We present a theoretical study of laser-induced autoionization and laser-induced continuum structures, performed with the aim of deriving predictions on the effect of continuum-continuum transitions on the resonance line shape. The model, developed as an extension of models introduced for the description of laser-induced autoionization and laser-induced continuum structures, on one hand, and for above-threshold ionization, on the other hand, provides quantitative predictions. For specific atomic systems, the line shapes of the structures are shown not to be affected by continuum-continuum transitions, in contrast with previously reported results.

Measurements of absolute total cross sections for electron capture by ${\mathrm{C}}^{3+}$ in collisions with ground-state hydrogen and deuterium are reported in the energy range 0.3–3000 eV/u. In general, good agreement is obtained with published experimental measurements at the lower (10–110 eV/u) and higher (>1 keV/u) collision energies. However, the improved accuracy and the large energy range of these measurements made possible by the merged-beams technique indicate an energy dependence different than was suggested by interpolating previous published and unpublished measurements and by a theoretical calculation that attempted to reconcile the previous low- and high-energy total-capture cross-section data. The present measurements above 100 eV/u show excellent agreement with a more recent 22-state molecular-orbital calculation that predicts slight structure in the cross section at collision energies between 1000 and 2000 eV/u. Below 100 eV/u, the present measurements, which were performed with deuterium, deviate from an energy dependence suggested by earlier hydrogen measurements and by fully quantal calculations.

A theoretical study of various inelastic processes resulting from collisions of ${\mathrm{O}}^{5+}$ ions with He atoms is carried out by a semiclassical molecular-orbital expansion method with the inclusion of electron translation factors at collision energies of 60 eV/u to 15 keV/u. In addition to the single-electron-capture process, projectile excitation and transfer excitation and ionization processes are also studied, and the corresponding cross sections are determined. Dominant processes are found to be a single-electron capture to the ${\mathrm{O}}^{4+}$(2s3s) and ${\mathrm{O}}^{4+}$(2s3p) states. Other channels are found to make weak but non-negligible contributions. All processes proceed on the outgoing part of the collision, after the transitions from the initial channel to the single-electron-capture channels are completed. Hence, direct processes to projectile excitation and transfer excitation are nearly negligible. The agreement of the present calculation for single-electron capture with measurements is very good.

s-wave scattering of positrons on atomic hydrogen is treated by solving the Faddeev equations. Elastic phase shifts below the Ps(n-1) excitation threshold and the K-matrix elements within the ore gap between the Ps(n=1) and H(n=2) thresholds are calculated as well as the zero-energy 2γ annihilation rate of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pair.

In this paper we concentrate on the most prominent points describing string curves: the detachment point and the localization threshold. We show that the detachment point is identical to the potential height for symmetric potentials and to the lowest barrier height in the general asymmetric case. Furthermore, we show that the existence of a detachment point is due to spectral concentration at the barrier top. Concerning the localization threshold, we show that the localization properties alter drastically at this point; the classical mechanism of a particle being trapped between potential barriers breaks down at this point. We then derive a condition for the existence of the localization threshold in the framework of the Wenzel-Kramers-Brillouin method and consider the dependence of the localization threshold on physical potential parameters; it is shown that there is no simple correspondence of the localization threshold to such parameters, since the localization threshold also depends on the explicit analytical form of the potential.

For a two-electron transition it was shown recently that the product amplitude of the independent-electron model can be derived as an approximation to the corresponding time-dependent second-order amplitudes. In this derivation the time-ordering effects were lost by use of frozen energies and wave functions for the intermediate states of second-order amplitude. Here we derive contributions to the product amplitude using relaxed intermediate state energies. We find that if inner shells are involved, these relaxation terms are small except for the lightest atomic targets.

