Cross sections for ionization of He by highly charged ${\mathrm{Cl}}^{\mathit{q}+}$, ${\mathrm{Cu}}^{\mathit{q}+}$, and ${\mathrm{I}}^{\mathit{q}+}$ (q=6–10) impact at velocities from 1.6 to 3.1 a.u. were measured. These results are compared with other experimental and theoretical results available over a wide velocity range. A universal scaling rule for target ionization by nearly bare, highly charged ions at low to intermediate velocities (0.2–3.5 a.u.) is reported. © 1996 The American Physical Society.

Electron-impact detachment cross sections have been measured from threshold to about 30 eV for ${\mathrm{D}}^{\mathrm{-}}$ and ${\mathrm{O}}^{\mathrm{-}}$. The purpose was to investigate the cross section near threshold and to reinvestigate earlier claims of resonances due to short-lived states of the doubly charged negative ion. Initial results for ${\mathrm{D}}^{\mathrm{-}}$ showed no resonances [L. H. Andersen, D. Mathur, H. T. Schmidt, and L. Vejby-Christensen, Phys. Rev. Lett. 74, 892 (1995)]. Here we show that there is no detectable resonance structure for ${\mathrm{O}}^{\mathrm{-}}$ either. The applied experimental technique is discussed in detail. Classical and semiclassical models for the detachment process are presented and compared with the data. © 1996 The American Physical Society.

The spectra of electrons emitted in ${\mathrm{Ar}}^{\mathit{q}+}$ on Ar (8≤q≤16) collisions at 2.3q keV were measured in the 30–400-eV energy range. ${\mathrm{Ar}}^{\mathit{q}+}$ ions were produced by the Cornell superconducting solenoid, cryogenic electron-beam ion source, and the emitted electrons analyzed by a √π/2 cylindrical electrostatic analyzer at 90° to the ion beam. The observed spectral features are interpreted in terms of energy differences between total electronic energies of final states of the collision system [${\mathrm{Ar}}^{\mathit{q}+}$+ (electron configuration)] + ${\mathrm{Ar}}^{\mathit{r}+}$ and the ${\mathrm{Ar}}^{\mathit{q}+}$+ Ar ground-state total electronic energy. The measured spectra appear to have a common interpretation. © 1996 The American Physical Society.

Doubly differential cross sections for electron ejection in H-He collisions have been measured in the incident atom energy range 2.3–10 keV. Analysis of the experimental data on the basis of the adiabatic theory of direct ionization has been made and parameters of the quasimolecule, namely, the energy of the initial 2pσ state coupled with the continuum, the effective charge, and the real and imaginary parts of the internuclear distance at which the ‘‘superpromoted’’ diabatic term crosses boundary of continuum, have been determined. The values of the initial-state energy and the effective charge agree well with the corresponding parameters of the Li united atom, whereas the dynamical characteristics ReR(E) and ImR(E) reveal considerable influence of electron correlations. The possibility for the development of an alternative method of quantitative spectroscopy of quasimolecules based on the analysis of experimental data on differential cross sections for direct ionization is discussed. © 1996 The American Physical Society.

For the case of inelastic electron-atom scattering, it has been known for some time that significant spin effects may be observed even if spin-dependent forces on the projectile can be ignored. These spin effects result from the Pauli principle and this phenomenon has become known as the fine-structure effect. Recently, the question of whether or not these same types of effects should be observed for atomic ionization has been considered and the initial indications are that significant spin asymmetries can also be expected for atomic ionization if the final ion satisfies LS coupling and the final J state of the ion can be resolved. In this paper, we consider this problem for electron-impact ionization of inert gases. The theory of the fine-structure effect is presented for ionization and first-order distorted-wave results are compared with very recent experimental data. © 1996 The American Physical Society.

