The near-threshold electron-impact ionization cross section for the tungsten atom is calculated in a distorted-wave approximation. A shape resonance is found to have a large effect on the total cross sections for the 6s and 5d subshells. The shape resonance in the l=2 scattering channel is most clearly defined when one examines the differential cross section with ejected energy.

Nineteen γ lines in the energy range 465–2842 keV, produced by the Ni(n,γ) and Fe(n,γ) reactions, were scattered from a natural Pb target at angles of θ=1.02° for the Ni source and θ=1.80° for the Fe source. Differential cross sections were measured relative to carbon and are dominated completely by Rayleigh scattering. The results were found to agree well with the modified relativistic form-factor calculations within 5%.

We have measured a hyperfine-induced spin-exchange frequency shift in the atomic-hydrogen ground-state hyperfine transition. A recent quantum-mechanical treatment of low-energy hydrogen-hydrogen scattering by Koelman et al. [Phys. Rev. A 38, 3535 (1988)] predicts such frequency shifts to become large at low temperature, and to affect the performance of atomic clocks such as the cryogenic hydrogen maser. The experiment reported here was performed with a hydrogen maser operating near room temperature, where the reported hyperfine effects are predicted to be small, but measurable. Using an adiabatic fast passage (AFP) technique to vary the incoming atomic population in the masing states from approximately 100% (AFP on) to 50% (AFP off), we determined the change in the dimensionless hyperfine-induced frequency-shift parameter Ω to be ${\mathrm{\Omega }}_{\mathrm{on}}$-${\mathrm{\Omega }}_{\mathrm{off}}$=5.38 (1.06)×${10}^{\mathrm{-}4}$. The theoretical prediction at this temperature is ${\mathrm{\Omega }}_{\mathrm{on}}$-${\mathrm{\Omega }}_{\mathrm{off}}$=-0.76×${10}^{\mathrm{-}4}$ to -1.12×${10}^{\mathrm{-}4}$, for the range of masing-state populations used in the present experiment. We review the relevant theory, report our experimental method and results, and discuss possible reasons for the discrepancy between experiment and theory.

The generalized oscillator strength (GOS) of an atom is an essential factor in the differential cross section for inelastic scattering of fast charged particles. Recently, Takayanagi et al. [Phys. Rev. A 41, 5948 (1990)] have obtained the GOS for the excitation of atomic krypton to the 4${\mathit{p}}^5$${(}^2$${\mathit{P}}_{1/2}$)5s and 4${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)5s states from inelastic-scattering measurements using electron-energy-loss spectroscopy. The present study was undertaken with the twofold objective to determine the results of theoretical computations of the scattering parameters of the above experiment and to examine the suitability of a recently developed relativistic local-density-potential method [M. Vijayakumar, N. Vaidehi, and M. S. Gopinathan, Phys. Rev. A 40, 6834 (1989)] to study atomic-collision processes. Calculations have been done to obtain GOS for electron-krypton and electron-xenon collisions in the squared-momentum-transfer range of 0.01 to 10 atomic units. The well-known theory of Bethe has been used for the determination of the GOS in the first-order Born approximation. The present results are in fair agreement with previous Hartree-Slater and Hartree-Fock calculations and with the experimental data available in the literature. Furthermore, results of the present calculations predict that the GOS goes through a minimum, similar to the ‘‘Cooper minimum’’ in the photoabsorption cross section, as a function of the value of the momentum transfer. Experiments at slightly higher values of momentum transfer are suggested to verify the position of this minimum.

We calculate double-differential cross sections for the impact ionization of He targets by 1.5-MeV ${\mathrm{H}}^{+}$ and 28.5-MeV ${\mathrm{F}}^{9+}$ ions using the distorted-wave strong-potential Born approximation. The theory is compared with the experimental electron spectrum. Both theory and experiment show the low-energy electron distribution, the electron capture to the continuum cusp, and the binary-encounter peak.

The time evolution of subexcitation electron degradation spectra in ${\mathrm{N}}_{2\mathrm{-}}$${\mathrm{O}}_2$ mixtures has been investigated by using the time-dependent Spencer-Fano equation. The calculations were performed for low-energy electrons whose energies fall below 6 eV. The present study showed that strong resonances in vibrational excitation channels in ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ greatly affect the spectrum at early times, and that the stationary spectrum (time →∞) is more readily achieved in energy regions with a large stopping power. The evolution of the characteristic features of spectra of pure ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ and their presence to various degrees in mixtures are also discussed.

