and justified to delineate the region of validity of the theory. Limiting cases of the QKME include the quantum Boltzmann master equation and the Uehling-Uhlenbeck equation, as well as an equation analogous to the Gross-Pitaevskii equation.

We study the thermodynamic behavior of an ideal gas of bosons trapped in a three-dimensional anisotropic harmonic-oscillator potential. The condensate fraction as well as the specific heat is calculated using the Euler-Maclaurin approximation. For a finite number of particles there is no phase transition, but there is a well-defined temperature at which the condensation starts. We also consider condensation in lower dimensions, and, for one-dimensional systems, we discuss the dependence of the condensate fraction and heat capacity on the ensemble used.

The behavior of a trapped atomic gas undergoing a Bose-Einstein condensation is strongly influenced by the attractive interaction between particles. The role played by such an interaction is analyzed by means of a variation method. The critical number of atoms for a metastable state to exist is determined. The aspect ratio of condensed bosons in an anisotropic trap, the loss rate of atoms, and the excitation spectrum are studied. The vortex state is discussed.

We extend the Hartree-Bogoliubov theory to the case of a multicomponent Bose condensate and determine the resulting quasiparticle frequency spectrum. We show that interferences resulting from cross coupling between the condensate components can lead to a reversal of the sign of the effective two-body interaction and to the onset of spatial instabilities.

We report evidence of an interference process between two bound-free atomic transitions induced by laser fields. Sodium atoms, initially prepared in the ${3\mathrm{P}}_{1\mathrm{/}2}$ excited state, are ionized by a nonresonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The experiment has provided a fringe visibility as high as 42%.

In this paper, we study the behavior of a two-level atom in an electromagnetic wave of circular polarization. A general solution to the Schrödinger equation is obtained, and the action of the electromagnetic wave on the atom is analyzed in detail. It is shown that the levels of the atom would be split, with the energy difference between the split levels the same for the two states of the atom. The atom would acquire an average momentum, with its direction the same as the propagating direction of the electromagnetic wave, showing the expelling action of the electromagnetic wave on the atom; or with its direction opposite to the propagating direction of the electromagnetic wave, showing the trapping action of the electromagnetic wave on the atom. The properties of the action are dependent on the initial state of the atom. The average expelling and trapping forces exerted on the atom by the electromagnetic wave are determined. Two cases of ``resonance'' and ``strict detuning'' are discussed particularly. Under the resonance case, the transition probability reaches its maximum for a given time. In the strict detuning case, the atom is not influenced by the electromagnetic wave.

A theoretical investigation of the formation of the so-called single pulse NMR echo has been undertaken in the presence of both Larmor and Rabi frequency inhomogeneities. It is found that this echo phenomenon, together with weaker, equally spaced secondary echoes, evolves from multiple-pulse excitation under certain constraints with respect to interpulse time intervals compared with longitudinal and transverse relaxation times. These transient coherences arise from a complex superposition of oscillatory free induction decays, which extend further into the time domain because of cumulative dephasing within each pulse in a multiple-pulse sequence. The effect is present for large Larmor inhomogeneity in isolation, but is stronger in the simultaneous presence of both frequency inhomogeneities, characteristic of ferromagnets. Predictions of this model analysis are confirmed through experiment via conventional, zero-field, pulsed NMR on binary ${}_{}{}^{51}{}_{}{}^{}\mathrm{VFe}$ and pure elementary (enriched) ${}_{}{}^{57}{}_{}{}^{}\mathrm{FeFe}$ ferromagnets.

We have employed a sample of cold cesium atoms in a magneto-optical trap to study nonlinear spectroscopy and four-wave-mixing optical phase conjugation using a noncycling transition. Reflectivities of the order of 1% have been measured for moderate levels of pump power and large angular aperture. In particular, we have performed nonlinear spectroscopy in a three-level V-type system and observed directly the ac Stark splitting of the cesium ${6\mathrm{S}}_{1\mathrm{/}2}$, F=4 ground state.

We report an experimental investigation of selective reflection spectra using a bichromatic field with a resonant Λ-type coupling to the ${\mathrm{D}}_1$ transition of Rb vapor atoms. The reflection spectra are found to have a more complex structure than equivalent spectra in transmission experiments. This structure is traced back to the effect of wall collisions on velocity selective optical pumping and saturation processes. The spectral line shapes are well described by a theoretical model developed in the text. In addition we report the observation of dark resonances in reflection spectroscopy.

The paper presents an analytic study of a two-state system which interacts with an external pulsed field. The excited state is supposed to decay out of the system with a decay rate Γ. We present two exact analytic solutions, viz., the finite Landau-Zener model and the Demkov-Kunike model. We also derive an approximate formula for arbitrary pulse shapes and detunings in the limit of a large decay rate. It gives the exact result for the original Landau-Zener model and the correct asymptotics for large Γ in the general case. We conclude that the independence of the final ground-state population on the decay rate, recently established for the original Landau-Zener model [V. M. Akulin and W. P. Schleich, Phys. Rev. A 46, 4110 (1992)], is only valid for the latter and not for any other model. The results show that the population loss ultimately decreases at large decay rates which can be viewed as a quantum analog of the overdamped oscillator.

We study the dynamics of ultracold atoms in a periodic optical potential submitted to a constant external force. Bloch oscillations in the fundamental and first excited bands of the potential are observed. In addition to a solid-state analysis we give a quantum-optics interpretation of this effect in terms of photon exchanges between the atoms and the laser waves. The dynamics of Bloch oscillations are equivalent to a sequence of adiabatic rapid passages between momentum states and can be described using the dressed-atom approach. We demonstrate efficient and dissipation-free acceleration of atoms by coherent transfer of a large number of photon momenta (≈100). This technique produces atomic beams with a very small longitudinal velocity spread that may find applications in atom optics and precision measurements.

We report on measurements of the population of Rydberg states of Na in a constant electric field that result when 300-K blackbody radiation induces transitions from initial states with |m|=0,1 to neighboring states. Our measurements included comprehensive ranges of initial states (${\mathrm{n}}_{\mathrm{i}}^{\mathrm{*}}$=24–29), repopulation times (${\mathrm{t}}_{\mathrm{d}}$=0–50 μs), and fields (${\mathrm{F}}_{\mathrm{s}}$=175–712 V/cm). We find that the population is redistributed nonuniformly among states neighboring the initial state, reflective of sharp resonances between the states. By comparing calculations of transition probabilities in Na to the data for ${\mathrm{F}}_{\mathrm{s}}$=200 V/cm we were able to identify the populated states. Comparisons with calculations in H reveal that these resonances are hydrogenic.

We have studied numerically the excitation of a model diatomic molecule by ultrashort pulses which have large envelope areas. In situations where the dynamics of the wave packet during the excitation process is expected to suppress the oscillatory behavior predicted by the area theorem, we observe a revival of the oscillations due to the freezing of the wave packet dynamics by fast Rabi oscillations. Furthermore, we show that with hyperbolic secant pulses one can obtain large excitation probabilities, but with Gaussian pulses one can keep the excited state wave packet reasonably well-defined even when the dynamics imposed by the excited state potential is expected to deform it strongly.