We have measured hyperfine-structure splittings and isotope shifts in the $4s{}^4{P}_J\to 4p{}^4{D}_{J^{\prime }}$ multiplet in atomic chlorine using Doppler-free saturated absorption spectroscopy with an external cavity diode laser. Spectra of seven of the eight components of this multiplet, which have wavelengths between 798 and 859 nm, were obtained. Values for the magnetic dipole and electric quadrupole hyperfine-structure constants of the ${}^4{P}_{1/2-5/2}$ and ${}^4{D}_{1/2-7/2}$ states in both ${}^{35}\mathrm{Cl}$ and ${}^{37}\mathrm{Cl}$ were found, and the isotope shifts of the seven transitions studied were measured. The data were also analyzed in an attempt to extract values for magnetic octupole coupling constants for some of the levels investigated, but these were found to be zero at the level of the experimental uncertainty of our measurements.

We present potential-energy curves for highly excited electronic states of diatomic molecules showing spectacular undulations including multiple barriers and wells. Those undulations unrelated to avoided crossings are closely correlated with the oscillations of atomic radial electron density in the Rydberg states. The LiHe, LiNe, and LiH cases are examined with an accurate quantum chemical calculation. For the ${\Sigma }^{+}$ states originating from the ns, np, nd, or nf states of lithium atom, $n-2$ potential barriers and the same number of potential wells exist. The $4{}^1{\Sigma }_g^{+}\mathrm{}\left(F\right)\mathrm{}$ state of ${\mathrm{Li}}_2$ also shows the energy barrier of the same origin. This spectroscopic property is supposed to be more general in diatomic molecules and other small molecules.

We present a hybrid method to solve the relativistic Hartree-Fock-Roothan equations where the one- and two-electron radial integrals are evaluated numerically by defining the basis functions on a grid. This procedure reduces the computational costs in the evaluation of two-electron radial integrals. The orbitals generated by this method are employed to compute the ionization potentials, excitation energies, and oscillator strengths of alkali-metal atoms and elements of group IIIA through second-order many-body perturbation theory. The computed properties are in excellent agreement with the experiment and other correlated theories.

The three-body distorted-wave Born and impulse approximations with spin-averaged static exchange potential have been used to calculate the electron-impact triple-differential ionization cross sections (TDCS’s) of ${\mathrm{Li}}^{+},$ ${\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ ions in different geometries and kinematics. In coplanar geometry at high incident energy (>500 eV) and scattering angle $\sim 10°,$ both recoil and binary peaks split into two in case of p-electron targets $({\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ ions). In noncoplanar geometry a strong interference resulting in a dip in TDCS’s has been noticed. In the perpendicular plane geometry, single- and double-scattering mechanisms together produce an oscillatory structure in the TDCS’s of a ${\mathrm{K}}^{+}$ ion. The values of the binary-to-recoil peak ratio for specific values of the momentum transfer have been determined to understand collision dynamics.

Elastic differential, integral, and momentum-transfer cross sections are reported for electron scattering by ${\mathrm{GeH}}_4$ at impact energies ranging from 0.2 to 100 eV. The Schwinger iterative variational method in the fixed-nuclei, static-exchange plus correlation-polarization approximation is used to calculate the scattering amplitudes. Large-angular-momentum contributions from the correlation-polarization potential to the scattering amplitude are taken into account via a closed formula for higher incident energies. We have observed a $T_2$ shape resonance in the ICSs and MTCSs at around 3.0 eV and we have predicted a Ramsauer-Townsend minimum at around 0.6 eV. Our calculated cross sections are compared with recent experimental data and other theoretical results.

We present measurements of cold collision processes in an alkaline-earth species. Collisions of ${}^{88}\mathrm{Sr}$ pairs in a magneto-optic trap are found to be dominated by very-long-range excitation to the nonradiating ${}^1{\Pi }_g$ state of ${\mathrm{Sr}}_2^{*}.$ We find good agreement between measured loss rate coefficients and a modified form of the semiclassical model that includes retardation effects that allow excitation to metastable molecular states.

We calculate the asymptotic high-energy amplitude for electrons scattering at one ion, as well as at two colliding ions, by means of perturbation theory. We show that the interaction with one ion eikonalizes and that the interaction with two ions causally decouples. We are able to put previous results on perturbative grounds and propose further applications for the obtained rules for interactions on the light cone. We discuss the implications of the eikonal amplitude on the pair production probability in ultrarelativistic peripheral heavy-ion collisions. In this context the Weizsäcker-Williams method is shown to be exact in the ultrarelativistic limit, irrespective of the produced particles’ mass. A new equivalent single-photon distribution is derived, which correctly accounts for Coulomb distortions. The impact on single-photon induced processes is discussed.

