We present measurements on fine-structure-changing ${\mathrm{Ne}}^{\mathrm{*}\mathrm{*}}${(2p${)}^5$(3p)}+He collisions at superthermal energies with an average value of 〈E〉=470 meV. For an extensive set of transitions absolute values of polarized-atom cross sections are given. A wide range of cross section magnitudes and polarization effects is observed. Even higher order polarization effects are evidently present. A suitable set of anisotropy parameters is introduced which has large advantages for a direct comparison of experimental and calculated cross sections. Although the experimentally observed polarization effects are reproduced by quantum calculations, the calculated magnitude of the cross section is, in general, larger than the experimental value. Moreover, in contrast with the thermal energy range, the current semiclassical model fails in describing the most prominent cross sections in terms of the ‘‘visible’’ avoided crossings in the adiabatic potentials. In the quantum calculations, clear indications are found for an additional transition mechanism of radial matching. In this mechanism a change in the rotational quantum number of the nuclear motion matches the asymptotic energy gap Δ${\mathit{E}}_{\mathit{k}\mathit{l}}$ between the initial and final state k and l, respectively. This process is most efficient at large internuclear distances, thus enhancing the magnitude of the cross section considerably. Radial matching is most effective when 〈E〉/Δ${\mathit{E}}_{\mathit{k}\mathit{l}}$≫1. The calculated magnitude of the cross section is thus sensitive to details of the long-range part of the potentials used as input. At thermal energies this sensitivity is masked, which explains the good agreement of the calculations with the model potentials of Hennecart and Masnou-Seeuws [J. Phys. B 18, 657 (1985)] with the experiments in the thermal range.

The asymptotic behavior of the optical potential, describing elastic scattering of a a charged particle α off a bound state of two charged, or one charged and one neutral, particles at small momentum transfer ${\mathrm{\Delta }}_{\mathrm{\alpha }}$ or, equivalently, at large intercluster distance ${\mathrm{\rho }}_{\mathrm{\alpha }}$, is investigated within the framework of the exact three-body theory. For the three-charged-particle Green function that occurs in the exact expression for the optical potential, a recently derived expression, which is appropriate for the asymptotic region under consideration, is used. We find that for arbitrary values of the energy parameter, the nonstatic part of the optical potential behaves for ${\mathrm{\Delta }}_{\mathrm{\alpha }}$→0 as ${\mathit{C}}_1$${\mathrm{\Delta }}_{\mathrm{\alpha }}$+o(${\mathrm{\Delta }}_{\mathrm{\alpha }}$). From this we derive for the Fourier transform of its on-shell restriction for ${\mathrm{\rho }}_{\mathrm{\alpha }}$→∞ the behavior -a/2${\mathrm{\rho }}_{\mathrm{\alpha }}^4$+o(1/${\mathrm{\rho }}_{\mathit{a}}^4$), i.e., dipole or quadrupole terms do not occur in the coordinate-space asymptotics. This result corroborates the standard one, which is obtained by perturbative methods. The general, energy-dependent expression for the dynamic polarizability ${\mathit{C}}_1$ is derived; on the energy shell it reduces to the conventional polarizability a which is independent of the energy. We emphasize that the present derivation is nonperturbative, i.e., it does not make use of adiabatic or similar approximations, and is valid for energies below as well as above the three-body dissociation threshold.

Energy losses and fractions of ${\mathrm{H}}_3^{+}$ and ${\mathrm{H}}_2^{+}$ ions emerging from carbon foils of 1–8.5 μg/${\mathrm{cm}}^2$ thickness, after incidence of 9.6-MeV/amu ${\mathrm{H}}_3^{+}$ ions, have been measured. From the decrease of the transmitted fraction of ${\mathrm{H}}_3^{+}$ ions with increasing foil thickness, the dissociation cross section for 9.6-MeV/amu ${\mathrm{H}}_3^{+}$ due to the ${\mathrm{H}}_3^{+}$-C collision has been determined to be (2.2±0.1)×${10}^{\mathrm{-}17}$ ${\mathrm{cm}}^2$. Further, the potential energy of the ${\mathrm{H}}_2^{+}$+${\mathrm{H}}^{+}$ system has been derived from the energy spectra of the fragment ${\mathrm{H}}_2^{+}$ ions, being 9.45±0.74 eV, with interproton separations being equal to those known for stable ${\mathrm{H}}_3^{+}$ ions. For the stopping power of carbon for 9.6-MeV/amu ${\mathrm{H}}_3^{+}$ ions a value of 83.3±5.5 eV/(μg/${\mathrm{cm}}^2$) has been obtained from the energy-loss data. This result corresponds to an effective charge of 1.45±0.09 for ${\mathrm{H}}_3^{+}$ in carbon. The observed stopping power and the derived effective charge agree well with the values calculated by using the first-order Born approximation.

