The results of high-precision, variational, bound-state calculations for the ground state in the negatively charged hydrogenlike ions ${}^{\infty }{\mathrm{H}}^{\mathrm{-}}{,\mathrm{}\mathrm{T}}^{\mathrm{-}}{,\mathrm{}\mathrm{D}}^{\mathrm{-}},$ ${}^1{\mathrm{H}}^{-},$ and ${\mathrm{Mu}}^{-}$ are presented. The mass dependence for various properties is studied. The results are formulated in the form of relatively simple analytical expressions. The probabilities of finding the final He atom in its ground and low-lying excited states (after the nuclear ${\beta }^{-}$ decay in the $T^{-}$ ion) have been determined numerically. It is shown that the total ionization probability has a very large value (≈30%). A possible explanation may include the spin conversion between the ${\beta }^{-}$ particle and remaining ${}^3\mathrm{He}$ atom. This means that the final ${}^3\mathrm{He}$ atom can be found not only in its singlet states, but also in the triplet states.

Ab initio quantum scattering calculations are carried out for the ${\mathrm{O}}_3$ molecule, in its ground electronic state, from which low-energy electrons are scattered in the gas phase. The results of the computations are compared with the existing experimental angular distributions for elastic (rotationally summed) scattering processes and for rotationally inelastic cross sections. The agreement of the present calculations with the available experiments is found to be remarkably good and the corresponding efficiency of rotationally inelastic processes is also discussed and analyzed in terms of the role played by the small permanent dipole moment of this target molecule.

The saturation effect in the excitation cross sections has been investigated for ${\mathrm{Si}}^{12+}$ ions colliding with gas target atoms (atomic number $Z_t$ between 2 and 54) in the (8–12)-a.u. velocity range, both experimentally and theoretically. The experimental excitation cross sections have been deduced from the measured Ly x rays following the decay of the excited atoms. Theoretical calculations have been performed using the first Born approximation, the symmetric eikonal continuum distorted wave (SECDW) approximation, and the Schwinger variational principle (SVP). Similar to the earlier observations on the heavier ion species, the saturation has been found in both the experimental and the SVP cross sections in the entire velocity range. The SECDW approximation, however, has been found to underestimate the cross sections at large values of $Z_t$ in the presently investigated systems. In the light of these findings, the role of higher-order effects within different theoretical models has been reviewed and the possible interaction mechanisms have been discussed.

We perform a systematic study of photoionization with excitation and double photoionization of ${\mathrm{H}}^{-},$ He, and ${\mathrm{Li}}^{+}$ using the convergent close-coupling formalism. The present calculations cover the photon energy range from the double-ionization threshold to 10 keV where the results go over continuously to the nonrelativistic limit of infinite photon energy. By consideration of scaling properties, tested by application to ${\mathrm{O}}^{6+},$ accurate nonrelativistic results for photoionization with excitation to arbitrarily high n and double photoionization may be obtained for all heliumlike targets.

Absolute cross sections for electron-impact excitation of the $2s{}^2\overrightarrow{S}2p{}^{2}P$ transition of ${\mathrm{B}}^{2+}$ measured between 5.4 and 7.0 eV are presented. The results are in good agreement with the R-matrix-with-pseudostates (RMPS) calculation of Marchalant et al. [J. Phys. B 30, L435 (1997)]. Also presented are cross sections for electron-impact single ionization of ${\mathrm{B}}^{2+},$ including measurements between 25 and 200 eV and calculations using the RMPS and time-dependent close-coupling methods. The measured ionization cross sections are about 14% higher near the peak than previous measurements by Crandall et al. [Phys. Rev. A 34, 1757 (1986)], but agree well with experimental data of Hofmann et al. [Z. Phys. D 16, 113 (1990)] and with the present and other theoretical predictions.

We have measured the channel-specific cross sections for dielectronic recombination (DR) via the $\mathrm{KLn}\hspace{0.5em}(n=2,\dots ,5)$ resonance series of He-, Li-, and Be-like krypton ions. The measurements were made with an electron beam ion trap, and the DR cross sections were determined relative to the cross section for nonresonant radiative recombination. In the present electron-beam ion-trap experiment, up to five charge states (He- to C-like krypton) were detected in the trap. A fit procedure was used to compare the experimental data with theoretical calculations. The results agree well with the predictions. Additionally, the radiative relaxation mechanism following the stabilization transition in the dielectronic-recombination process was analyzed. The experimental approach used for measuring the DR excitation function opens up a spectroscopic method for the determination of the relative abundance of the highly charged ions.

