Absolute K-shell ionization cross sections of B, O, and F have been obtained for incident ${\mathrm{He}}^{+}$ ion energies of 0.4–2.0 MeV in steps of 0.2 MeV. These ionization cross sections were obtained from Auger-electron yields measured for atomic boron in ${\mathrm{BF}}_3$; for atomic oxygen in CO, ${\mathrm{CO}}_2$, and ${\mathrm{O}}_2$; and for atomic fluorine in ${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$, ${\mathrm{C}}_2$ ${\mathrm{F}}_6$, and ${\mathrm{C}}_4$${\mathrm{F}}_8$. For F-containing molecules, the K-shell ionization cross sections per atom are found, both in this laboratory and at other laboratories, to decrease as the number of F atoms in the molecule increases. The experimental K-shell ionization cross sections are compared to existing experimental K-shell ionization cross sections as well as to various theoretical predictions. © 1996 The American Physical Society.

Ab initio calculations are carried out for the photoionization cross sections and oscillator strengths for Fe III in the close-coupling approximation employing the R-matrix method. A 49-state eigenfunction expansion for Fe IV, with states dominated by the ground configuration 3${\mathit{d}}^5$ and the excited configurations 3d${\mathrm{}}^4$4s and 3d${\mathrm{}}^4$4p, is employed to ensure an extensive treatment of autoionizing resonances and other electron correlation effects related to channel couplings that considerably affect the effective cross sections. Of particular interest are the wide resonances due to photoexcitation within the core states that enhance the background cross sections by orders of magnitude at high energies. Coupled wave functions are obtained for 805 bound states of Fe III below the first ionization threshold; all of the 199 LS terms observed experimentally have been identified. Photoionization cross sections, with detailed autoionizing resonances, are obtained for the computed bound states as a function of photoelectron energy. The present results also include the oscillator strengths for 11 979 transitions between the bound states of Fe III that lie below the ionization threshold. In addition, the partial photoionization cross sections for 239 bound states, with the residual ion in the ground state, are also obtained. These cross sections are computed for applications to the determination of electron-ion recombination rate coefficients and population of levels in collisional-radiative models that do not assume local thermodynamic equilibrium. The present radiative data are compared with the observed energy values, lifetimes, and the photoionization cross sections calculated by others. © 1996 The American Physical Society.

We have performed ab initio calculations of scattering length, differential, total, and momentum-transfer cross sections for elastic scattering of electrons from open-shell atoms; in particular, the chlorine atom. The polarization of chlorine atoms due to the scattered electrons and the electron correlation effects that are very important in the calculation are taken into account through the configuration-interaction procedure using the multiconfiguration Hartree-Fock method for continuum wave functions. Phase shifts for various partial waves calculated in this approximation have been used to calculate elastic differential, total elastic, and momentum-transfer cross sections for low energies ranging from 0–27.2 eV. The scattering length is calculated with wave functions computed exactly at zero energy. The results are compared with other available theoretical data. © 1996 The American Physical Society.

The linear polarization of resonance vacuum-ultraviolet vuv radiation emitted from collisionally excited helium atoms has been measured in the threshold region below 25 eV. The apparatus consisted of a hemispherically analyzed electron beam interacting with a gas jet; the normally emitted radiation was analyzed using reflection optics. The threshold polarization was measured to be unity, in agreement with arguments based on the conservation of angular momentum. The measured vuv excitation functions and corresponding polarization curve were compared to 11-state R-matrix calculations of the excitation of the 2${\mathrm{}}^1$P state and excellent agreement was observed up to the n=3 threshold. Negative-ion resonances and cascade had pronounced effects on the observed sublevel excitation functions and on the polarization. © 1996 The American Physical Society.