This paper presents a detailed description of KLL Auger spectra arising from collisions of hydrogenic ${\mathrm{N}}^{6+}$ ions on clean Si(100) and Al(110) surfaces at energies ranging from 78 eV up to 60 keV and at incident angles from 2.5° up to 45°. Atomic structure calculations of KLL Auger energies and decay rates for hollow atoms, together with simulations of the projectile trajectories, support a model in which the de-excitation of the ions proceeds essentially via two mechanisms. Before the ion enters the ‘‘close collision range,’’ a few of its L-shell vacancies are filled by Auger cascades involving outer shell electrons. But as seen as the projectile suffers close collisions, L-shell vacancies are rapidly filled by direct capture of target core electrons. The time scale of the latter mechanism is determined by the collision frequency.

We have constructed an atom interferometer with interfering beams that are physically isolated by a metal foil. By applying an interaction to one of the two interfering beams, we can measure ground-state energy shifts with a spectroscopic precision of 6.6×${10}^{\mathrm{-}14}$ eV/√min , or 16 Hz/√min . Applying an electric field to one beam of the interferometer, we have measured the phase shift induced from the quadratic Stark effect. By analyzing these phase shifts, we have determined the ground-state polarizability of sodium, with much improved accuracy, to be 24.11(6${)}_{\mathrm{statistical}}$(6${)}_{\mathrm{systematic}}$×${10}^{\mathrm{-}24}$ ${\mathrm{cm}}^3$.

Spectral filtering of the time-separated neutrons passing through an interferometer reconstitutes the interference pattern through the formation of a macroscopic superposition out of the neutron beams and produces squeezed states. In this paper we use matter-waves squeezed states produced in this way to manipulate the rate of expansion of the neutron wave packet. We demonstrate that these quantum states also have nontrivial correlations 〈ΔkΔx〉. The Heisenberg relation for the spread of the wave packet holds even in this case and the reversal of the wave packet spreading is impossible. However, one can manipulate the rate of the wave packet expansion by changing the phase shift in one of the arms of a neutron interferometer.

We derive the dispersion relation for the propagation of quasiresonant light with frequency ${\mathrm{\omega }}_{\mathit{L}}$ in an ultracold gas of bosonic atoms in the dilute regime, i.e., for an atomic density ${\mathrm{\rho }}_0$≪(${\mathrm{\omega }}_{\mathit{L}}$/c${)}^3$. In our calculation, valid up to order 2 in density, two types of corrections to the Lorentz-Lorenz formula for the refractive index appear. The first one is due to the bosonic nature of the atoms and its contribution is related to the two-body correlation function. The second correction originates from multiple scattering of photons within pairs of close atoms, giving rise to the resonant van der Waals interaction. The temperature dependence of the refractive index gives a clear signature of quantum statistical effects, even if the degeneracy threshold for Bose-Einstein condensation is not reached.

We use the Wigner random-matrix model and describe the transformation of the electron density spectrum of warm metallic clusters. We calculate the effect of temperature on the magic numbers, the cluster stability, and the kinetics of evaporation. Comparison of the calculated dependencies with the experimental ones shows good agreement.

The plateau in the harmonic-generation (HG) spectrum of a bound quantum system is first associated with classical chaotic dynamics. The molecular cutoff in the HG spectra was obtained at Ω=mω, where ω is the frequency of the time-periodic field and m is the area of the bound chaotic region in the phase space divided by 2πħ.

An analytic expression of the polarization induced by a polychromatic weak probe field in a two-level medium, driven by a strong polychromatic pumping field, is obtained. This expression allows one to obtain both the absorption and dispersion spectra at any frequency of a probe field. The numerically calculated graphs of the central-component probe-field absorption and dispersion spectra, depending on the pumping-field amplitude in the cases of the one-, two- three-, four-, and five-component pumping field, are presented. It is shown that when the probe-field detuning is close to the strong-field Rabi frequency, the absorption spectrum changes to the dispersionlike form, while the dispersion spectrum changes to the absorptionlike one.