The collision system of 19-MeV ${\mathrm{F}}^{9+}$ on Ne has been studied using recoil and projectile momentum spectroscopy. For each event, identified by final recoil and projectile charge state, the three-dimensional momentum vector of the recoil ion and the transverse momentum vector of the projectile ion were measured. The transverse momenta of the recoil and projectile ions were found to be equal in magnitude and opposite in direction, indicating that the transverse momentum exchange is dominated by interactions between the two ion cores. The transverse momentum distributions are well described by nCTMC calculations. The longitudinal momentum distributions of the recoil ions show that a large fraction of the momentum transferred to the projectile is carried off by continuum electrons. The recoil ions are scattered slightly backward, in partial agreement with predictions of nCTMC calculations. © 1996 The American Physical Society.

It is shown that a method for calculating unified total electron-ion recombination rates, incorporating both the radiative and dielectronic recombination processes, can be applied to substantially complex atomic systems where a large number of recombination channels need to be coupled in an ab initio manner. Calculations are carried out for the astrophysically important ion Fe III, and total and state-specific recombination rate coefficients are obtained for e+ Fe IV → Fe III, in the close coupling approximation employing the R-matrix method. The calculations correspond to a 49-state eigenfunction expansion for the Fe IV core that includes states from the ground configuration 3${\mathit{d}}^5$ and the excited configurations 3${\mathit{d}}^4$4s and 3${\mathit{d}}^4$4p for proper treatment of the electron correlation effects. This is an accurate and detailed atomic calculation for the recombination rates for Fe III. The present rates are up to an order of magnitude lower than the previous values obtained through the Burgess general formula at electron temperatures where dielectronic recombination dominates. © 1996 The American Physical Society.

We present semiempirical Gaunt factors for the widely used Van Regemorter formula [Astrophys. J. 136, 906 (1962)] for allowed transitions from <ital>l=0 or <ital>l=1 levels in atoms with the LS coupling. Cross sections calculated using these Gaunt factors agree with measured cross sections to within experimental error. © 1996 The American Physical Society.

To the best of our knowledge, no examination of the Berry connection in association with resonance scattering processes has appeared. In the present study we examine the contribution of the Berry connection to the transition amplitude of the scattering processes. It is demonstrated that the nonadiabatic contribution of the Berry connection can be significant in resonance scattering processes by affecting both resonance factor and intermediate transitions between the open and closed channels. The nonadiabaticity of the Berry connection is also important in direct scattering processes by influencing only intermediate transitions between open and closed channels. © 1996 The American Physical Society.

The single-rescattering contribution to the amplitude pertaining to three-body charge exchange reactions (triangle amplitude) contains the off-shell Coulomb T-matrix ${\mathit{T}}^{\mathit{C}}$ describing the intermediate-state Coulomb scattering of charged subsystems. For ease of computation, the latter is usually replaced by the potential ${\mathit{V}}^{\mathit{C}}$ which, however, is unsatisfactory in many cases. An alternative approximation, obtained by ‘‘renormalizing’’ the ‘‘triangle’’ contribution with ${\mathit{V}}^{\mathit{C}}$ instead of ${\mathit{T}}^{\mathit{C}}$ by a simple analytic expression, is shown to yield results in excellent agreement with the numerically calculated exact amplitude, for atomic elastic exchange reactions, over a wide range of (medium to high) projectile energies and scattering angles (including the forward direction). © 1996 The American Physical Society.