The formation of ${\mathrm{O}}_2$ by radiative association and by inverse predissociation of ground-state oxygen atoms is studied using quantum-mechanical methods. Cross sections, emission spectra, and rate coefficients are presented and compared with prior experimental and theoretical results. At temperatures below 1000 K, radiative association occurs by approach along the 1 ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathit{u}}^{}{}_{}{}^{}$ state of ${\mathrm{O}}_2$ and above 1000 K inverse predissociation through the B ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{\mathrm{-}}{}_{}{}^{}$ state is the dominant mechanism. This conclusion is supported by a quantitative comparison between the calculations and data obtained from hot oxygen plasma spectroscopy.

A detailed discussion is given of the general behavior of differential cross sections for radiative electron capture in relativistic and nonrelativistic heavy-ion–atom collisions. From angular-momentum conservation and a simple approximate treatment, the basic qualitative features can be derived in an analytic form. Subsequently, the results of rigorous relativistic calculations are presented graphically for a sequence of projectile charges and projectile energies. It is shown that in radiative electron capture, the influence of the electron spin manifests itself in a particularly clear-cut way.

The scattered-energy spectra of monochronized synchrotron-radiation photons toward 90° by C, Al, Fe, Cu, Au, and Pb samples were measured using high-purity Ge detectors to obtain incoherent scattering function (S) in the 1.14<x<2.28-A${\mathrm{̊}}^{\mathrm{-}1}$ region. The multiple-scattering intensity was evaluated by a Monte Carlo calculation while considering the Doppler broadening of the Compton-scattered photons and the photon linear polarization. The measured S of C, Al, Fe, and Cu agreed with that based on the Waller-Hartree theory (${\mathit{S}}^{\mathit{W}\mathit{H}}$), as well as that based on a relativistic impulse approximation (${\mathit{S}}^{\mathit{I}\mathit{A}}$) within the experiment error (2.5%). The measured S of high-Z atoms (Au and Pb) agrees with ${\mathit{S}}^{\mathit{I}\mathit{A}}$ and is smaller than ${\mathit{S}}^{\mathit{W}\mathit{H}}$ by 3–6 %.

Double ionization of the ground state of helium by Compton scattering is investigated using many-body perturbation theory. Representative calculations of cross sections differential in the scattered photon energy and angle are given to illustrate the nature of the process. Total cross sections for single ionization and double ionization of helium and the ratio of these cross sections are presented for energies from 2.5 to 20 keV. Current experimental data for the ratio due to both Compton scattering and photoionization are in reasonable agreement with the corresponding calculations presented here. However, comparisons of the present results with other calculations reveal substantial discrepancies. Possible reasons for these differences at finite energies are proposed. Our results for double ionization by Compton scattering have not reached an asymptotic limit.

The autoionizing states of ytterbium (Yb) were investigated by three-photon polarization spectroscopy. Ytterbium atoms were excited to autoionizing states through the transition 6${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–6s6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$–4${\mathit{f}}^{13}$${(}^2$${\mathit{F}}_{7/2}$)6${\mathit{s}}^2$6${\mathit{p}}_{3/2}$ (J=2). The total angular momenta J of the autoionizing states were identified by comparing the line intensities from the spectra recorded with different combinations of the laser polarization. In this work, 17 autoionizing states including many new ones were investigated. For all these states Fano parameters and J values are measured.

A quasiclassical formula for inelastic energy losses in collisions of slow ions and atoms has been derived. The electronic stopping power is assumed to be due to a momentum transfer during scattering of the target atom’s electrons in the screened Coulomb field of the projectile and vice versa. The Coulomb repulsion between the nuclei of colliding particles is shown to decrease greatly the electronic stopping in the limiting case of small, reduced energies, E≪1. This leads to inelastic energy losses being no longer linearly dependent on the particle velocity, as predicted by the linear-response theory. The latter effect explains the experimentally observed strong dependence of the kinetic electron emission well below the threshold on the energy of ions bombarding a solid surface. At relatively large energies (E>1) the expression obtained reduces to the result found earlier by Lindhard and Scharff [Phys. Rev. 124, 128 (1961)]. The total energy losses of slow atoms calculated on the basis of the formula derived for the inelastic stopping power and the expression for the elastic energy losses available in the literature are in good agreement with the experimental data.

