Electron-impact excitation of the ${\mathit{L}}_{2,3}$${\mathit{M}}_1$${\mathit{M}}_1$ Auger process in magnesium has been observed in a triple-coincidence experiment. A 3.5-keV incident electron is used and the scattered electron, the ejected 2p electron, and the 35-eV Auger electron are detected. The experiment was performed for ejected electron energies of 35, 45, and 100 eV. Three-dimensional angular distributions for the ejected and Auger electrons were recorded for a fixed scattered-electron direction. A model is presented which describes the angular distribution by additive terms representing the anisotropy in the distribution of ejected and Auger electrons relative to the momentum transfer direction, and the angular correlation between the ejected and Auger electrons. Three corresponding parameters are extracted from the data and compared with the results of a calculation based on a two-step model of ionization and Auger emission.

A comprehensive survey is presented of the behavior of all correlations ${\mathit{C}}_{\mathit{i}\mathit{j}}$ between the polarization of the incoming photon and the spin of the outgoing photoelectron, for ejection from all ${\mathit{np}}_{\mathit{J}}$ subshells of uranium for photoelectron energies ranging from 1 eV to 100 keV. The numerical results for these polarization correlations are obtained within the independent-particle approximation in a relativistic self-consistent atomic field. At low photon energies the dipole predictions are generally valid, predicting large degrees of spin polarization of photoelectrons down to the threshold for all ${\mathit{np}}_{\mathit{J}}$ subshells with a specific J, even for the outer shells where the spin-orbit coupling is weak. At some energies where the dipole correlations vanish, deviations from dipole predictions may be visible even at low photon energies. As the photon energy grows higher, multipoles eventually dominate and the dipole-type symmetry in the ejection angle is lost, while the curves of a given correlation for the initial states with the same total and orbital angular momenta and different n merge into a common curve. We also discuss the comparison with the usual nonrelativistic dipole limit, in which summing over the J=1/2 and J=3/2 substates results in np photoionization with no spin polarization. In the relativistic case the spin-orbit effects, magnified in the differences in the positions of Cooper minima for the outer shells at low energies, lead to finite spin polarization of photoelectrons even after summing over J.

The periodic trends in the vibrational structure of core-level photoelectron spectra are studied by multiconfiguration self-consistent-field ab initio and local-density-functional calculations on ${\mathrm{SiH}}_4$ and ${\mathrm{GeH}}_4$. Contributions from both symmetric stretching and bending vibrations to the vibrational structure are studied. These periodic trends are rationalized by the screening of the core charge, and in terms of electrostatic and relaxation contributions to core ionization, which determine the change in the M-H (M=Si,Ge) bond length.

We have measured the total photoattenuation cross section of helium for photons in the energy range of 3–14 keV. At these energies, the photoionization cross section is rapidly decreasing, so that Compton scattering is significant at 4 keV and dominates at the highest energies. Our measurements verify the dominance of Compton scattering in this energy range, and its importance in recent measurements of the ratio of double to single photoionization of helium. The measured cross sections are close to the combined calculated cross sections for Compton scattering and photoionization; we are able to distinguish the contributions of the two effects.

A simple optical experiment is discussed that implements the ‘‘quantum time-translation machine’’ suggested by Aharonov et al. [Phys. Rev. Lett. 64, 2965 (1990)] for photons. The time-translation effect was observed experimentally as the shift of the fringe pattern in a modified Mach-Zehnder interferometer. The possibility of describing the effect classically, in terms of Maxwell’s equations, suggests an alternative interpretation that does not invoke time-translation effects.

A many-body-theory (MBT) formulation for electron scattering by excited electronic states is applied to excitation from the 2 ${}_{}{}^{1,3}{}_{}{}^{}$S metastable states of helium. The lowest-order MBT for these transitions involves a ‘‘chanel coupling’’ to the ground state of helium as well as a distortion of the free-electron wave function by the ground state of the target, while the distorted-wave approximation (DWA) involves a distortion of the incident and scattered electron wave functions by the initial and final target states, respectively. Results using the first-order many-body theory and DWA are reported here for integral and differential cross sections for excitation from the 2 ${}_{}{}^{1,3}{}_{}{}^{}$S states to (2,3) ${}_{}{}^{1,3}{}_{}{}^{}$P, 3 ${}_{}{}^{1,3}{}_{}{}^{}$S, and 3 ${}_{}{}^{1,3}{}_{}{}^{}$D states, and are compared with published experimental and theoretical data.

