Using semiclassical methods we investigate the threshold behavior for three-particle breakup of a system with one particle of charge $Z$ and two other particles of charge $-q$. For the particular case where the ratio of the charges of the third particle to the wing particles is $Z/q=1/4\mathrm{}$, the Wannier exponent for breakup diverges and the threshold law changes from a power law to an exponential law of the form $\exp (-\lambda /\sqrt{E})$. The threshold behavior is tested above the region of divergence and it is found that for $Z/q<\mathrm{}0.3\mathrm{}$ a power law does not hold. Ionizing trajectories show that the dynamics within the near zone can become crucial to the energy dependence of the cross section. Cases are found to arise where more than one trajectory contributes to the same final state giving rise to semiclassical interference effects.

We have carried out theoretical studies of low-energy elastic electron collisions with nitrous oxide $\left({\mathrm{N}}_2\mathrm{O}\right),$ obtaining differential, integral, and momentum-transfer cross sections. Polarization effects are incorporated in the electron-molecule scattering dynamics. A simple, objective, and physically motivated criterion is introduced for constructing a compact set of configurations that accurately accounts for polarization in resonant symmetries while avoiding overcorrelation. Our cross sections are in generally good agreement with experiment and with earlier high-level calculations, and most differences are readily understood. However, certain discrepancies between the calculated and measured differential cross sections remain puzzling.

We have applied the laser-ablation technique for the study of depolarization effects of the $5d6p$ ${}^3{P}_1$ and ${}^3{D}_1$ states of Ba with He gas in the wide temperature range of 10–300 K. The depolarization cross sections of both states have been found to have a positive temperature dependence, indicating that the responsible interaction is not a van der Waals type. A tentative and qualitative explanation for the difference of the temperature dependence between $5d6p$ ${}^3{P}_1$ and ${}^3{D}_1$ is given in terms of electron density distribution.

We examine the long-range part of the rare-gas diatomic potentials that connect to the $R\{(n-{1)p}^5\mathrm{ns}\}+R\{(n-{1)p}^5\mathrm{np}\}$ atomic states in the separated atom limit ($n=3,$ 4, 5, and 6 for Ne, Ar, Kr, and Xe, respectively). We obtain our potentials by diagonalization of a Hamiltonian matrix containing the atomic energies and the electric dipole-dipole interaction, with experimentally determined parameters (atomic energies, lifetimes, transition wavelengths, and branching ratios) as input. Our numerical studies focus on Ne and Kr in this paper, but apply in principle to all other rare gases lacking hyperfine structure. These diatomic potentials are essential for applications in which homonuclear rare-gas pairs interact at large internuclear separations, greater than about 20 Bohr radii. Among such applications are the study of cold atomic collisions and photoassociative spectroscopy.

Triple differential cross sections have been measured for the electron impact ionization of a $3p$ shell of argon in coplanar geometries. Results are presented for 40, 20, 8, and 2 eV excess energy with respect to ionization threshold, this energy being equally shared between both outgoing electrons in the final state. Comparison is made with distorted wave Born approximation calculations that include initial channel polarization and final channel post-collisional interaction. Results are shown to be very sensitive to few-body effects in both incident and final channels; however, agreement remains poor and the experimental results present a substantial challenge to theory.

Cross sections for dissociative recombination and dissociative excitation of ${\mathrm{NO}}^{+}$ in the vibrational ground state have been measured at a heavy-ion storage ring. Branching ratios for dissociative recombination into different electronic states of the atomic fragments have also been determined by an imaging technique. More than 80% of the dissociative recombination events at E = 0 eV produced $\mathrm{N}{(}^{2}D)$ + O${(}^{3}P)$. The rotational temperature of the ${\mathrm{NO}}^{+}$ ion beam was changed by a factor of 2 by employing different types of ion sources. The branching ratios and the dissociative recombination cross section as a function of energy were independent of the rotational temperature. The dissociative recombination cross section was proportional to $E^{-1}$ at low energies and had peaks in the energy regions 3–8 eV and 10–16 eV. We obtain a total dissociative recombination rate $\alpha =4\mathrm{}\times {10}^{-7}$ ${\mathrm{cm}}^3/\mathrm{s}$ at $T=300\mathrm{}$ K for ${\mathrm{NO}}^{+}$ in the vibrational ground state.

