We report measurements of the absolute differential cross sections for charge-transfer scattering of 0.5-, 1.5-, and 5-keV protons by atomic oxygen at scattering angles between 0.01° and 2.6° in the laboratory frame. Absolute integral cross sections are also reported and compared with previously published total cross sections. The measurements were made using a flowing gas target, which consisted of a mixture of atomic and molecular oxygen produced by passage of ${\mathrm{O}}_2$ through a microwave discharge. The cross sections for atomic oxygen were obtained by appropriate subtraction of the signal due to molecular oxygen from that due to the mixture of O and ${\mathrm{O}}_2$. © 1996 The American Physical Society.

Using the adiabatic theory and the dipole close-coupling and classical impulse approximations, scaling relations are derived for the cross sections of dipole-allowed and dipole-forbidden 1${\mathrm{}}^1$S→n${\mathrm{}}^1$L excitation transitions in atoms induced by multiple charged ions of intermediate to high reduced energies. The cross-section scalings incorporate the ionic charge q, the electron transition energy ${\mathrm{\omega }}_{1\mathit{n}}$, and the oscillator strength (in the case of optically allowed transitions). The derived scalings describe well the available experimental data for H and He atoms for reduced energies E/q${\mathrm{\omega }}_{1\mathit{n}}$ greater than about 25 keV/amu. These scalings also show that, for a fixed energy, the nonscaled excitation cross section as a function of q does not saturate at high q values but decreases after reaching a maximum. © 1996 The American Physical Society.

The different kinematical and geometrical arrangements that may be used in (e,2e) studies are briefly reviewed. The ionization of H(1s) is considered, and within the confines of a relatively simple theoretical model, it is shown how to define experimental setups where one may extract information on the role of Coulomb three-body effects in the incident and final channels. Theoretical and experimental results are presented for coplanar constant geometry where the focus is primarily on incident channel effects. © 1996 The American Physical Society.

Very-low-energy collisions between two atoms are usually suppressed, in that the probability of close approach of the atoms becomes greatly reduced as the collision energy vanishes, even if the potential is completely attractive (with the exception of the Coulomb interaction). The suppression is a quantum effect, related to the Wigner threshold law. It is gauged by comparing the ratio of the probability of being inside the well to the probability of being outside for both the classical and quantum regimes. As the asymptotic kinetic energy vanishes, the approaching atoms reach a minimum distance of typically 20 or 30 a.u. Here we study attractive interaction potentials of the form -α/${\mathit{r}}^{\mathit{n}}$, and give some numerical results for accurate X${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ and a${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ states of ${\mathrm{Li}}_2$ and ${\mathrm{Na}}_2$ molecules. We show that in some circumstances it is possible to use Wentzel-Kramers-Brillouin theory in the suppression regime (where it fails) and to correct for its failure with a simple factor. © 1996 The American Physical Society.

An electron capture at the cyclotron energy has been explored by using a high-resolution magnetic spectrograph. The K-shell electron capture from heavy elements (V, Cr, Cu, Ge, Nb, Ag, and Sn) by ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ projectiles at 52 and 72 MeV was measured. The dependence of the cross sections on the atomic number of the target obtained at these two energies are compared with current electron-capture theories. © 1996 The American Physical Society.

Total cross sections for electron scattering by ${\mathrm{CO}}_2$ molecules in the energy range 400–5000 eV have been measured with experimental errors of ∼3%. The present results have been compared with available experimental and theoretical data. The dependence of the total cross sections on electron energy shows an asymptotic behavior with increasing energies, in agreement with the Born-Bethe approximation. In addition, an analytical formula is provided to extrapolate total cross sections to higher energies. © 1996 The American Physical Society.