We calculate the electronic stopping power and the corresponding straggling for ions of arbitrary charge number, penetrating matter at any relativistic energy. The stopping powers are calculated by a simple method. Its starting point is the deviation of the precise theory from first-order quantum perturbation. We show that this deviation can be expressed in terms of the transport cross section, ${\mathrm{\sigma }}_{\mathrm{tr}}$, for scattering of a free electron by the ion. In the nonrelativistic case the deviation is precisely the Bloch correction to Bethe’s formula; we look into the nonrelativistic case in order to clarify both some features of our method and a seeming paradox in Rutherford scattering. The corresponding relativistic correction is obtained from ${\mathrm{\sigma }}_{\mathrm{tr}}$ for scattering of a Dirac electron in the ion potential. Here, the major practical advantage of the method shows up; we need not find the scattering distribution, but merely a single quantity, ${\mathrm{\sigma }}_{\mathrm{tr}}$, determined by differences of successive phase shifts. For a point nucleus our results improve and extend those of Ahlen. Our final results, however, are based on atomic nuclei with standard radii. Thereby, the stopping is changed substantially already for moderate values of γ=(1-${\mathit{v}}^2$/${\mathit{c}}^2$${)}^{\mathrm{-}1/2}$. An asymptotic saturation in stopping is obtained. Because of finite nuclear size, recoil corrections remain negligible at all energies. The average square fluctuation in energy loss is calculated as a simple fluctuation cross section for a free electron. The fluctuation in the relativistic case is generally larger than that of the perturbation formula, by a factor of ∼2–3 for heavy ions. But the finite nuclear radius leads to a strong reduction at high energies and the elimination of the factor ${\gamma }^2$ belonging to point nuclei. ©1996 The American Physical Society.

The energy and width of one-electron multicharged-ion states interacting with an Al surface are determined using the nonperturbative coupled angular mode method. The case of the ${\mathrm{He}}^{+}$, ${\mathrm{Li}}^{2+}$, and ${\mathrm{O}}^{7+}$ states is studied. The various states within the n manifolds mix to form hybrids with very different energies and widths. The hybridization within the manifolds can be analyzed in terms of Stark mixing. © 1996 The American Physical Society.

We consider the behavior of an atomic Bose-Einstein condensate in the presence of an atom in an (internal) excited electronic level. We analyze the boson-accumulation regime, defined by the relation ${\mathit{N}}_0$≫a,N-${\mathit{N}}_0$, where N is the total number of atoms in the ground electronic state, ${\mathit{N}}_0$ is the number of atoms in the condensate, and a is the number of levels to which the excited atom can decay. In this regime, quantum statistical effects related to the boson nature of the atoms are predominant in the process of spontaneous emission. Simple arguments suggest that the proportion of atoms in the condensate decreases after the spontaneous decay. A more elaborated model based on a master equation description shows that, in general, these arguments may lead to erroneous conclusions. Under certain conditions it predicts that the proportion of atoms in the condensate increases. We give an interpretation of this phenomenon in terms of quantum interferences between processes that include reabsorption of the emitted photons, and dipole-dipole interactions between the atoms. The physical effect considered here may help to pump atoms into a condensate and to compensate for atomic losses in the atomic trap. © 1996 The American Physical Society.

We solve the Gross-Pitaevskii equations for a dilute atomic gas in a magnetic trap, modeled by an anisotropic harmonic potential. We evaluate the wave function and the energy of the Bose-Einstein condensate as a function of the particle number, both for positive and negative scattering length. The results for the transverse and the vertical size of the cloud of atoms, as well as for the kinetic and potential energy per particle, are compared with the predictions of approximated models. We also compare the aspect ratio of the velocity distribution with experimental estimates available for ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$. Vortex states are considered and the critical angular velocity for production of vortices is calculated. We show that the presence of vortices significantly increases the stability of the condensate in the case of attractive interactions. © 1996 The American Physical Society.

A geometric phase factor is derived for a split-beam experiment as an example of cyclic evolutions. Poincaré sphere descriptions for the split-beam experiment show its geometric property. We observe this geometric phase with a two-loop neutron interferometer, where a reference beam can be added to the beam from one interference loop. The combination of phase shifters and partial absorbers permitted the compensation of the dynamical phase and measurement of the geometric phase. All the experimental results show complete agreement with our theoretical treatment. We discuss a situation where a geometric phase exists even when the dynamical phase becomes zero. © 1996 The American Physical Society.

