We show that a theory that neglects dynamic correlation in ion-atom collisions (the frozen correlation approximation, FCA) is able to provide accurate multiple-excitation cross sections from one-electron amplitudes. Important requirements for the success of the FCA are (i) inclusion of correlation in both the initial and final states, and (ii) inclusion of screening effects during the collision. We find an excellent agreement between fully correlated and FCA results for double excitation of He by bare ions, thus showing that the FCA can be efficiently used in the evaluation of multiple-excitation cross sections when fully correlated calculations are not accessible.

The ratios of the cross sections for the processes of transfer ionization (TI) and single capture (SC) were measured for 2 MeV/u ${\mathrm{Cl}}^{7+,9+,13+,14+,15+}$ and ${\mathrm{Ti}}^{15+,18+}$ projectile ions on He targets. The ratio was determined using a cold He gas target and measuring the coincidences between the projectile and the charge states of the emerging recoil ions. The measured ${\mathrm{\sigma }}_{\mathrm{TI}}$/${\mathrm{\sigma }}_{\mathrm{SC}}$ ratio shows a strong dependence with the projectile charge state that is well described by calculations based on the independent electron model. A model to take into account the effect of the electron screening of partially dressed projectiles in the target ionization is also presented.

The unitarity conditions upon the scattering amplitudes for the elastic scattering of spin- particles from spin-0 targets at energies below the first inelastic threshold transcribe to a set of coupled nonlinear integral equations for the phase functions of two helicity amplitudes and thence, by simple linkage, to the non-spin-flip and spin-flip scattering amplitudes. From the latter set, by Legendre integrations, one obtains the scattering phase shifts, ${\mathrm{\delta }}_{(\mathrm{l},\mathrm{}\mathrm{j}\mathrm{=}\mathrm{l}\pm 1\mathrm{/}2)}$. Input to the study are the differential cross section and the polarization, {(dσ/dΩ)(θ),P(θ)}. An iterative method of solution based upon Frechét derivatives and with generalized cross validation (GCV) smoothing of the variations between iterates can give convergent, stable, and accurate results. Two test cases, the first built upon a model set of (small) phase-shift values and the second for an optical model calculation of 1-MeV neutrons scattered from an α particle, have been used to demonstrate convergence and accuracy. There are natural ambiguities (fourfold, in fact) for the phase functions of the scattering amplitudes since data are invariant to complex conjugation of, or the Minami transform on, the phase shifts of the mirror data set {(dσ/dΩ)(θ),-P(θ)}, as well as to the combined action of complex conjugation and Minami transformation of the phase shifts given by the initial solution. Those ambiguities are presented herein and are shown not to pose numerical problems in solution, provided the initial guesses are not near to the symmetry ``lines'' of the four solutions, and the GCV process is used to prevent branch flips occurring at scattering angles where the allowed solutions intersect.

An integral representation of wave functions for the Temkin-Poet model of electron impact on atomic hydrogen is given. Approximate wave functions are evaluated analytically for large hyperradius to extract the ionization S-matrix element. An ionization cross section of the form exp[-${\mathrm{aE}}^{\mathrm{-}1\mathrm{/}6}$+${\mathrm{bE}}^{1\mathrm{/}6}$], where a and b are positive constants, is derived. The exponential suppression of ionization for small E appears to be the quantum counterpart of the delayed onset of ionization in the classical theory for this model.

Relative photodetachment cross sections for rotational thresholds of the hydroxide anion were measured using a coaxial laser-ion beam spectrometer. The thresholds correspond to transitions from the J=1 and J=2 states of X${\mathrm{}}^1$${\mathrm{\Sigma }}^{+}$ ${\mathrm{OH}}^{\mathrm{-}}$ to the J=3/2 state of X${\mathrm{}}^2$${\mathrm{\Pi }}_{3\mathrm{/}2}$ OH. Best fits of the data give cross sections that scale with energy above threshold as (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.18\pm 0.06}$ and (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{0.21\pm 0.05}$ for the two thresholds, respectively. This non-Wigner law energy dependence of the cross section results from the long-range ${\mathrm{r}}^{\mathrm{-}2}$ interaction between the dipole and the departing electron. Experimental results were compared with the predictions of a strong-coupling model of charge-dipole interaction devised by Engelking and good qualitative agreement was observed. When the resolution and range of the data for transitions that terminate in the lower Λ-doublet component is such that the Λ-doublet splitting is resolved, one observes the (E-${\mathrm{E}}_{\mathrm{th}}$${)}^{1\mathrm{/}2}$ behavior predicted by the Wigner law. This result is explained in terms of the effect of the Λ doubling upon the ${\mathrm{r}}^{\mathrm{-}2}$ charge-dipole interaction. From the threshold frequencies, an OH electron affinity of 14 741.02(3) ${\mathrm{cm}}^{\mathrm{-}1}$ was determined.