Retarded van der Waals forces involving electric quadrupole polarizabilities are investigated in detail in order to resolve the recently published conflicting predictions of the forms of dispersion potentials. One of the predictions was based on an approximate evaluation of the energies obtained from dispersion theory techniques. These techniques avoided explicit reference to the detailed structure of the individual systems. In applications to specific cases, the scattering amplitudes were related to the multipole polarizabilities in an approximate way. The second prediction used conventional nonrelativistic quantum electrodynamics in which a proper account of electronic structure was made. In this paper two particular cases involving an electric quadrupole polarizable body A are analyzed in detail. In one case the second body B has an electric dipole polarizability, and in the other case it has an electric quadrupole polarizability. For these two cases explicit expressions for the true intermolecular dispersion potentials are derived. The analysis shows that in general it is necessary to take into account the internal structures of the interacting systems, both their geometry and their dynamics, in order to obtain the complete form of van der Waals forces in terms of the multipole polarizabilities for the systems. © 1996 The American Physical Society.

By means of a crossed-beam technique, we have conducted absolute differential cross-section measurements for the excitation of ozone’s Hartley band by electron impact. The angular range covered was from 12° to 156°, while the impact energies employed were 7, 10, 15, and 20 eV. The angular distributions indicate that the Hartley band has a composite nature, arising from at least one optically allowed and at least one optically forbidden transition. Candidates for these transitions are indicated with help from the results of theoretical calculations. Unfortunately, aside from the well known 1${\mathrm{}}^1$${\mathit{B}}_2$←X${\mathrm{}}^1$${\mathit{A}}_1$ allowed transition observed in optical spectra, definitive assignments for these transitions cannot yet be made. Integrated cross sections were calculated from the measured differential cross sections. The former exhibit a maximum near 15-eV impact. © 1996 The American Physical Society.

This paper reports measurements of absolute differential cross sections for the direct scattering of oxygen atoms by He, Ne, Ar, Kr, Xe, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, CO, ${\mathrm{CO}}_2$, ${\mathrm{H}}_2$O, ${\mathrm{SO}}_2$, ${\mathrm{NH}}_3$, ${\mathrm{CH}}_4$, ${\mathrm{CF}}_4$, and ${\mathrm{SF}}_6$ targets. The measured cross sections include contributions from all elastic and inelastic processes that result in a fast neutral oxygen atom product. Cross sections are presented for 0.5- and 1.5-keV projectile energies over the laboratory angular range 0.2°–5°. When compared in the center-of-mass reference frame, these cross sections exhibit a high degree of similarity in both amplitude and angular dependence. The cross sections for ${\mathrm{N}}_2$, CO, ${\mathrm{CO}}_2$, and ${\mathrm{H}}_2$O are inverted using a partial-wave analysis to yield empirical interaction potentials, which can then be used to extrapolate the measurements down to lower energies. Using these potentials, cross sections are evaluated at 0.1 keV. © 1996 The American Physical Society.

Using a cryogenic hydrogen maser, suitably modified to have electronic control of both the resonance frequency and the quality factor of the external cavity, we have measured a number of spin-exchange parameters for an atomic-hydrogen (H) gas at a temperature of 0.5 K. These results are relevant to the ultimate achievable frequency stability for cryogenic H masers and, when coupled with accurate calculations of the spin-exchange parameters, serve as a sensitive test of the H-H interatomic potentials. We find evidence for a frequency shift not predicted by semiclassical theories of spin exchange. In the context of a fully quantum mechanical hydrogen-atom spin-exchange theory [B. J. Verhaar et al., Phys. Rev. A 35, 3825 (1987) and J. M. V. A. Koelman et al., Phys. Rev. A 38, 3535 (1988)], this frequency shift is attributed to the influence of hyperfine interactions during spin-exchange collisions. Our findings are generally in agreement with these predictions; however, the sign of the hyperfine-induced frequency shift appears to differ from theory. © 1996 The American Physical Society.