The Faddeev approach is used to study s-wave scattering and fusion in flight for the muonic-atomic systems p+pμ, d+dμ, t+tμ, d+tμ, and t+dμ in a broad energy range below the n=2 threshold of the target muonic atom. Clear manifestations of near-threshold virtual states in the p+pμ and t+tμ systems are seen, e.g., in large amplifications of the fusion-in-flight reaction rates near zero energy. Peaks and minima (essentially exact zeros for the symmetric systems) in the fusion-in-flight rates are also seen at finite energies. In the case of d+tμ, the fusion-in-flight rate (at liquid-hydrogen density) reaches a value of 3.40×${10}^6$ ${\mathrm{s}}^{\mathrm{-}1}$ for collisions at 76.3 eV, which may be compared with the rate 1.14×${10}^5$ ${\mathrm{s}}^{\mathrm{-}1}$ at thermal energies. © 1996 The American Physical Society.

We have investigated a two-color coherent-control process at high densities of the absorbing medium. Irradiating a vapor of atomic mercury with a coherent field, with one component nearly resonant with the three-photon 6s${\mathrm{}}^1$${\mathit{S}}_0$→6p${\mathrm{}}^1$${\mathit{P}}_1^{\mathit{o}}$ transition, and the other component the third harmonic of the first, we observe the interference between the two excitation pathways for this transition. Measurements of this interaction as the density of the mercury vapor is increased show evidence of nonlinear coupling (phase and amplitude locking) of the two field components with each other. This is manifested as a decrease in the magnitude of the photoionization signal, as well as a decrease of the depth of modulation of the photoionization signal. © 1996 The American Physical Society.

An absolute measurement of the rate coefficient for dielectronic recombination (DR) of ${\mathrm{C}}^{3+}$, via the 2s-2p core excitation, in an external electric field of 11.4±0.9(1σ) V ${\mathrm{cm}}^{\mathrm{-}1}$ is presented. An inclined-beam arrangement is used and the stabilizing photons at ∼155 nm are detected in delayed coincidence with the recombined ions. The full width at half maximum of the electron energy spread in the ion rest frame is 1.74±0.22(1σ) eV. The measured DR rate, at a mean electron energy of 8.26±0.07(1σ) eV, is (2.76±0.75)×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$. The uncertainty quoted for the DR rate is the total uncertainty, systematic and statistical, at the 1σ level. In comparing the present results to theory, a semiempirical formula is used to determine which recombined ion states are ionized by the 4.65 kV ${\mathrm{cm}}^{\mathrm{-}1}$ fields in the final-charge-state analyzer and not detected. For the present results, any DR of the incident electrons into n levels greater than 44 is assumed to be field ionized in the final-charge-state analyzer. A more precise treatment of field ionization, which includes the lifetime of the ${\mathrm{C}}^{2+}$ ions before they are ionized and the time evolution and rotation of the fields experienced by the recombined ions, is needed before a definitive comparison between experiment and theory can be made. Our DR measurement, within the limits of that approach, agrees reasonably well with an intermediate coupling calculation that uses an isolated resonance, single-configuration approximation, but does not agree with pure LS-coupling calculations. © 1996 The American Physical Society.

The deexcitation of the resonantly excited Kr 3${\mathit{d}}_{3/2,5/2}^{\mathrm{-}1}$5p states has been studied by recording the electron spectra with very high photon and electron energy resolution and by comparing the results with multiconfiguration Dirac-Fock calculations. The high resolution has enabled us to resolve the fine structures in detail by utilizing the Auger resonant Raman effect. High-quality experimental results are used to confirm the roles of exchange interaction and of initial- and final-state interactions in distributing the intensity to the fine structures in the spectra. © 1996 The American Physical Society.

Absolute cross sections, differential in energy and angle of ejection of the secondary electrons, were determined for electron emission from ${\mathrm{He}}^{+}$+${\mathrm{H}}_2$ collisions at 28–114 keV by measuring electron energy spectra from 1.5 to 300 eV at several angles from 15° to 160°. A rotatable electrostatic analyzer was used with an energy resolution of 5% and an angular acceptance of 4.6° full width at half maximum. The double-differential cross sections were integrated over angle, energy, or both to obtain single-differential and total ionization cross sections. The latter are in excellent agreement with previous experimental results. Agreement of the experimental double-differential cross sections with plane-wave Born approximation calculations is generally poor. The cross section for ejection of high-energy electrons by ${\mathrm{He}}^{+}$ is considerably larger than that for ${\mathrm{H}}^{+}$ at the same velocity. This is attributed mostly to electron emission from the projectile. The presence of prominent Doppler-shifted helium autoionization peaks in the spectra indicates an appreciable probability for electron capture by the helium ions with simultaneous double excitation. © 1996 The American Physical Society.