The many-body decomposition of the interaction energy for ${\mathrm{Be}}_{\mathit{N}}$ and ${\mathrm{Li}}_{\mathit{N}}$ (N=2 to 4) clusters is calculated in two approximations: the self-consistent-field method and the Mo/ller-Plesset perturbation theory up to the fourth order. This allows us to estimate the electron-correlation contributions to the many-body forces. The explicit expressions for these contributions in the perturbation theory formalism are obtained. We present a comparative analysis of the role of electron correlations in the ${\mathrm{Be}}_{\mathit{N}}$ and ${\mathrm{Li}}_{\mathit{N}}$ cluster formations and in the many-body interactions in these clusters. As follows from our results, the contribution of electron correlation to many-body interactions is essential for both the ${\mathrm{Be}}_{\mathit{N}}$ and ${\mathrm{Li}}_{\mathit{N}}$ clusters, especially for the latter ones, where nonadditivities are surprisingly large. © 1996 The American Physical Society.

In this paper we consider the adiabatic transfer in three-level systems using the counterintuitive pulse sequence, the stimulated Raman process (STIRAP). We consider explicitly the nonadiabatic corrections to the ideal case. The problem simplifies when we use certain pulse types extending to infinity, ramp pulses. In this case we find models which are analytically solvable. In the smooth pulse case, we find simple exponential corrections to adiabatic behavior. For a resonant intermediate level, the problem is equivalent to a two-level problem, where adiabatic corrections have been investigated extensively. We utilize these results to interpret the behavior of the three-level system exposed to the counterintuitive pulse case. When the pulse separation is in a certain range, exponential adiabatic behavior is found, and the numerical data can be explained by the analytic theory taken from the two-level work. We cannot explain all results analytically, but the computations suggest that this description of the three-level system gives a good understanding of the STIRAP. © 1996 The American Physical Society.

The high-frequency Floquet theory (HFFT) of laser-atom interactions solves the space-translated version of the Schrödinger equation by an iterative procedure, leading to corrections of increasing order in the inverse photon energy. The lowest-order approximation (high-frequency limit) has been often evaluated before, but its accuracy at finite frequencies has not been established. To explore this issue we have computed the corrections yielded by the first-order iteration to the energy levels, and the ionization rates of a one-dimensional atomic model with a Gauss attractive potential. We have then compared, at frequencies above the field-free ionization potential ‖${\mathit{W}}_0$‖, the HFFT results with those of a full Floquet calculation. We show that the agreement is substantially improved by the inclusion of the HFFT corrective terms. The agreement is good at all intensities for photon energies larger than several times ‖${\mathit{W}}_0$‖. Even when the photon energy is marginally larger than ‖${\mathit{W}}_0$‖ the discrepancies vanish at sufficiently high intensities. © 1996 The American Physical Society.

We study the dynamics of laser-cooled two-level atoms in a time-dependent standing wave. In the absence of spontaneous emission, our model reduces to the much studied quantum standard mapping. For a sample of atoms initially cooled to the Doppler limit, the momentum variance increases linearly with interaction time. A sample of atoms initially cooled to the recoil limit will display momentum diffusion at a rate that is determined by two numbers: the classical chaos parameter and a dimensionless Planck parameter. This diffusion ceases after the quantum-mechanical break time. Spontaneous emission will modify the diffusion rate of both Doppler- and recoil-cooled atoms. We show that one component of the new diffusion constant is due to spontaneous recoil and that there is a correction to the deterministic diffusion rate due to the destruction of correlations in the stroboscopic position. Spontaneous emission will destroy the dynamic localization of atoms cooled to the recoil limit. The new momentum diffusion constant can be related to the quasistationary state-decoherence rate. © 1996 The American Physical Society.

We present a theoretical investigation of the temperature and of the magnetization of atoms in one-dimensional ‘‘gray’’ optical lattices. Such lattices consist of counterpropagating beams having crossed linear polarizations, tuned on the blue side of a J→J or a J→J-1 transition, and a static longitudinal magnetic field. The variation of the atomic temperature with the magnetic field recently observed (an increase at low magnetic field; a decrease towards an asymptotic value at high magnetic field) is found for any atomic transition admitting one or two uncoupled states. This change of behavior of the temperature results from a qualitative change of the cooling mechanism and is correlated with a transition from paramagnetism to antiparamagnetism in the optical lattice. © 1996 The American Physical Society.