Measurements are presented for the annihilation rates of thermalized positrons in a variety of substances, including noble gases, simple inorganic molecules, hydrocarbons, substituted hydrocarbons, and aromatics. The measurements were performed using trapped clouds of room-temperature positrons, into which substances under test were introduced as low-pressure gases, so that only two-body interactions were involved. These data are compared with other values in the literature, and a compilation of annihilation rates is presented. The measuremetns illustrate the importance of both chemical composition and the vibrational modes of excitation of the molecules in determining the annihilation rates. The anomalously high annihilation rates observed for large molecules provide evidence for the existence of long-lived resonances. The nature of these resonances is not yet understood, and the data presented are expected to provide useful constraints for the development of theoretical models.

We present Breit-Pauli R-marix results for the resonance-enhanced 4${\mathit{s}}^2$${(}^1$${\mathit{S}}_0$)→4s4p${(}^3$${\mathit{P}}_{0,1,2}$) excitation cross section in ${\mathrm{Kr}}^{6+}$. A comparison with a recent merged-beams experiment [Bannister et al., Phys. Rev. Lett. 72, 3336 (1994)] is made in the near-threshold region. From a 14-level calculation, we find fairly good agreement. The resonance structure in this region is predominantly due to 4s4p${(}^1$P)nl(n=9,10) contributions, which are strongly perturbed by the 4s4d4f configuration. A sensitivity of the resulting cross section to the exact positions of these perturber resonances is demonstrated by varying the 4s4d${(}^{1,3}$D) target thresholds.

We describe a method for calculating cross sections for double ionization of a two-electron atom (or ion) by one or more photons, and we present results of an application to double ionization of He by one photon. The method does not require the explicit inclusion of the asymptotic boundary conditions for two-electron escape; rather, the contribution for the single ionization channel to the inclusive rate is removed by using the projection operator that projects onto this channel. Energy and angular distributions for double ionization can be obtained by modifying this projection operator.

We derive geometric electriclike and vectorlike potentials arising in studies of the Berry phase and the Born-Oppenheimer approximation, using a local inertial frame. The potentials result from a careful averaging over fast degrees of freedom. This method is applied to two exactly integrable models: coupled harmonic oscillators and a fast gyroscope, which illustrate the general results.

We apply the convergent close-coupling (CCC) method to calculation of 40-eV electrons scattering on the ground state of helium. We present the differential cross sections up to the n≤3 levels, as well as the electron-impact coherence parameters for the 3 ${}_{}{}^1{}_{}{}^{}$D state. We find our results to be in excellent agreement with the measurements. It is shown that at this energy treating the target continuum has a large effect on the presented results, and for this reason the CCC theory is the only one that is able to obtain agreement with experiment. Integrated, total ionization, and total cross sections are also presented, and are found to be in excellent agreement with experiment.

A method is proposed for studying collisions between two atoms or ions with one or more electrons each. An effective charge density as a function of a physically unobservable impact parameter b→’ is introduced to describe the spatial nonlocalization of the electron cloud of the projectile atom. It is shown that the probability amplitude at a given (real) impact parameter may be reduced to a convolution over b→’ of the effective charge density with target excitation amplitude per unit charge. This method, which we call the virtual-impact-parameter method, enables one to identify in a physical way in impact-parameter space the contribution of the electron-electron interaction as well as that of the electron-nucleus interaction. A node structure in the target excitation probability amplitude is found due to the electron-electron interaction. Generalization of the method to the description of many-electron transitions is discussed within the independent-electron approximation.

The eigenchannel R-matrix approach, in conjunction with the multichannel quantum-defect theory and the LS→jj recoupling frame transformation, is used to calculate the photoionization spectrum of Al i below the 3s3p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ ionization threshold (photon energies in the range 0.44≤ħω≤0.98 Ry). Relativistic channel mixing is incorporated in the calculations by a recoupling frame transformation and by the inclusion of experimental fine-structure threshold energies. This mixing enables autoionization of resonances whose decay would otherwise be forbidden in the pure LS-coupling scheme. The calculated J-dependent energies and widths agree well with those of experimental resonances. The complicated relativistic spectrum, with up to 11 interacting channels, provides an experimentally realizable testing ground for studies of statistical properties of resonances. The spectrum below the 3s3p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ ionization threshold exhibits the Wigner and Porter-Thomas distributions of positions and widths of resonances, respectively. While portions of the aluminum spectrum appear to be random, according to these measures, there remains much underlying regularity in the level spacing and width distributions.

Kinetic energies and charge-state fractions of specularly reflected fragments have been studied at glancing-angle incidence of (0.8–2.5)-MeV ${\mathrm{HeH}}^{+}$ ions on a clean (001) surface of SnTe. It is shown that the kinetic energies of fragments agree well with those calculated from Coulomb repulsion of ${\mathrm{He}}^{+}$ and ${\mathrm{H}}^{+}$ fragments, while most of the outgoing fragments are bare nuclei. The final kinetic energies and charge-state fractions of fragments are explained by a computer simulation: ${\mathrm{HeH}}^{+}$ dissociates into fragments where at least one electron is retained in the He-H system and there follows electron loss from the fragments after most of the initial potential energy has been transferred to the kinetic energies of fragments. The cross section for dissociation of ${\mathrm{HeH}}^{+}$ ions in collisions with electrons at the surface is estimated to be of the order of ${10}^{\mathrm{-}16}$ ${\mathrm{cm}}^2$.