The algebraic coupled-state method is used to investigate the various anomalies of the cross sections near the positronium-formation threshold in positron-hydrogen scattering. In particular, our calculations confirm the existence of the S-wave Wigner’s cusp of the elastic $e^{+}-\mathrm{H}$ cross sections that was published in the recent literature. We show that these so-called anomalous behaviors can actually be understood well within the framework of the theory of scattering near thresholds and that they should exist independently of whether the cross sections are calculated accurately or not. The complete results of a fairly comprehensive calculation in the vicinity of the positronium-formation threshold using several different coupling schemes (large and small) are presented to demonstrate our finding.

$\mathrm{}(e,2e)\mathrm{}$ cross sections for argon have been measured for binding energies in the valence regime of the ion at energies of 500, 1000, 1500, and 1800 eV. The kinematic arrangement is noncoplanar symmetric. Cross sections are relatively normalized for each energy. The assumptions of electron-momentum spectroscopy are verified in detail. The distorted-wave impulse approximation with Hartree-Fock orbitals describes the data within experimental error. It gives the correct inner valence s orbital manifold cross section relative to the outer valence p manifold at all energies. Orbital manifolds of ion states are identified, within which cross-section ratios are independent of energy and recoil momentum. Spectroscopic factors are defined by the ratio of the cross section for an ion state to the cross section for the orbital manifold. The spectroscopic sum rule is verified.

A number of studies have been made over the years of the near-threshold behavior of phase shifts for the scattering of electrons by spherically symmetric atomic targets, allowing for the important effect of long-range polarization forces. For targets that are not spherically symmetric, the theory must be extended to account for a superposition of asymptotic power-law potentials, including an inverse-cube potential for targets with a permanent quadrupole moment, which couple the various channels. Such an extension is described here. A degenerate perturbation theory, extended to arbitrarily high orders through the introduction of continued-fraction representations, is developed for the construction of the asymptotic states. Results of illustrative numerical calculations are reported. The asymptotic solutions, along with a variational principle for the reaction matrix, provide the basis for a modified effective-range theory. A minimum principle, valid at threshold, is available for the calculation of scattering length and effective range matrices.

Cross sections for single electron emission have been measured in collisions of 95-MeV/u ${\mathrm{Ar}}^{18+}$ projectiles with atomic Li for electron energies ranging from 3 to 1000 eV and angles ranging from $25°$ to $155°.$ Models based on the Born approximation are introduced to separate two- and three-body effects in the angular distributions of the ejected electrons. Both experiment and theory provide information about the separability of the two- and three-body effects. The high projectile velocity and the use of the Li target are shown to be essential for the present analysis. The emission of the $1s$ electron is attributed mainly to three-body effects. The cross section for three-body collisions rapidly decreases with the electronic energy transfer involving a power law with an exponent of $-\mathrm{3.5.}$ Consequently, two-body effects dominate at high electron emission energies. Remarkably large contributions from two-body collisions were also observed for the low-energy emission of the $2s$ electrons. Demonstrating the analogy in ionization by photons and ions, the two- and three-body processes are associated with Compton scattering and photoabsorbtion, respectively.

State-selective charge transfer from ${\mathrm{H}}_2$ by ${\mathrm{C}}^{4+}$ and ${\mathrm{N}}^{5+}$ in the low-energy region $(0.01\mathrm{}<~E<~2{\mathrm{keV}\mathrm{}\mathrm{amu}}^{\mathrm{-}1})$ has been investigated employing the semiclassical, impact parameter, close-coupling method based on a molecular expansion augmented with the plane-wave translation factor. Freezing the molecular features of the target, the colliding system is approximated by a pseudo-one-electron entity. The method of pseudopotential is applied to account for the binding of the electron in the transient diatomic quasimolecule. A large number of close-coupled equations are solved to obtain probabilities of transitions, and thereby evaluate the partial capture cross sections. Both the total and partial capture cross sections agree well with the recent experimental results. The low-energy behavior of the total cross sections due to ${\mathrm{C}}^{4+}$ and ${\mathrm{N}}^{5+}$ is very different owing to their different core structure, albeit both are He-like ions; the former shows some fluctuations whereas the latter registers a monotonic rise with E. The possible role of the projectile core size on the final l distribution of the estimated cross section is discussed. The inclusion of the rotational couplings is essential for the correct low-energy behavior of the cross sections. For ${\mathrm{N}}^{5+}{+\mathrm{H}}_2,$ the low-energy l distribution of the capture cross sections has been presented.

Triply differential cross sections and spin asymmetries for $\mathrm{}(e,2e)\mathrm{}$ reactions of the ${1s}_{1/2}$ state of intermediate- and high-$Z$ atoms in coplanar asymmetric geometry are analyzed within the framework of the relativistic distorted wave Born approximation. Specific signatures of the relativistic nature of the electron-electron interaction, and of the atomic-number-dependent electron-residual ion interaction, are identified.