Absolute ionization cross sections for 28–114-keV helium ion impact on atomic hydrogen, differential in energy and angle of the ejected electrons, have been obtained from crossed-beam measurements and previously measured cross sections for molecular hydrogen targets. A radio frequency discharge source produced a mixed atomic and molecular target with a typical dissociation fraction of 74%. Energy spectra were measured from 1.5 to 130 eV by an electrostatic analyzer with a resolution of 5%. The angular range was 15°–160°. Results are compared with calculations based on the first Born, continuum-distorted-wave–eikonal-initial-state, and classical trajectory Monte Carlo methods. Total electron yields are obtained by combining calculations that are separately performed for liberating the target and projectile electrons. Model potentials are used to represent the interparticle-separation-dependent screening of the nuclear charges experienced by the electrons. Though projectile electron emission is a negligible component of the total ionization cross section for the present collision energies, its contribution is significant for particular regions of the spectrum of ejected electrons. Through comparison with our proton-impact data [Kerby et al., Phys. Rev. A 51, 2256 (1995)], differences and similarities are demonstrated owing to the common asymptotic charge of these two projectiles but differing nuclear charges and the electron carried by the ${\mathrm{He}}^{+}$ ion. Comparisons are also made illustrating the differences between atomic and molecular hydrogen targets. © 1996 The American Physical Society.

We demonstrate that for atom interferometers with lasers serving as beam splitters state independence (including freedom from Doppler effects) of the phase shift is obtained if the paths of the two interfering beams are closed and have obtained the same momentum transfer. This is also the case when the center-of-mass motion of the atoms is subject to a constant force field (acceleration). Among these interferometers the one with the simplest geometry is deduced for which the phase shift does not depend on acceleration. Its remaining sensitivity to rotation is discussed. Finally the geometry of a state-independent interferometer that is practically insensitive to both acceleration and rotation is worked out. As an application its relevance for the detection of the influence of space-time curvature on the atomic motion is studied. © 1996 The American Physical Society.

We investigate the time-dependent Schrödinger equation for the case of a plane matter wave incident on an oscillating potential step. Approximate solutions are given for different regimes of the problem. The resulting energy spectra are quantized. This may either be interpreted as a phase modulation of the wave or as a coherent multiphonon exchange. A comparison to the results that apply for a beam of classical particles is made. In particular, the transition from the quantum-mechanical to the classical case is examined. We have performed an experiment with very cold neutrons that clearly revealed the discussed effects. Energy splittings of 2.8 to 9.1 neV of the reflected waves were analyzed with an energy resolution of ∼1 neV, thus demonstrating the feasibility of such high-resolution experiments with neutron reflectometry. © 1996 The American Physical Society.

We study the coherent scattering of weak light from a system of cooled bosonic atoms in a trap. We derive and discuss in detail the scattering equation for such a process that describes multiple scatterings from different atoms. Our theory accounts for atom-atom interactions and determines the density profile of the atomic fields from the self-consistent Bogoliubov-Hartree method. We present two kinds of analytic solutions to the scattering equation using the on-shell approximation and the generalized diffraction theory. We discuss the validity of those approaches, and demonstrate that they both lead to similar results. Our studies indicate that the scattering cross sections exhibit non-Lorentzian shapes with narrow features close to resonance. Coherent scattering occurs mainly in the forward direction, and the angular distribution is limited by the size of the atomic sample in the trap. We present results for various ranges of relevant parameters such as trap size, interaction strength, number of atoms, and temperature, etc. © 1996 The American Physical Society.