Crossed magnetic and electric fields are observed to extend the lifetime of high molecular Rydberg states of DABCO (1,4-Diazabicyclo[2.2.2]octane) well into the microsecond range. The experimental and computational (using classical mechanics and for a diatomic polar core) results agree both on the magnitude of the effect and on its decrease with increasing electrical field. Theoretical considerations suggest that this time-stretching effect is only present for high Rydberg states and/or for not too weak fields. The computed lifetime increases as the magnetic-field strength is decreased but it requires a finite magnetic field for the onset of the effect. The experimental technique of detection of the surviving Rydberg states via their ionization in a delayed field pulse (known as ZEKE spectroscopy) is most easily implemented for high (say, n>100) but not too high (n<400) Rydberg states. In this regime, the magnetic field required for the additional time stretching is larger than that due to the earth but can be significantly weaker than that required to induce extensive chaotic behavior. The results of the numerically exact classical simulations are interpreted using equations of motion, cast in the form of a mapping, which retain terms up to second order in the fields. (The first-order terms are qualitatively and quantitatively not, by themselves, sufficient.) As is to be expected on physical grounds, the origin of the effect is the slow, periodic modulation of the magnitude of the magnetic quantum number ${\mathit{m}}_{\mathit{l}}$ of the electron due to terms second order in the magnetic field. Since the angular momentum l of the electron is bounded from below by ${\mathit{m}}_{\mathit{l}}$, and since it requires a low l for the electron to couple effectively to the molecular core, the presence of the magnetic field provides for an elongation of the time scale which is over and above that made possible due to the periodic motion of l due to the dc electric field.

The ionization of hydrogenic atoms by a static electric field is investigated via a classical Hamiltonian model which includes the effects of nonzero angular momentum corresponding to magnetic quantum number m. A rigorous treatment of the saddle-point criterion is employed to derive a sufficient condition for the existence of bound states. For particle energies above the saddle-point energy, where the potential well is open, orbits may be trapped by the existence of an isolating integral (the Runge-Lenz invariant), which is identified as the separation constant of the Hamilton-Jacobi equation in parabolic coordinates. Relations between the two criteria are established and parallels drawn with the (integrable) problem of two centers. We also treat the nonintegrable case of parallel electric and magnetic fields, and apply the saddle-point criterion to the effective potential. Perturbation by a circularly polarized field does not possess an effective potential, so that the saddle-point criterion does not apply. It is shown that in this case the full Hamiltonian dictates stability and that circular orbits can lose stability via Krein collision of eigenvalues.

When a pump field and probe field simultaneously drive an electronic-state atomic transition, the probe field absorption spectrum can consist of an absorption peak centered near Δ’=0 and amplification peak centered near Δ’=2Δ (Δ and Δ’ are the pump and probe field detunings from the atomic transition frequency, respectively). This type of spectrum is seen in the limit ‖Δ‖≫χ≫Γ, where χ is the Rabi frequency associated with the pump field and 2Γ is the homogeneous width associated with the atomic transition. For atoms cooled below the recoil limit of laser cooling, the qualitative nature of the probe absorption spectrum can undergo a dramatic change. Provided that the recoil splitting ${\mathrm{\omega }}_{\mathit{k}}$ is larger than the homogeneous decay rate (as might occur in the case of a forbidden transition), the absorption and amplification features each split into an absorption-amplification doublet. In addition, structure is found in the probe absorption spectrum near Δ’=Δ; this structure consists of two absorption-amplification doublets. Both doublets can be resolved if ${\mathrm{\omega }}_{\mathit{k}}$>Γ. If ${\mathrm{\omega }}_{\mathit{k}}$<Γ, one of the doublets can be resolved provided that ${\mathrm{\omega }}_{\mathit{k}}$>${\mathrm{\Gamma }}_{\mathit{A}}$, where ${\mathrm{\Gamma }}_{\mathit{A}}$ is some effective atomic ground state width in the problem. The positions, widths, and relative weights of all the components are readily predicted using a dressed-atom theory in which quantization of the center-of-mass momentum is included. An analytical expression for the probe field spectrum is obtained for a simple case in which spontaneous decay to the lower level is neglected. Validity criteria for the results are discussed.

We develop models for the motion of two trapped ions in one dimension, including both harmonic trapping forces and the Coulomb interaction between the ions. Mathematical and physical consistency dictates that the divergence of the Coulomb interaction at small distances be tempered with a soft core. A split-operator method is used to analyze numerically the energies and eigenstates of the motion of the two ions. Classification and closed-form expressions are also presented for the energies and the eigenstates in the limits when the equilibrium distance due to the Coulomb repulsion is either much smaller or much larger than the amplitude of the zero-point motion of the ions. Implications of the results for the properties of two identical ions with Bose-Einstein or Fermi-Dirac statistics are discussed.