Absolute measurements with an accuracy of 22% and theoretical results in a distorted-wave Born approximation (DWBA) are reported for the triple-differential cross section for 26.6-eV electron-impact ionization of helium. An apparatus is used that allows all scattering angles to be independently varied for both coplanar and noncoplanar geometries. The measurements are compared with a DWBA calculation that includes exchange distortion in the calculation of the distorted waves, as well as with earlier calculations by Crothers [J. Phys. B 19, 463 (1986)] and Pan and Starace [Phys. Rev. Lett. 67, 185 (1991)]. Emphasis is placed on understanding the mechanisms for near-threshold ionization.

Electron collisional excitation of the 4D, 5D, 4P, and 6S states of Na has been measured with about 30-meV energy resolution. Very rapid, unresolved threshold onsets are seen for all but the 4P state, and a near-threshold resonance is suggested by the 5D data. However, only weak undulations in the cross sections are observed above threshold.

The knowledge of scattering parameters for positronium–lithium-ion scattering is required when the positronium atom is used as a probe to study the surface properties of ionic crystals. Elastic para-para (ortho-ortho) and excitation (spin-flip) processes are studied using the first-order Born approximation. Two wave functions are used to find their relative importance for the elastic processes. The differential and total cross sections for all the processes are reported at intermediate energies.

We present differential and total cross sections for charge transfer in proton-hydrogen collisions at 5, 2.8, and 1 MeV, derived from extensive nonperturbative coupled-channel calculations. We use expansions of target and projectile bound and continuum states of angular momenta up to l=2 in terms of Gaussian-type orbitals. Satisfactory convergence is reached. Good agreement is achieved with the experimental data at 5 MeV but not at 2.8 MeV.

Cross sections for K x-ray production in solid targets (Z=13, 22, 26, 29, 32, 40, 42, 46, and 50) by 30-MeV/amu beams of H, N, Ne, and Ar were measured. The cross sections were determined by recording the spectra of K x rays with a Si(Li) detector in coincidence with beam particles detected in a microchannel plate assembly. The K-shell-ionization cross sections deduced from these data agree quite well with the predictions of the perturbed-stationary-state theory with energy loss, Coulomb deflection, and relativistic corrections. Detailed analyses of the projectile and target Z dependences of the cross sections were performed. Also, the relative intensities of K x rays from double K-shell ionization of the higher-Z targets are presented for N, Ne, and Ar projectiles.

We recast the multichannel quantum-defect theory in a full operatorial form where all the spectral information is naturally incorporated into a density-of-states matrix. This matrix is expressed as a function of two quantities: One of them (the reactance matrix) is associated with the short-range interaction; the other one is associated with the long-range behavior of the potential seen by the Rydberg electron, where any external interaction can be included. The general results obtained in this work give an important unification of phenomena occurring in Rydberg systems. However, they can be applied to other systems as well, as soon as one can split the total system into two entities, like in molecular dissociation or reactive collisions.

We have measured cross sections for true double-electron capture (DC) and transfer ionization (TI) in slow ${\mathrm{Xe}}^{\mathit{q}+}$-(Xe,He) collisions in the charge-state regime 15≤q≤42. We find that the probabilities for radiative stabilization ${\mathit{P}}_{\mathrm{rad}}$=${\mathrm{\sigma }}_{\mathrm{DC}}$/(${\mathrm{\sigma }}_{\mathrm{DC}}$+${\mathrm{\sigma }}_{\mathrm{TI}}$) increase rapidly with q for q≥28 and are very similar for two-electron transfer from Xe and He. This surprising similarity indicates that the core structure of the projectile, rather than the initially populated capture state, is decisive for the electronic relaxation process.

The trajectories of fast molecules during and after penetration through foils are governed by Coulomb explosion and distorted by multiple scattering and other penetration phenomena. A scattering event may cause the energy available for Coulomb explosion to increase or decrease, and angular momentum may be transferred to the molecule. Because of continuing Coulomb explosion inside and outside the target foil, the transmission pattern recorded at a detector far away from the target is not just a linear superposition of Coulomb explosion and multiple scattering. The velocity distribution of an initially monochromatic and well-collimated, but randomly oriented, beam of molecular ions is governed by a generalization of the standard Bothe-Landau integral that governs the multiple scattering of atomic ions. Emphasis has been laid on the distribution in relative velocity and, in particular, relative energy. The statistical distributions governing the longitudinal motion (i.e., the relative motion along the molecular axis) and the rotational motion can be scaled into standard multiple-scattering distributions of atomic ions. The two scaling laws are very different. For thin target foils, the significance of rotational energy transfer is enhanced by an order of magnitude compared to switched-off Coulomb explosion. A distribution for the total relative energy (i.e., longitudinal plus rotational motion) has also been found, but its scaling behavior is more complex. Explicit examples given for all three distributions refer to power-law scattering. As a first approximation, scattering events undergone by the two atoms in the molecule were assumed uncorrelated. A separate section has been devoted to an estimate of the effect of impact-parameter correlation on the multiple scattering of penetrating molecules. That effect is by and large unrelated to Coulomb explosion, but some attention is indicated since it is an unavoidable feature in all scattering phenomena involving molecular ions.