Strong resonance features were observed in the near-threshold excitation of ${\mathrm{Ar}}^{6+}$. Absolute total cross sections for electron-impact excitation of the ${3\mathrm{s}}^2$${\mathrm{}}^1$S→3s3p${\mathrm{}}^3$${\mathrm{P}}^{\mathrm{o}}$ and ${3\mathrm{s}}^2$${\mathrm{}}^1$S→3s3p${\mathrm{}}^1$${\mathrm{P}}^{\mathrm{o}}$ transitions in ${\mathrm{Ar}}^{6+}$ were measured by using the merged electron-ion beams energy loss technique. The results are compared with the R-matrix close-coupling theory (CCR) and the independent-process isolated-resonance distorted-wave approximation. Observed disagreement between CCR theory and experiment at the near-threshold peak for the ${3\mathrm{s}}^2$${\mathrm{}}^1$S→3s3p${\mathrm{}}^3$${\mathrm{P}}^{\mathrm{o}}$ transition is interpreted to be due to very sensitive resonance interference.

Energy- and angle-resolved resonant Auger spectra were recorded on the first 12 Mg 2p→nl resonances. Utilization of a high-resolution monochromator has allowed us to selectively excite resonant states which could not be resolved in earlier electron spectrometry measurements. The branching ratios and values of the angular anisotropy parameter Β were extracted for all resonances. The measurements provide a first basis for a systematic test of the spectator model for the angular anisotropy. Our experimental values are compared with ab initio multiconfiguration Dirac-Fock calculations. The sensitivity of the calculations to the configuration basis size is discussed in detail.

We study the nonadditive part of the long-range interaction among three alkali-metal atoms in their ground states. Using nondegenerate perturbation theory, up to the third order, we have computed the dispersion coefficients C for three alkali-metal atoms interacting via their electric dipole moments. Both heteronuclear and homonuclear cases are considered. The numerical values for the C coefficients suggest that such three-body dipole interaction effects may not be neglected in the description of the long-range surface potential interaction for alkali-metal atoms. Furthermore, we show that approximate formulas for C of Midzuno and Kihara [J. Phys. Soc. Jpn. 11, 1045 (1956)] give results in excellent agreement with results of our calculations. Comparisons are given of our results with results of others and with results of other approximate formulas. The effect on our results for the C coefficient of uncertainties in the experimental values of the static dipole polarizabilities, on which our results depend, is also analyzed.

The time evolution of deexcitation of ${\mathrm{O}}^{7+}$ ions penetrating the (111) surface of Cu is studied by means of secondary electron spectroscopy. It is found that the filling of the K shell proceeds faster at 102 keV than at 51-keV ion energy, by about 10%. This velocity dependence is determined from a comparison of emission depth profiles, assuming straight-line trajectories for the ions. No further model is needed for the ion deexcitation or the solid-state interactions of the emerging Auger electrons. The model independence is based on three steps: (i) the emission depth profiles for different ion energies and angles are evaluated only in conditions where they are practically equal; (ii) the full K Auger electron spectra with their inelastic parts are obtained using matching pairs of measurements with ${\mathrm{O}}^{6+}$ projectiles; and (iii) the observation angles are adjusted to equalize, for both ion energies, the laboratory energies of the Auger electrons at emission as well as their solid-state interactions on their way to the surface. The present method can provide benchmark values for multistep cascade models of highly charged ion deexcitation in solids.

Electronic stopping powers for fixed-charge-state He ions, ${\mathrm{He}}^{2+}$ and ${\mathrm{He}}^{+}$ , in Al target are calculated within the framework of linear-response dielectric theory by using the dynamic local-field correction in the dielectric function and the concept of local plasma density approximation. An average stopping power has been defined by weighting the fixed-charge-state results with theoretical occupation probabilities. Satisfactory agreement between this average stopping power and experimental data, measured for He-ion beam, is found.

Within the framework of quantum scattering theory, we present a consistent calculation of the stopping power for slow protons and antiprotons moving in two-dimensional electron gases. The Friedel sum rule is used to determine the screening constant in the scattering potential. For the stopping power our results are compared with that of the random-phase approximation dielectric theory and that predicted by the linear Thomas-Fermi potential.

The Coulomb explosion of 173-MeV ${\mathrm{HeH}}^{+}$ molecular ions traversing thin carbon foils has been measured for foil thicknesses ranging from 2 to 200 μg/${\mathrm{cm}}^2$. In contrast with measurements at lower energies, the energy spectra for protons observed emerging in the incident beam direction show distinct components that correspond to the partner helium ions being in charge states 0, 1, and 2. From an analysis of the variation of the yields of these components as functions of the target thickness, we extract electron-loss cross sections that are in good agreement with theoretical estimates. ``Wake effects'' that increase with increasing target thickness are observed as asymmetries in the yields and energy shifts for ``backward-going'' as compared to ``forward-going'' protons.