We analyze a type of cavity, or trap, for atoms: a hollow optical fiber bent into a vertical U shape. The atoms are confined by gravity and by light forces due to the evanescent wave on the fiber’s interior surface. A unique feature of this cavity is its mechanical flexibility, which allows tailoring of the gravitational potential experienced by the atoms. In particular a cycloid shape gives simple harmonic motion along the fiber. It can achieve confinement times similar to the parabolic reflector type of gravitational cavity. Quantized motion and an intracavity cooling scheme are considered.

We study the near-resonant Kapitza-Dirac diffraction of two atoms interacting via the dipole-dipole interaction within the framework of an effective one-dimensional model. We concentrate mostly on the Stern-Gerlach regime of diffraction, where the atomic wave functions are sufficiently well localized that a good physical understanding of the system dynamics in terms of local potentials can be achieved. In general, the dipole-dipole interaction can lead to substantial modifications of the Stern-Gerlach diffraction pattern. We find in particular that under appropriate conditions, bound states of the atomic system can be established, with the two atoms separated by a distance of the order of hundreds of nanometers. However, spontaneous emission eventually destroys the binding between these states via a heating mechanism somewhat similar to strong-field Sysiphus heating. In this respect, the behavior of the ‘‘diatom’’ bound state under the influence of spontaneous emission is similar to that of the atomic solitons predicted to occur in near-resonant Kapitza-Dirac diffraction in the framework of nonlinear atom optics.

Selective excitation of laser-induced breakdown of xenon by nanosecond laser pulses in the spectral range of 420–600 nm and light intensity of ${10}^{11}$ W/${\mathrm{cm}}^2$ has been studied. A number of multiphoton resonances have been identified in the breakdown excitation spectra and the excitation conditions within the focal volume have been determined from the ac Stark shift of the resonances. It has been shown that the breakdown initiation is influenced by odd harmonics of the laser light, which are generated in the focal volume. The phase-matched harmonic field induces the breakdown near some of the multiphoton resonances, while this field is able to suppress the breakdown under on-resonance excitation due to destructive interference between optical transitions. Taking into account the internally generated harmonics of the laser field, the interpretation of some of the previous experimental results is reconsidered.

Using solutions of the time-dependent Schrödinger equation for the ${\mathrm{H}}_2^{+}$ molecular ion, we examine various scenarios for controlling high-order harmonic generation with combinations of laser fields. It is found that (i) the harmonic generation ‘‘plateau’’ can be modified by combining a laser field with its third harmonic field, (ii) even harmonics are controllable using the combination of a laser field and a weaker second harmonic field, and (iii) the phases of high-order harmonics in a strong driving field can also be changed by additional fields. In particular, when a weak dc electric field is applied, we prove that the phase shifts of all harmonics linearly depend on the dc field strength. In addition, when an ultrastrong static magnetic field is applied collinearly with the driving laser polarization, harmonic generation efficiency is enhanced and higher harmonics beyond the usual ‘‘cutoff’’ are generated as a result of the squeezing of the transversely spreading electron wave packets. The enhancement is a confirmation of the electron-ion recollision mechanism for high-order harmonic generation.

Time-dependent unrestricted Hartree-Fock (TDUHF) theory is formulated for the multiphoton ionization of a model atom. The single-particle states are nonorthogonal and the quantum variational principle requires additional normalization terms. For a model He atom, the three-dimensional (3D) electrostatic interaction is replaced by a 1D soft-core interaction that allows the construction of the two-electron atom in only two dimensions. The resulting 1D formulation of the TDUHF equations contains numerous exchange and overlap terms. The TDUHF multiphoton ionization rates are found to be in better agreement with the results of a direct solution of the time-dependent Schrödinger equation on a 2D lattice than the rates predicted by a 1D formulation of the standard TDHF equations.

This paper deals with the motion of a dipole molecule in a standing wave of a microwave field. Depending on the frequency of the field, a molecule is shown to have very different dynamics. The range of the exhibited phenomena spreads from the regular nonlinear oscillation near the nodes (antinodes) of the standing wave to the chaotic acceleration of a molecule. The possibility of monitoring a molecular position by a strong microwave field is numerically demonstrated.