The interaction of 800-MeV ${\mathrm{H}}^{\mathrm{-}}$ ions with a thin foil produces protons, the ground state and excited states of neutral ${\mathrm{H}}^0$ atoms, and unstripped ${\mathrm{H}}^{\mathrm{-}}$ ions. We investigated the distributions of individual ${\mathrm{H}}^0$ Stark states within the $n=3\mathrm{}$ and 4 levels produced by C and ${\mathrm{Al}}_2{\mathrm{O}}_3$ foil stripping of ${\mathrm{H}}^{\mathrm{-}}$ ions. Foils of various thicknesses were placed upstream of a magnet with a linearly increasing transverse field along the beam direction producing a motional electric field strong enough to ionize ${\mathrm{H}}^0$ states with $n>~3.$ We consider three questions: (i) What are the populations of individual ${\mathrm{H}}^0$ Stark states produced in the interaction of 800-MeV ${\mathrm{H}}^{\mathrm{-}}$ ions with thin C and ${\mathrm{Al}}_2{\mathrm{O}}_3$ foils, (ii) how do the relative population distributions change with foil thickness, and (iii) how is the population distribution produced in an ${\mathrm{Al}}_2{\mathrm{O}}_3$ foil modified when the foil is placed in a magnetic field? A simple qualitative model is presented to explain the major trends.

We have calculated the total dielectronic recombination (DR) coefficients for the ${}^2{P}_{1/2}$ and ${}^2{P}_{3/2}$ states in B-like ${\mathrm{Ti}}^{17+},$ ${\mathrm{Fe}}^{21+},$ and ${\mathrm{Mo}}^{37+}$ ions for electron temperatures $0.1<~T<~10000$ eV. The calculations are carried out using the multiconfiguration Dirac-Fock method in intermediate coupling with a configuration interaction. We find that accurate Coster-Kronig energies are critical for a successful determination of low-temperature DR coefficients. We also find that the DR involving fine-structure excitations can be as important as the $2s-2p$ excitation channels in the low-temperature regime for some ions.

Recently, we demonstrated that the enhancement of dielectronic recombination by an electric field, or crossed electric and magnetic fields, may be reduced by the interaction between resonances through a common continuum [Phys. Rev. Lett. 80, 1402 (1998)]. However, these calculations were carried out in a configuration-average (CA) approximation and the question then remained as to how these predicted reductions might be affected when one removes the degeneracy inherent in the CA approximation and includes the interactions between the resonant states associated with individual intermediate-coupled levels. In the present paper we address that issue for the case of dielectronic recombination in the presence of an electric field. Indeed we have found that these more detailed calculations support the general conclusions of our earlier work and provide additional insight into the effects of interacting resonances on dielectronic recombination in the presence of fields. Results for the Li-like ions ${\mathrm{C}}^{3+},$ ${\mathrm{Ne}}^{7+},$ and ${\mathrm{Si}}^{11+}$ are presented.

We consider the production of ultrarelativistic positronium (Ps) in $\gamma \overrightarrow{A}\mathrm{Ps}+A$ and $e\overrightarrow{A}\mathrm{Ps}+eA$ processes, where A is an atom or a nucleus with charge $\mathrm{Ze}.\mathrm{}$ For the photoproduction of para-and ortho-Ps and the electroproduction of para-Ps we obtain the most complete description compared to previous works. It includes high-order $Z\alpha$ corrections and polarization effects. The accuracy of the obtained cross sections is determined by omitted terms of the order of the inverse Ps Lorentz factor. The high-order multi-photon electroproduction of ortho-Ps studied dominates for the collision of electrons with heavy atoms over the bremsstrahlung production from the electron via a virtual photon proposed by Holvik and Olsen [Phys. Rev. D 35, 2124 (1987)]. Our results complete and correct the studies of those authors.

Electron capture by ${\mathrm{Cs}}^{+},$ ${\mathrm{K}}^{+},$ or ${\mathrm{Li}}^{+}$ from a $\mathrm{Na}$ Rydberg target in an extreme Stark state of $n=24,\hspace{0.5em}m=0\mathrm{}$ has been measured and compared with both a classical and a quantal calculation as a function of the reduced velocity $\tilde{v}=0.15\mathrm{}-\mathrm{1.6.}$ Peaks and shoulders are found at $\tilde{v}=0.25,$ 0.5, and 1.0 in the ratio of the measured capture cross sections for electronic charge polarization antiparallel and parallel to the ion-beam direction. A close-coupling calculation reveals similar features in the total capture cross sections near those same scaled velocities. A classical trajectory calculation attributes this structure to swapping of the electron between the two positive charge centers as they pass one another during the collision. The total classical capture cross section of a Rydberg atom having an upstream-directed charge distribution is shown to be the sum of five-and-greater-swap, three-swap, and one-swap contributions, which result in the structure near $\tilde{v}=0.25,\hspace{0.5em}0.5,$ and 1.0, respectively.