We have measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d≊4–700 μg/${\mathrm{cm}}^2$) bombarded with swift (13.6 MeV/u) highly charged (q=16–18) argon ions. The dependence of the electron yields on target thickness and charge state of the ions is analyzed within the framework of an extended semiempirical model. Due to the high velocity of the ions, it is possible to distinguish electron production in primary ionization (related to the stopping power and the effective charge of the ions) from secondary electron production due to the transport of so-called δ electrons (cascade multiplication). By combining the experimental results with numerical simulations of electron transport in matter by a Monte Carlo method, we have obtained electron transport lengths of high energy (E≫100 eV) δ electrons parallel and perpendicular to the ion trajectory, as well as diffusion lengths of slow electrons (E≪100 eV). In order to study the velocity dependence of these transport lengths, we have not only investigated 13.6 MeV/u Ar ions, but also 1 MeV/u C and 3.9 MeV/u S, for which experimental results are available [Koschar et al., Phys. Rev. A 40, 3632 (1989)]. We discuss the origin of electron yield reductions (compared to a simple scaling with the square of the nuclear charge) with heavy ions and present measurements of double differential energy and angular electron distributions of 13.6 MeV/u ${\mathrm{Ar}}^{17+}$ ions.

We show theoretically that an atomic pattern with period d can be obtained with 100% visibility even for an infinitely extended source by sending atoms through two transmission gratings with periods d and d/2, respectively, and separated by half the Talbot length ${\mathit{L}}_{\mathit{T}}$/2=${\mathit{d}}^2$/2${\lambda }_{\mathrm{dB}}$, where ${\lambda }_{\mathrm{dB}}$ is the atomic wavelength and the source is infinitely far away. For a finite source distance, as would be attainable in any real experiment, a small correction to the grating periods and separations restores the period-d pattern. This effect is closely related to the Talbot and Lau effects in classical optics and can be used to localize atoms to a submicrometer scale without a compromise in atomic flux. We first derive compact analytical formulas for the idealized case of a monochromatic source and large gratings and then verify numerically that a finite grating size and velocity dispersion in the beam do not decrease the fringe visibility considerably. Finally, we briefly present an experiment in preparation to exhibit this localization.

Dissociation induced by an external strong electromagnetic field has been considered for linear triatomic molecules such as HCN utilizing a model Hamiltonian. The classical dynamics of this model system is shown to include chaotic trajectories. Dissociation probabilities have been calculated as a function of the intensity and the frequency of the external electromagnetic field. The effect of various pulse shapes on the threshold intensity for dissociation has been studied. Sensitivity of the dissociation probabilities with the variation of the isotopic masses has also been demonstrated.

The photodetachment of the negative ion ${\mathrm{H}}^{\mathrm{-}}$ by an uv photon in the presence of a low-frequency (LF) electric field and a static magnetic field is investigated. The case is treated in which the ejected electron may exchange a large number of LF photons and the LF field period is much larger than the time required for the electron ejection. The LF field polarization is taken along the magnetic field direction, with the uv photon polarization either along or perpendicular to B. The intensities of the two external fields are taken to be sufficiently weak to neglect the Stark and diamagnetic shifts of the initial state. The reported results support the physical picture according to which (a) the main role of the LF field is that of creating a repulsive barrier in the direction of the ejected electron free motion and (b) the oscillations observed in the experiment [M. C. Baruch, T. F. Gallagher, and D. J. Larson, Phys. Rev. Lett. 65, 1336 (1990)] are due to repeated reflections by such a barrier. The presence of a magnetic field enhances the interference effects and photodetachment experiments carried out with the additional presence of a magnetic field could provide more information on the precise role the LF field plays in the process.