Spectra of $L$ x rays emitted by 5.2- to 14.4-MeV/nucleon Xe ions traveling in gaseous targets of He, Ne, and Ar have been measured with a curved crystal spectrometer. Detailed spectral analysis provided estimates of the average projectile charges and the average $L$- and $M$-electron populations inside the gases. In comparisons of the present results for gases with those obtained previously for solids, it was found that Xe ions emitting $L$ x rays in solids have, on average, many more $L$ vacancies than those that emit $L$ x rays in gases. Average charges deduced for Xe ions traveling in Ar gas were 2.3 units lower than the average charges of Xe ions traveling in solid KCl.

Theoretical double photoionization cross sections are predicted for He initially in the $1s2s{}^{1}S$ or ${}^{3}S$ state. The probability for double photoionization is a factor of 6 higher for the ${}^{1}S$ state at 20 eV above the threshold for double ionization and a factor of 3 larger at 80 eV above threshold. We compare these results with each other and with predictions for the high-energy limit to assess the influence of exchange on the double ejection process. Similar comparisons are also made for the process of photoionization-induced excitation of ${\mathrm{He}}^{+}\mathrm{}\left(n\right)\mathrm{}$.

Finite nuclear size is found to cause reduction in photoproduction of $\mu$ and $\tau$ pairs with bound negative lepton. The reduction is less dramatic for muons than claimed previously but very severe for experimental production of tauonic atoms. For both types of leptons, $L$-shell production exceeds $K$-shell production in heavy targets, and light targets are more favorable for ground-state $\tau$-atom production. Implications for heavy-ion collisions are outlined.

Triply differential cross sections obtained in relativistic $\mathrm{}(e,2e)\mathrm{}$ experiments on inner shell states in coplanar asymmetric geometries show a number of characteristic features, most notably a secondary maximum for emission of both electrons into the same quadrant. Referring to a recent experiment [K.-H. Besch, M. Sauter, and W. Nakel, J. Phys. B 30, L73 (1997)] an interpretation of these findings in terms of destructive interference of the contributions to the S matrix describing exchange of one longitudinal photon with those representing single transverse photon exchange is proposed. An additional feature of the triply differential cross section for backward emission of the slow electron observed in this experiment is attributed to a double-scattering mechanism.

We studied forward and backward electron ejection from carbon foils penetrated by swift ions (energy range, 20 keV/nucleon–74 MeV/nucleon; projectiles, H–U) over four orders of magnitude of electronic energy loss (0.01–25 keV/nm). Ejection of fast electrons from primary ionization and subsequent energy dissipation by secondary cascades are found to be asymmetric. Electronic energy deposition by swift ions is thus different at the entrance surface, in the solid’s bulk, and at the exit surface. The effect strongly increases with projectile atomic number due to an increasing contribution of close collisions and enhanced fast-electron ejection.

The deexcitation of slow, hydrogenlike nitrogen ions through interaction with a Au(111) surface is studied by $\mathrm{KLL}$ Auger electron spectroscopy. Special emphasis is given to processes occurring above the first atomic layer in projectiles that graze the electron gas at different depth. It is found that the increasing electron density around the jellium edge causes acceleration of $L$-shell filling. No difference is seen for projectiles that do or do not penetrate the first atomic layer. A cascade model of hollow atom deexcitation at the border of the electron gas is presented. The model includes a depth dependence of the $L$ shell filling rate caused either by interaction with the electron continuum or by a violent collision with a single target atom. Good agreement with experiment is found, along with some evidence for transfer of electrons from states below the Fermi level.

Collisions of Ne with endohedral $\mathrm{La}@{\mathrm{C}}_{82}$ at a center-of-mass energy of 1.44 keV have been investigated using molecular dynamics. The interactions among the C atoms are modeled using the empirical potential of Takai et al., which includes two- and three-body terms. All other interactions are treated using two-body potentials. A total of about 7200 trajectories are calculated, using randomly selected initial orientations of the fullerene and impact parameters in the range 0.5 Å to 6.0 Å. The trajectories are analyzed to determine the energy transferred to the fullerene target and the pattern of fragmentation of the target following the impact. The average energy transferred at a given impact parameter is well correlated with the density of C atoms in the path of the incident projectile. The fragmentation data can be compared with recent experiments of Lorents et al. on the similar system $\mathrm{Ne}-(\mathrm{La}@{\mathrm{C}}_{82}{)}^{+}$. We find a narrower distribution of fragment sizes than these experiments. We also see no preference for the production of fullerene fragments with even numbers of C atoms. However, we analyze the fragments only about 80 fs after impact, and at that time they have sufficient internal energy to split even further. Since the experimental mass analysis takes place about a microsecond after the collision, our results suggest that further fragmentation and equilibration into the more stable fragments with an even number of C atoms takes place well after the collision.