Through the use of Fourier-transform spectroscopy, the electron-impact optical cross sections for the infrared emissions from the $4F,$ $5F,$ and $6F$ levels of the helium atom have been measured for gas pressures between 3 and 50 mTorr and incident electron energies from threshold to 200 eV. The very strong pressure dependence of these measured emission cross sections at different energies is in excellent quantitative agreement with the mechanism of excitation transfer from the $n^{1}P$ levels through collisions with ground-state helium atoms. From our data, we determine the electron-impact cross sections for excitation into the $4F,$ $5F,$ and $6F$ levels. The effects of the singlet-triplet mixing in the nF states on the excitation cross sections are discussed. The excitation data of the $5F$ level are compared with those of $5{}^{1}S$ and $5{}^{1}D.\mathrm{}$ We have also obtained the $n{}^1\overrightarrow{P}\mathrm{nF}$ collisional excitation transfer cross sections for $n=4\mathrm{}–7.$

Eigenphase shifts and eigentime delays near a resonance for a system of one discrete state and two continua are shown to be functionals of the Beutler-Fano formula using appropriate dimensionless energy units and line profile indices. Parameters responsible for the avoided crossing of eigenphase shifts and eigentime delays are identified. Similarly, parameters responsible for the eigentime delays due to a frame change are identified. With the help of new parameters, an analogy with the spin model is pursued for the S matrix and time delay matrix Q. The S matrix is found to be put into $\exp \left[i\right(a+b\overrightarrow{\sigma }\cdot \mathrm{n̂}\left)\right].$ The time delay matrix Q is shown to be given as $Q=\frac{1}{2}{\tau }_r(1+P_a\cdot \overrightarrow{\sigma }+P_f\cdot \overrightarrow{\sigma }),$ where the first term is the time delay due to resonance, the second term is the one due to avoided crossing interaction, and the last term is the one due to a frame change. It is found that $P_a^2{+P}_f^2=1.\mathrm{}$

Ab initio calculations are presented for total and partial recombination cross sections and rate coefficients for $e+\mathrm{Fe}V\to \mathrm{Fe}\mathrm{IV},$ employing a unified treatment that incorporates both the radiative and dielectronic recombinations (RR and DR) in a self-consistent and accurate manner. The theoretical treatment is based on the close-coupling approximation using the R-matrix method. Recombination calculations for heavy atoms such as Fe IV, with $3d$ open-shell ground configuration, require an extensive eigenfunction expansion for the target ion, as well as a carefully optimized basis set of electron-ion bound configurations that represent short range correlation effects. The large-scale calculations are preceded by electron scattering and photoionization calculations using a 31-term eigenfunction expansion dominated by the configurations ${3d}^4,$ ${3d}^{3}4s,$ and ${3d}^{3}4p$ of Fe V. Photorecombination cross sections and DR collision strengths are thereby obtained, including individual photorecombination cross sections for a large number of bound states of Fe IV that couple to the ground state ${3d}^4{}^{5}D$ of the target ion Fe V—all possible bound states up to $n=10\mathrm{}\hspace{0.5em}(740\mathrm{}\hspace{0.5em}\mathrm{LS}$ terms of Fe IV). The cross sections include autoionizing resonances, also up to the $n=10\mathrm{}$ complex, accounting for the unified (RR + DR) contribution into the $n<~10$ (low-$n)$ bound states of Fe IV. Recombination into the high-$n$ states, $10<~n<~\infty ,$ is obtained through the DR collision strengths for the corresponding series of resonances in the electron-ion continua. The convergence of the close-coupling expansion, with respect to the target ion states and the (electron plus ion) correlation functions is discussed with reference to other highly complex atomic systems. Maxwellian average at a range of temperatures yields the total rate coefficient, as well as partial contributions directly to state-specific recombination rate coefficients. The new close-coupling rates differ considerably from those heretofore obtained from simpler approximations. We expect the present data to be of importance in the modeling of astrophysical and laboratory plasmas where iron is often a prominent constituent.