Electronic stopping power of aluminum, carbon, and krypton were calculated for energetic ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{+}$ ions together with a diproton (${2\mathrm{H}}^{+}$) and a triproton (${3\mathrm{H}}^{+}$) with specific energies ranging from 1 keV/amu to 10 MeV/amu in a frozen charge state, based on the degenerate electron-gas model and the wave-packet model. It was found that for ${\mathrm{H}}_2^{+}$ and ${2\mathrm{H}}^{+}$ ions a strong orientation effect can be theoretically predicted on the stopping power S and the effective charge ${\mathit{Z}}_{\mathrm{eff}}$. It was also clarified that the bound electrons significantly diminish the values of S and ${\mathit{Z}}_{\mathrm{eff}}$, and that the velocity dependence of ${\mathit{Z}}_{\mathrm{eff}}$ is considerably weaker than that expected from the existing data. The values of S and ${\mathit{Z}}_{\mathrm{eff}}$ calculated for ∼10 MeV/amu ${\mathrm{H}}_2^{+}$ ion incident on carbon foils are found to be in good agreement with the recent experimental data.

Two interferometric complementarities are formulated and demonstrated, and relations between them are analyzed. The first relates the distinguishability D(scrP) of the path of propagation of a particle (where scrP is the preparation of an ensemble) to the fringe visibility ${\mathit{v}}_1$ when amplitudes from two paths are combined. It is shown that [maxD(scrP)${]}^2$+${\mathit{v}}_1^2$=1, where the maximum is taken over all preparations compatible with a fixed density operator. The second complementarity relates the visibility of one-particle interference fringes to the visibility ${\mathit{v}}_{12}$ of two-particle fringes: ${\mathit{v}}_{12}^2$+${\mathit{v}}_1^2$≤1. With a suitable extension of operations on the pair of particles, this inequality is strengthened to an equality.

A recently presented formalism for the computation of energy-loss spectra of penetrating particles in the presence of charge exchange has been extended to include analytical expressions for energy-loss spectra in the diffusion approximation. For a two-state system the case of charge transfer in one direction only, as well as in both directions, is investigated. Expressions obtained for charge-state distributions are evaluated, using theoretically and experimentally determined cross sections.

Secondary-ion yields have been measured for Si and ${\mathrm{SiO}}_2$ targets bombarded by C, Si, Ge, and Ag projectiles over an energy range beween 0.4 and 10 MeV, where the atomic-collision process changes from a nuclear to an electronic one. Obtained yields of secondary ${\mathrm{Si}}^{\mathit{q}+}$ (q=1,2,3,4) ions for the C projectiles are generally decreasing functions of incident energy. On the other hand, the yields for Ag increase with increasing energy except for ${\mathrm{Si}}^{+}$. The possibility of multiple-charged recoil-ion production through the simultaneous process of ionization and recoil caused by the projectiles is discussed on the basis of an independent-electron model, which describes multiple ionization of atoms by energetic heavy-ion impact.

As an electron is freed from an atom by an intense laser it is accelerated by the oscillating field. Before escaping entirely, it may recollide with its parent ion. During this rescattering, the remaining electrons of the ion might be excited or ionized. The influence of the laser field on these impact excitation and ionization processes is investigated. Model calculations for helium at 780 nm and intensities near ${10}^{15}$ W/${\mathrm{cm}}^2$ show that the effect of the field on the total probability of promoting an electron from the ionic ground state is small. However, the laser alters the final excited-state populations, rapidly ionizing any excited bound states. We discuss the importance of these results in the search for the mechanism responsible for direct double ionization in the tunneling regime.

Three typical excited-state nonlinear absorption effects in molecular systems with weak ground-state absorption (at 532 nm) are experimentally demonstrated: (a) reverse saturable absorption (RSA) caused by strong triplet first-to-second excited-state absorption in a copper phthalocyanine solution with 15-ns laser pulses; (b) RSA caused by strong singlet first-to-second excited-state absorption in a ${\mathrm{C}}_{60}$ solution with 21-ps laser pulses; (c) the RSA-SA turnover caused by strong singlet first-to-second excited-state absorption and weak singlet second-to-third excited-state absorption in a metallo-porphyrin-like solution with 21-ps laser pulses. To simulate these excited-state nonlinear absorption behaviors we propose a general ten-level density-matrix model and its simplified models for the three cases. All simulations are in good agreement with the experimental results.