The probabilities of electron-positron pair creation with K-shell capture, of ionization, and of excitation are nonperturbatively calculated with a finite-difference method. Calculations for a collision of ${\mathrm{U}}^{92+}$ on ${\mathrm{U}}^{91+}$ at ${\mathit{E}}_{\mathrm{lab}}$=10 GeV/nucleon at small impact parameters are presented. The results show an enhancement of the probability for pair creation with the capture of about two orders of magnitude in comparison to the outcome of a calculation in first-order perturbation theory. This proves the expected strong nonperturbative character of such relativistic collisions for very heavy ions.

Al atomic LMM Auger electron emission from clean and oxygen- or argon-covered Al surfaces has been studied for low-keV ${\mathrm{Ar}}^{+}$-ion bombardment along normal and off-normal incidence directions. We show that, for a clean Al surface, many transitions are suppressed by the surface-neutralization process and that the quenching of such a neutralization mechanism and the activation of a surface-ionization mechanism by oxygen chemisorption or argon physisorption can unveil these hidden peaks. Our results provide some insights into the peculiarities of ion-solid interactions and offer an interesting explanation to the observed large difference in the relative transition ratios in experiments with solid and gas targets.

Convoy electrons produced at glancing-angle scattering of MeV H, He, Li, and C ions from a clean (001) surface of SnTe single crystal are studied. Acceleration of the convoy electrons is observed. The observed acceleration increases with increasing atomic number of the projectile ion as predicted by the surface-wake-acceleration model. The classical-trajectory Monte Carlo simulation is performed with the surface-wake-acceleration model in order to study the acceleration of the convoy electrons. The agreement between the calculated results and the experimental ones is reasonably good.

Intervals between the 10G, H, I, K, and L (L=4–8) states of helium have been measured with high precision using a fast-beam microwave-laser technique. The uncertainties in the measured intervals are less than 1 kHz (or about ${10}^{\mathrm{-}13}$ a.u.) for each of the 16 intervals. The measurements are compared with precise theoretical predictions and are used to test relativistic, QED, and retardation effects in these states. The measurements confirm the contributions from ‘‘retardation forces’’ to better than 10%.

The collisional-broadening rate coefficients, for both helium and argon buffer gases, have been measured for transitions from the metastable 6s5d ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$ levels in barium. A weak cw diode laser was used to populate the ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$ manifold through optical pumping of the 6${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$→6s6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$ transition, followed by radiative decay into the ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_{2,1}$ states. The absorption coefficient for the transition of interest was then measured by monitoring the transmission of a very weak cw probe laser beam as its frequency was scanned over the line shape. Such scans were carried out for several diode (pump) laser powers, and using the standard line-shape normalization integral, we were able to construct density-independent line shapes. Each spectral line shape consists of many hyperfine components due to the presence of five barium isotopes (including two with nonzero nuclear spin). Each component is affected by Doppler, pressure, and natural broadening and, therefore, can be represented by a Voigt function. Thus, the measured line shapes (recorded with 50, 100, 200, and 500 Torr of either He or Ar buffer gas) were fitted with a sum of Voigt functions in which the only free parameter was the pressure-broadening rate. Using a least-squares fit of the broadening rates versus buffer-gas number density, the broadening-rate coefficients ${\mathit{k}}_{\mathrm{br}}$ were found. In addition, collisional line shifts for these transitions were measured, and shift rate coefficients ${\mathit{k}}_{\mathrm{shift}}$ were determined.

We conduct a numerical experiment on a regular system which typically describes molecular vibrational modes. We find that randomly missing levels do not affect spectral properties significantly, but the effect of levels missing systematically in quantum number space can distort the statistics considerably, yielding results apparently consistent with the Berry-Robnik prediction for mixed systems. This observation underscores the important point that misleading interpretation of data is possible, and is of particular relevance in an experimental context.