We present a theoretical analysis of the above-surface neutralization of highly charged ions in front of LiF. The study is based on the assumption that the dominant electron transfer occurs in the classically allowed region. Estimates of critical distances and corresponding quantum numbers for capture from an ionic crystal within the classical-overbarrier (COB) model are presented, which differ considerably from corresponding results for metals. The role of the dielectric response of LiF is investigated. In addition, classical-trajectory simulations are performed for a slow highly charged ion approaching an insulator. It is shown that capture effectively begins ∼ 3 a.u. closer to the surface than estimated from the COB model. This correction can be incorporated into a modified COB model. The energy gain for grazing incidence ions is obtained using a staircase model which includes the deceleration due to charge-up of the surface.

We study theoretically the macroscopic interference of two independent Bose condensates released from a double potential trap. The observation of fringes could serve as a test for the paradigm of broken gauge symmetry. By numerical solution of the nonlinear Schrödinger equation in three dimensions, the consecutive stages of expansion, overlap, and interference are investigated in order to facilitate the design of future experiments. The phase-space dynamics of the condensates is analyzed by means of the Wigner function. It turns out that the distance of interference fringes grows linearly in time with a velocity inversely proportional to the initial distance of the two condensates. The collisional reduction of the fringe visibility is estimated.

It is shown that the correlation between momenta of two atoms cooperatively deflected from an off-resonant quantized standing-wave light field arises due to the fluctuations of the field in the photon number. The normalized cross-correlation functions are calculated. The Cauchy-Schwartz-type inequalities for the joint distribution of the atomic transverse momenta are derived. The simultaneous deflection of the atoms allows one to determine the mean photon number and the width of the photon distribution and to produce predetermined correlations between atomic de Broglie waves.

We consider the leading corrections to the Thomas-Fermi approach to the description of the properties of a magnetically trapped, Bose-condensed cloud of atoms. Simple analytical expressions are derived for the kinetic energy in terms of an effective cutoff length, which we calculate numerically by considering the one-dimensional problem for a linear-ramp potential. We also determine the lowest angular velocity at which it is energetically favorable for a vortex to enter the cloud. For large clouds, our results are in excellent agreement with available numerical calculations.

We describe an approach of spectral type for numerically integrating the time-dependent Schrödinger equation associated to the interaction of a one active electron atom with an electromagnetic pulsed field whose polarization may be arbitrary. The wave function is represented on a Coulomb-Sturmian basis. The time propagation method is based on a parallel-iterated Runge-Kutta method of predictor-corrector type. This method is in fact fully implicit and of very high order, ensuring a high stability of the time propagation. Moreover, it has the following advantages: it provides a scheme for an adaptive time step and it is particularly well suited to parallel computing. We discuss the performance of the present approach and compare it to already existing ones. In the case of linearly polarized fields, most of our results are in good agreement with those obtained with other approaches. In the case of circularly polarized fields, we compare our results with those obtained by, so far, the only existing method which is based on the single state Floquet approximation. Finally, and for the sake of illustration, we treat the case of the interaction of atomic hydrogen with a strong pulsed electromagnetic field whose polarization depends on time.

An analytical study is presented of two models of intense-laser ionization from an isolated initial state via a band of high-Rydberg states in a system which bears many features of the hydrogen atom. The models differ in the way the population from the directly excited Rydberg states can migrate to the states of higher angular momenta. In one of them it migrates via nonresonant and in the other via resonant degenerate Raman coupling. Within these models we draw conclusions about the efficiency of migration, the initial-state lifetime, the redistribution of population, the laser intensities stabilizing the atom against ionization, the effect of the initial state on the threshold stabilizing intensity, as well as about stabilizing mechanisms versus the initial-state choice.

We report here an effect in a four-level ladderlike system, which is in contrast to the usual quantum interference effects such as electromagnetically induced transperency (EIT) or coherent population trapping: we predict the occurrence of a narrow absorption peak within the EIT window when an EIT atomic system interacts with an additional driving rf field. The Doppler-free-central absorption appears when the three-photon resonance condition is satisfied. In the limit of the rf field strength ${\mathrm{\Omega }}_{\mathrm{rf}}$→0, the usual EIT profile is recovered.

Calculations based on coupled Mathieu-Coulomb equations have been performed to investigate the dynamical behavior of two particles in a Paul trap. It is found that the average transient lifetime increases exponentially with the increase of trap voltage or with the decrease of damping. The stationary chaos does not exist in this dissipative system. Transient chaos is a common phenomenon before trajectories are settled in frequency-locked orbits. We also study the basins of attraction and find riddled basins as well as Wada basins in the system.