A numerical method for the solution of the time-dependent Dirac equation to describe atomic processes in relativistic heavy-ion collisions is presented. Different from previous approaches found in the literature, we are working entirely in momentum space to propagate the electron wave function in time. Due to the localization of the electron in momentum space the wave function can be confined to a finite discretization volume without the stringent violation of the boundary conditions which is encountered in configuration-space methods. From the final state, we can extract probabilities on inner-shell ionization, excitation, electron transfer, and bound-free pair production. Results are presented in the energy range from 0.24 to 10 GeV/nucleon. At 0.24 GeV/nucleon we demonstrate that our formalism incorporates both the ionization and the transfer channel, which are equally important at this energy. At higher energies from 0.93 to 10 GeV/nucleon we focus on ionization and bound-free pair production. We find that the enhancement of bound-free pair production as compared to perturbation theory is much smaller than reported previously by others. © 1996 The American Physical Society.

We have measured the polarization of fluorescence emitted by the noble gases He, Ne, Ar, Kr, and Xe following impact excitation by polarized electrons. In He, the $3^3$P→$2^3$S transition was studied; in the heavy noble gases the ${\mathit{np}}^5$(n+1)p${\mathrm{}}^3$${\mathit{D}}_3$→${\mathit{np}}^5$(n+1)s${\mathrm{}}^3$${\mathit{P}}_2$ transitions were analyzed. We investigated these transitions as candidates for efficient optical electron polarimetry and found that, because of their larger excitation cross sections and analyzing power, the heavy noble gases are superior to He, which had been used previously as a polarimetric target. Several issues with regard to the implementation and accuracy of optical electron polarimetric techniques are discussed. © 1996 The American Physical Society.

Proton-induced electron emission from sputter-cleaned thin carbon targets was investigated under ultrahigh vacuum conditions using time-of-flight energy analysis. These data emphasized the low-energy portion, e.g., ${\mathit{E}}_{\mathit{e}}$<20 eV, of the spectrum. Energy distributions of the emitted electrons were measured as a function of the emission angle, impact energy, and foil thickness. © 1996 The American Physical Society.

The electronic energy loss for low-velocity protons channeled in the 〈100〉 direction of single-crystal Au is calculated. The spatial distribution of valence electronic density in Au is calculated by using the tight-binding linear muffin-tin method [O. K. Andersen and O. Jepsen, Phys. Rev. Lett. 53, 2571 (1984); O. K. Anderson, Z. Pawlowska, and O. Jepsen, Phys. Rev. B 34, 5253 (1986)]. The proton trajectories are determined by numerical integration of the classical equation of motion, and the energy loss is evaluated by using a quantum-mechanical transport cross-section approach with a self-consistent model based on Friedel’s sum rule. The results allow us to describe the nonlinear behavior of energy loss with ion velocity observed experimentally. © 1996 The American Physical Society.

We have directly determined the impact-parameter-dependent stopping powers for 2.0- and 2.5-MeV He ions passing through GaAs single crystals. The points reside in the preparation of the single-crystal sample with a dopant of a heavy element located in some definite interstitial site at definite depth and in synthesis of a Monte Carlo program to simulate accurately the ion trajectories. Er-doped homoepitaxial GaAs layers grown by molecular-beam epitaxy and by metal-organic chemical-vapor deposition were used for this purpose. As previously reported, fine single-crystal clusters of ErAs are formed in the GaAs host and Er takes the position exactly equivalent to the tetrahedral interstitial site. The present Monte Carlo simulation has revealed the fact that some definite impact-parameter region dominates the backscattering Er peak position and this region shifts continuously by tilting the incident beam axis slightly from a major crystal axis. The results obtained are compared with the Oen-Robinson [Nucl. Instrum. Methods 132, 647 (1976)] model and other theoretical predictions and clearly show that inner-shell excitations and ionizations contribute significantly to the stopping power even for large impact parameters. © 1996 The American Physical Society.

We consider a waveguide for cold neutral atoms with a magnetic moment proportional to their spin angular momentum. The waveguide consists of a thin wire carrying a constant current. For the spin-1 case, we find bound states for the two-dimensional part of the motion around the wire, and present numerical and approximate analytic results for these bound states. In an experiment, the bound states can decay due to various effects, and we calculate the time scales involved. © 1996 The American Physical Society.