We have computed the above-threshold ionization and the emitted harmonic spectra of H interacting with short laser pulses, with photon energies ranging from 1.16 to 5.44 eV and with peak intensities ranging from 6×${10}^{13}$ to 7×${10}^{14}$ W/${\mathrm{cm}}^2$, by solving the time-dependent Schrödinger equation (TDSE). The method of solution involves the expansion of the time-dependent wave function Ψ(r→,t) over the exact wave functions of the discrete and the continuous spectrum, computed numerically, and the subsequent integration of the resulting coupled first-order differential equations by a Taylor series expansion technique. This state-specific approach (SSA) to the solution of the TDSE allows systematic understanding of convergence as a function of the number and type of the field-free states for each value of the laser frequency (ω) and peak intensity (${\mathit{I}}_0$). For example, the method allows practical numerical study of the degree of participation of high (n,l) (l=0,1,...,n-1) Rydberg, as well as of high-energy scattering states for each partial wave. For the harmonic spectra, comparisons are made between the results obtained by the SSA and those obtained in recent years by a number of researchers from the application of finite-difference grid methods. As regards economy, a general observation is that in the SSA the necessary number of partial waves is smaller than that required in the grid methods. Predictions are made for the case of ħω=2 eV, ${\mathit{I}}_0$=2×${10}^{14}$ W/${\mathrm{cm}}^2$, in the context of a study of the effect of the pulse shape on the harmonic-generation spectrum. It is shown that the number of harmonics and the appearance of the plateau depend on the duration of the peak intensity.

We describe a variant of the quadrupole rf (Paul) ion trap capable of localization of a trapped ion to much less than an optical wavelength (Lamb-Dicke regime). The trapping potentials are generated by a high-Q, vacuum-compatible, quarter-wave resonator driven at about 240 MHz. The binding strength of the trap has been characterized. The trap contains compensation electrodes which allow the cancellation of stray static electric fields within the trap. Secular frequencies of tens of megahertz have been achieved for trapped magnesium and beryllium ions.

We develop a method for calculating the (quasi)energy eigenvalue E(F) of a hydrogen atom in a nonperturbative ac field of strength F starting from a knowledge of the coefficients ${\mathit{E}}^{\left(2\mathrm{}\mathit{m}\right)}$ of the Rayleigh-Schrödinger perturbation series E(F)=${\mathit{tsum}}_{\mathit{m}=0\mathrm{}}^{\mathit{M}}$${\mathit{E}}^{\left(2\mathrm{}\mathit{m}\right)}$${\mathit{F}}^{2\mathit{m}}$. We first use the coefficients ${\mathit{E}}^{\left(2\mathrm{}\mathit{m}\right)}$ (the unperturbed energy is ${\mathit{E}}^{\mathrm{}\left(0\right)\mathrm{}}$) to construct the inverse series ${\mathit{F}}^2$(E)=${\mathit{tsum}}_{\mathit{m}=1\mathrm{}}^{\mathit{M}}$${\mathit{F}}^{\left(\mathit{m}\right)}$(E-${\mathit{E}}^{\mathrm{}\left(0\right)\mathrm{}}$${)}^{\mathit{m}}$. We resum the latter series using the Padé method, and solve the implicit equation ${\mathit{F}}^2$(E)=F${\mathrm{¯}}^2$ for E(F¯). The reconstructed function E(F) has the singularity structure appropriate to the true E(F). We are able to obtain good results for the lifetime of a hydrogen atom in a high-frequency field up to very high intensities, well into the (highly nonperturbative) stabilization regime.

We investigate the modifications of the Pendello$iuml—sung of near-resonant Kapitza-Dirac diffraction due to many-body effects. For the case of ‘‘two-beam’’ doppleron (velocity-tuned) resonances, we find that in the absence of dissipation, the population oscillations between diffraction orders, which are complete in the linear case, can either be decreased in frequency, or become partial, or even be completely suppressed, in which case the system evolves to a quasistationary state where the atoms are in a coherent superposition of their upper and lower excited states. The influence of spontaneous emission on these predictions is then studied by numerically solving the effective single-atom Hartree-Fock master equation of nonlinear atom optics. We find that spontaneous emission is not detrimental in weakly allowed transitions, in which case the predicted effects survive for several spontaneous lifetimes.