The K-shell ionization cross section of boron has been obtained for incident 0.4–2.0-MeV ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ ions and for 0.6–1.2-MeV ${\mathrm{H}}_2^{+}$ ions in steps of 0.2 MeV. The cross sections were obtained from the boron KLL Auger yields of gaseous ${\mathrm{B}}_2{\mathrm{H}}_6.$ The present K-shell ionization cross sections are compared to existing experimental cross sections as well as to theoretical predictions.

We present the theoretical formalism required to interpret the electron-photon coincidence measurements of the helium $4F$ state by Cvejanović and Crowe [Phys. Rev. Lett. 80, 3033 (1998)]. The results of the convergent close-coupling theory are compared with experiment and found to be in good qualitative agreement. We demonstrate that the singlet-triplet mixing in the $4F$ states does not affect the calculated results significantly.

We compute the scattering properties of rubidium atoms at ultracold temperatures, placing special emphasis on the radioactive isotopes ${}^{82}\mathrm{Rb}{,}^{83}\mathrm{Rb}{,}^{84}\mathrm{Rb},$ and ${}^{86}\mathrm{Rb}.$ In combination with the more “conventional” isotope ${}^{87}\mathrm{Rb},$ these species yield a wide variety of scattering behavior, with consequences for creating mixtures of Fermi-Fermi, Fermi-Bose, and Bose-Bose gases. In each of these mixtures we find at least one case where the interspecies scattering lengths can be tuned using an external magnetic field. These results should greatly broaden the prospects for quantum degenerate gases in future experiments.

We consider two conceptional problems that arise in the quantum-mechanical calculation of inelastic atomic collisions. The definition of the coupling matrix elements, which govern the inelastic transitions, seems to be ambiguous and the matrix elements can remain nonzero as the internuclear distance goes to infinity. We show that there is no ambiguity in the coupled equations, which describe inelastic collisions, and we demonstrate that the asymptotic couplings are required for a correct description of the asymptotic part of the scattering wave function. We develop on this basis a straightforward and simple procedure to deal with the asymptotic couplings in the framework of a full quantum approach and we apply it to a simple example of H+Na collisions.

A five-body classical trajectory Monte Carlo model has been developed to study double electron removal from ${\mathrm{H}}_2$ by collisions with highly charged ions at impact energies ranging from 1 eV/u to 1 GeV/u. The longitudinal and transverse final-state correlation between ejected electrons is calculated for double ionization of ${\mathrm{H}}_2$ by impact of ${\mathrm{Se}}^{28+}$ at 3.6 MeV/u and ${\mathrm{U}}^{92+}$ at 1 GeV/u; the electron-electron interaction is dynamically included during the collision when one of the electron’s total energy becomes positive. Relativistic corrections are incorporated to reflect the Lorentz contraction of the projectile’s electric field. The cross section dependence on the alignment of the ${\mathrm{H}}_2$ molecular axis was investigated. Here, transfer ionization of ${\mathrm{H}}_2$ by ${\mathrm{O}}^{8+}$ at 500 keV/u is found to have a maximum for the molecular axis aligned perpendicular to the projectile velocity, while no orientation dependence is found for double ionization at 500 keV/u. In contrast, a minimum in the cross section at $90°$ is found for 1-GeV/u ${\mathrm{U}}^{92+}{+\mathrm{H}}_2$ collisions. A systematic study of the energy partitioning between the two product ${\mathrm{H}}^{+}$ ions has been made for ${\mathrm{Xe}}^{54+}{+\mathrm{H}}_2$ from 1 eV/u to 1 MeV/u. Large deviations from Franck-Condon behavior are found for impact energies $E\lesssim 10$ keV/u. At low energies the proton energies are very energetic with the main contribution arising from collisional transfer from the projectile, while the proton energy spectrum at high impact energy is due to the Coulomb explosion of the isolated molecule.

The range of conditions for which inclusion of the generalized Breit interaction is important in calculating the scattering matrix elements for 1s ionization is explored within the relativistic distorted-wave approximation. This approach is applied to the calculation of 1s ionization cross sections for a variety of ions with one to four bound electrons and nuclear charge Z in the range of $10<~Z<~92.$ These data are then interpolated with simple, but accurate, fit formulas. The resulting expressions are readily integrated over a relativistic Maxwellian electron distribution function to obtain rate coefficients for plasma modeling. A discussion of the high energy behavior of the cross sections for large Z is also given.

We have measured the resonant Auger decay spectrum of the water molecule at the oxygen K edge. Strong deviations from the expected linear dispersion of the participator lines (which correspond to single-hole valence states) are observed. In a simplified picture the effect can be attributed to the combined effect of the intermediate- and final-state potential curves.