Electron-impact emission cross sections for transitions from the ten ${5p}^{5}6p$ levels of xenon to the lower levels have been measured at different gas pressures between 0.1 and 2.0 m Torr, and with incident electron energy from threshold to 150 eV. The exceptionally strong pressure dependence of the measured emission cross sections reported previously is confirmed. The optical emission cross sections yield the apparent excitation cross sections for the ${5p}^{5}6p$ levels which vary significantly with pressure. Use of a Fourier-transform spectrometer makes it possible to measure previously uninvestigated infrared cascade transitions into the ${5p}^{5}6p$ levels. Our measured cascade cross sections increase with pressure because of radiation trapping effects, and their pressure dependence is similar to that of the apparent excitation cross sections. By subtracting the cascade contribution from the apparent cross sections, we obtain the direct excitation cross sections which are found to be independent of the pressure. The peak direct cross sections for the ${2p}_1{-2p}_4$ levels (Paschen’s notation) are much smaller than those of the ${2p}_5{-2p}_{10}$ group. This is explained on the basis that the ionization energies for the former group are about only one-half of those for the latter group. Within each of these two groups ${(2p}_1{-2p}_4$ and ${2p}_5{-2p}_{10}),$ the levels with even values of the total angular momentum J have larger cross sections than the levels with odd J.

We consider the effect of shape on the anisotropy of stopping of swift ions by molecular targets. By use of an anisotropic harmonic-oscillator model and frequencies based on jellium ellipsoids, we show that a meaningful estimate of molecular stopping anisotropy can be obtained from molecular shape alone.

For ions grazingly impinging on solid surfaces, the mechanism of binary collision with the free-electron gas is investigated by employing a semiclassical formalism. The model is applied to the calculation of the energy lost by fast protons colliding with aluminum surfaces. Different transitions that contribute to the energy loss are analyzed. The total energy loss by binary collisions is compared with recent experimental data and results from the dielectric formalism. The energy spectrum of the ejected electrons is also studied, finding good agreement with the available experimental data for emission angles separated from the direction of specular reflection of the projectile.

We present a theoretical study of the structural, vibrational, and electronic properties of small niobium clusters $({\mathrm{Nb}}_8,$ ${\mathrm{Nb}}_9,$ and ${\mathrm{Nb}}_{10})$ using density-functional theory with local as well as gradient-corrected exchange correlation density functionals. For ${\mathrm{Nb}}_8$ and ${\mathrm{Nb}}_9,$ several low-energy structures were obtained, whereas for ${\mathrm{Nb}}_{10}$ there seems to be only one isomer that is energetically preferred. Vibrational spectra are in the range 100–300 ${\mathrm{cm}}^{-1}$ and are sensitive to the structure. Electronic properties are investigated by means of the densities of states, which depend strongly on the geometry. The theoretical results for the ionization potentials and vertical detachment energies are in good agreement with available experimental data.

We study the interaction of ${\mathrm{C}}_{60}$ molecules with intense $(10\mathrm{}–1000{\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^2)$ femtosecond pulses of 790-nm wavelength. High charge states of carbon up to ${\mathrm{C}}^{4+}$ are produced in this intensity range, as determined by time-of-flight spectroscopy. These high charge states of carbon are produced at intensities more than an order of magnitude lower than would be required based on optical field ionization of isolated carbon atoms. From a line-shape analysis of the time-of-flight data, we derive the kinetic-energy distribution of the C ions. We find average energies of up to a few hundred eV, indicating that these ions are released through Coulomb explosion of the ${\mathrm{C}}_{60}$ molecules once the charge buildup due to ionization becomes sufficiently high. Ionic potentials of up to 200 V are derived from the kinetic-energy distributions. The ionic potential needed to retain electrons energetic enough to cause K-shell impact ionization is 392 V. This potential is not reached in case of ionization of ${\mathrm{C}}_{60}$ with 790-nm, 45-fs pulses since the disintegration of the cluster occurs on a time scale of a few femtoseconds, much shorter than the optical pulse. In this short time span, the heating of the cluster is not efficient enough for higher charge states to be ionized.

We explore Bragg diffraction from atomic lattices bound by light as a diagnostic tool for studying properties of optical lattices not accessible so far. A weak laser beam at a wavelength of about half the wavelength of the lattice field is diffracted from the (100) and (130) lattice planes of a Rb optical lattice. The observation of well-defined Bragg spots confirms the long-range order in optical lattices. From the acceptance angle for Bragg diffraction we deduce the range over which crystalline order is preserved. The comparison of two Bragg spots diffracted from different lattice planes allows us to directly measure the position spread of the atomic wave packets oscillating in the light-induced potential wells. By combining conventional probe transmission spectroscopy with Bragg diffraction we study the motion of atoms that are deeply bound inside the potential wells. We show experimentally and theoretically how backaction of the bound atoms on the trapping field influences the lattice constant. We develop a model for the modification of the refractive index by the highly ordered atomic medium. Based on first-order scattering theory the model explicitly includes the influence of atomic localization on the resulting lattice constant. By measuring the change of the Bragg angle, we observe a decrease of the lattice plane separation for increasing atomic density. We report experimental evidence for the correction to the refractive index due to the finite position spread of the atoms.