We did an extensive search for the lowest-energy isomers of ${\mathrm{Nb}}_n^{-}$ $(n=3\mathrm{}–8)$ with a local-spin-density method. We report the calculated optimum geometries for various cluster isomers, and their vibrational frequencies and electron binding energies. We describe two simple ways to account for final-state effects on electron binding energies, based on Slater’s transition state method, which yield results consistent with one another and with experiment.

The intensity dependence of the angular and energy distributions of photoelectrons is used to map the effect of channel closing in short-pulse, multiphoton ionization of xenon. At 800 nm, we observe the nine-photon channel to close at $\approx 3\times {10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ and the dominant ionization path to switch from $\mathrm{}(8+1)\mathrm{}$-photon resonant ionization via $f$-type Rydberg states to $\mathrm{}(9+1)\mathrm{}$-photon resonant ionization via $g$-type Rydberg states. Very low-energy electrons are observed prior to reaching the channel closing intensity, indicative of a contribution of nonresonant ionization.

With this experiment we demonstrate that excitation of a two-state system with radio-frequency fields differing in phase by 90° produces nonintuitively different results, even for very long pulses. In addition, we show how the phase dependence of the transition probability of long pulses can be easily understood by using the single cycle time propagator. Finally, we have found surprising results for real pulses in the strong-field regime, i.e., pulses having appreciable rise and fall times.

In the first Born approximation, the symmetrical double-electron-capture collision between an α particle and a helium atom in the presence of an intense laser field is studied. The capture cross section is promoted considerably and is an increasing function of the ratio of the laser amplitude to frequency. With increasing impact energy, the dressing modification becomes notable.

We have studied the dynamics of two-electron wave packets in barium consisting of $6snd{}^{1,3}{D}_2$ Rydberg states, which are degenerate with the doubly excited $5d7d{}^1{D}_2$ state. The configuration interaction manifests itself in the temporal oscillation of the wave packet among the three configurations in this system. We present in detail a quantum defect theory analysis that adequately describes the system, and we introduce a simple physical picture to understand the dynamics.

Analytical expressions are presented for the third-order diamagnetic corrections to the energy of nondegenerate hydrogen levels with arbitrary principal quantum number $n$ and the magnetic quantum number $|m|=n-1,n-2,n-3$. The leading term for the third-order energy correction for levels with high $n$ is determined to be $\Delta E^{\mathrm{}\left(3\right)\mathrm{}}\approx \frac{3}{128}{n}^{16}{B}^6$. Together with the well-known first- and second-order corrections $\Delta E^{\mathrm{}\left(1\right)\mathrm{}}\approx \frac{1}{8}{n}^4{B}^2$ and $\Delta E^{\mathrm{}\left(2\right)\mathrm{}}\approx -\frac{1}{32}{n}^{10}{B}^4$ it determines the upper and lower bounds for the level energy in field and also the range of magnetic fields where the first- and second-order perturbation theory terms are valid for calculating the Zeeman energy in hydrogenlike states of atoms.

We derive and apply an optical Bloch equation (OBE) model for describing collisions of ground and excited laser cooled alkali atoms in the presence of near-resonant light. Typically these collisions lead to loss of atoms from traps. We compare the results obtained with a quantum mechanical complex potential treatment, semiclassical Landau-Zener models with decay, and a quantum time-dependent Monte Carlo wave packet (MCWP) calculation. We formulate the OBE method in both adiabatic and diabatic representations. We calculate the laser intensity dependence of collision probabilities and find that the adiabatic OBE results agree quantitatively with those of the MCWP calculation, and qualitatively with the semiclassical Landau-Zener model with delayed decay, but that the complex potential method or the traditional Landau-Zener model fail in the saturation limit.