The validity of the Zintl-Klemm-Busmann (ZKB) principle is examined for the cluster series ${\mathrm{Sb}}_3$SnA, ${\mathrm{Sb}}_3$In${\mathit{A}}_2$ (A=alkali-metal atom) and ${\mathrm{Sb}}_3$TeHalo (Halo=halogen atom), which have been recently discovered by Knudsen effusion mass spectrometry. In the Zintl anion systems ${\mathrm{Sb}}_3$SnA and ${\mathrm{Sb}}_3$In${\mathit{A}}_2$, the alkali-metal atoms function, according to the ZKB principle, as electron donors. Since sizable electron transfer occurs from the alkali-metal-atom system to the ${\mathrm{Sb}}_3$Sn or ${\mathrm{Sb}}_3$In cluster nuclei, these units approach a 20-valence-electron configuration and therefore tetrahedral symmetry. From our ab initio treatment of the systems ${\mathrm{Sb}}_3$SnA and ${\mathrm{Sb}}_3$In${\mathit{A}}_2$ with A=Na, K, or Cs it turns out that for both types of Zintl clusters a geometry is energetically favored which allows for maximal electron transfer and thereby for the closest possible approximation to tetrahedral symmetry of the cluster nuclei. This finding confirms that the ZKB principle is applicable to free clusters. For the Zintl cationic system ${\mathrm{Sb}}_3$TeHalo (Halo=Cl,Br,I), again a clear correspondence between electron transfer and cluster nucleus geometry is observed, but in contrast to the Zintl anion systems discussed in this work, the tetrahedral structure of the cluster nucleus does not turn out to be the stablest one. Quantitative comparisons between theoretical and measured ionization energies yield excellent agreement. © 1996 The American Physical Society.

The multiphoton absorption mechanism of anharmonic molecular bonds in laser fields of high intensity is investigated. The time dependence of the absorbed energy as a function of field frequency is studied for a number of cases of varying anharmonicity and field strength. It is found that the short-time dynamics can be meaningfully described by an ‘‘apparent detuning.’’ The results obtained with a continuous-wave field are used for the design of Gaussian pulses. © 1996 The American Physical Society.

We have observed momentum transfer in cesium atoms using the process of adiabatic transfer between dark states. We have investigated the sensitivity of this process to Zeeman shifts and to the differences between the translational energies of the states involved. We have shown that, for a chosen velocity class, the resultant phase shifts of the magnetic substates can be canceled so a dark state is maintained. Two different laser polarization configurations were used to perform the experiments: two circularly polarized beams or a linearly and a circularly polarized beam. Results for each are presented. Finally we discuss the relevance of our results to atomic interferometry and consider methods to minimize the possible phase shifts introduced in an atomic interferometer based upon adiabatic transfer. © 1996 The American Physical Society.

We apply kinetic theory to the problem of evaporative cooling of a dilute collisional gas in a trap. Assuming ‘‘sufficient ergodicity’’ (phase-space distribution only a function of energy) and s-wave collisions with an energy-independent cross section, an equation for the evolution of the energy distribution of trapped atoms is derived for arbitrary trap shapes. Numerical integration of this kinetic equation demonstrates that during evaporation the gas is accurately characterized by a Boltzmann distribution of atom energies, truncated at the trap depth. Adopting the assumption of a truncated Boltzmann distribution, closed expressions are obtained for the thermodynamic properties of the gas as well as for the particle and energy loss rates due to evaporation. We give analytical expressions both for power-law traps and for a realistic trapping potential (Ioffe quadrupole trap). As an application, we discuss the evaporative cooling of trapped atomic hydrogen gas. © 1996 The American Physical Society.

We discuss the application of Gutzwiller’s semiclassical theory to a circular billiard with a singular magnetic flux line added at its center. The Aharonov-Bohm effect manifests itself through the cancellation of periodic orbits for particular flux strengths. Diffraction phenomena affect the gross-shell structure of the level density and require corrections of higher order in ħ. The full quantization of the level spectrum, however, is much less affected. © 1996 The American Physical Society.