Saturated absorption spectroscopy linewidths down to half the natural linewidth have been obtained for the sodium ${\mathit{D}}_1$ transition in a strongly absorbing gas cell. The strong-line-narrowing effect is achieved by using an experimental ‘‘tandem’’ approach with two absorbing cells in series, where the saturated absorption output from the first cell is used as an input beam for a saturated absorption measurement in the second cell. A physical interpretation of the line-narrowing mechanism in highly absorbing media is presented, which, among other things, explains why the narrowing effects are different for crossover transitions. It is further argued that in these experiments the line-narrowing effect is caused by optical pumping and can be appropriately described by rate equations. This is different from several other recent papers on high-contrast transmission spectroscopy, where full Bloch vector models were necessary to fully describe the line shapes. Here a rate equation model, which includes two spatial dimensions, is developed, and reasonable agreement is obtained between experimental and theoretical data. © 1996 The American Physical Society.

Collisions between laser-cooled metastable xenon atoms in the presence of laser fields provide a good experimental test for ultracold collision theories as the observed ionization can be directly linked to the atomic binary collisions. Hyperfine structure is not present for the even isotopes ${\mathrm{Xe}}^{132}$ and ${\mathrm{Xe}}^{136}$, which simplifies the theoretical work. We present our results and show how these and the results presented in Walhout et al. [Phys. Rev. Lett. 74, 605 (1995)] can be interpreted with suitably adapted two-state models. Our approach suggests one possibility why the strong shielding of collisions predicted by the simple two-state model of Suominen et al. [Phys. Rev. A 51, 1446 (1995)] has not been achieved in the xenon experiment. © 1996 The American Physical Society.

We investigate the characteristics of the coherent population trapping (CPT) states in a dense medium. We incorporate the local-field effects into the density-matrix equations and numerically integrate the resulting nonlinear equations. We find that the CPT state is essentially unaffected by the local-field effects for all ranges of field strengths; however, states close to the CPT state are sensitive to them. We demonstrate a dispersive behavior near the CPT state in a dense medium besides important asymmetries and shifts of the Autler-Townes peaks in absorption spectra. We also study the effect of local fields on the dynamics before CPT occurs and show that the evolution to the CPT state is delayed due to the local fields. We extend our calculation to analyze lasing without inversion in a dense medium of Λ systems. © 1996 The American Physical Society.

We investigate the mechanical potential of an atom due to a Raman transition. This is a generalization of the usual two-level dipole potential. With sufficiently large one- and two-photon detunings the forms of the potential and excitation probability are the same as for the two-level case, but with a scaled detuning. This allows greater effective blue detunings to be accessible with the Raman scheme. This means that with the same available laser power, atoms can move coherently in the light field for many orders of magnitude longer without spontaneously emitting. Furthermore the sign of the potential may be controlled independently of the one-photon detuning. © 1996 The American Physical Society.

We have confined magneto-optically trapped cesium atoms to densities of nearly ${10}^{12}$ ${\mathrm{cm}}^{\mathrm{-}3}$ by directing most of the population to a hyperfine level which does not interact with the trapping light. This compression represents almost an order of magnitude improvement on the best density we have been able to attain in a conventional magneto-optical trap. In previous work such improvements in density have only been reported for much lighter sodium. We discuss and demonstrate the difficulties in applying the technique to a heavy alkali-metal atom and the limits that are imposed on the best combination of density and temperature that can be produced. We present a simple model that accurately reproduces the density behavior, and justify its basic assumptions by Monte Carlo simulation. © 1996 The American Physical Society.

We present an analytic theory of three-dimensional Trojan (stable nonspreading) wave packets. The theory is valid beyond the harmonic approximation presented previously. We show that electron wave packets that are both radially and angularly well localized can be generated from an angularly completely delocalized state (a circular Rydberg state) by adiabatic switching of a circularly polarized electric field. Confinement of the Rydberg electron results from the suppression of the nonlinearity of the Coulomb spectrum, the same nonlinearity that is responsible for wave-packet spreading and revivals in the absence of the applied field. A comparison with the harmonic approach is also presented, as well as the results of numerical experiments based on integration of the time-dependent and field-dependent Schrödinger equation for the electron. © 1996 The American Physical Society.