We present a theoretical description of the formation of excited states H(n) due to the interaction of relativistic ${\mathrm{H}}^{\mathrm{-}}$ ions with thin foils. The classical transport theory based on a Monte Carlo solution of a Langevin equation is generalized to relativistic speeds of the projectile. Relativistic corrections are introduced to both collision dynamics and kinematics. The n distribution of excited states, H(n), is studied as a function of the foil thickness and compared with recent experimental data. © 1996 The American Physical Society.

Measurements of ${\mathrm{H}}^{-}$ stripping and ${\mathrm{H}}^0$ excited-state production for a wide range of foil thicknesses and experimental conditions are reported. An 800-MeV ${\mathrm{H}}^{-}$ beam was passed through carbon or aluminum oxide foils of thicknesses ranging from 10 to 550 μg/${\mathrm{cm}}^2$ and the excited states produced were analyzed by field stripping in a special magnet downstream of the foil. The foil thicknesses were independently determined. The ${\mathrm{H}}^0$ atoms emerging in excited states with $n>\mathrm{}2\mathrm{}$ can be stripped to protons in fields of up to 1.3 T. The yield of excited states as a function of foil thickness and the cross sections for the various interactions are presented. The cross-section ratio of double to single ionization of ${\mathrm{H}}^{-}$ in carbon is found to be (1.8±0.9)%.

The 4d absorption spectra of I, ${\mathrm{I}}^{+}$, and ${\mathrm{I}}^{2+}$ have been observed in the 45–300-eV region using the dual laser-produced plasma technique. At lower energies each spectrum is dominated by intense 4d→5p transitions, which are identified with the aid of multiconfiguration Hartree-Fock calculations. The excited states decay by direct autoionization involving 5s or 5p electrons and rates for the different processes and resulting linewidths were calculated and compared, where possible, with experiment. In the case of neutral iodine 4d→6p transitions are also observed in the region close to the ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{5/2}$ threshold. The dominant feature in each spectrum, however, arises from 4d-f excitation, which evolves from a 4d-ɛf shape resonance in neutral iodine to a feature containing discrete 4d→nf transitions in ${\mathrm{I}}^{2+}$. © 1996 The American Physical Society.

A fully relativistic distorted-wave Born procedure for collision strength of highly charged ions is presented. Multiconfiguration Dirac-Fock wave functions and intermediate coupling are used to describe the target-ion states, and the relativistic continuum wave functions are calculated using a hybrid mesh point in the field of frozen target-ion charge distribution with the Dirac-Fock-Slater exchange potential. Also a normalization procedure of the continuum wave function is used. The factorization form of collision theory of Sampson and Zhang [Phys. Rev. A 45, 1657 (1992)] is applied. The calculations of collision strengths of the ground state to singly excited states with an N-shell electron for nickel-like Gd and U ions are done and the results are compared with other work. There are several detailed considerations in our procedures, so the present results should be more reliable and accurate. © 1996 The American Physical Society.

Two theoretical treatments have been employed to investigate various angular distributions of ejected electrons following the photoionization of the msns Rydberg states in the alkaline-earth atoms. The specific analytic expressions of angular distributions in terms of reduced dipole matrix elements have been carried out. The characteristics of angular distributions are interpreted in detail. © 1996 The American Physical Society.

We show that two factors contribute to the forward-backward angular asymmetry in low-energy electron emission by ion impact: the deviation of the target potential from a pure Coulomb potential and the two-center effect. We perform calculations with various theories that include these two effects: the continuum-distorted-wave–eikonal-initial-state (CDW-EIS) and the CDW approximations based on distorted-wave perturbation theory and a close-coupling calculation using a discrete representation of the continuum. The various theories give consistent results on the asymmetry but discrepancies remain between theory and experiment. © 1996 The American Physical Society.

The photodetachment cross section of the ${\mathrm{O}}^{\mathrm{-}}$(2${\mathit{p}}^5$${\mathrm{}}^2$${\mathit{P}}^{\mathit{o}}$) ground state is studied near the O(2${\mathit{p}}^{43}$${\mathit{P}}_{{\mathit{j}}_{\mathit{c}}}$) fine-structure thresholds, using the eigenchannel R-matrix method, multichannel quantum-defect theory, and the recoupling frame transformation. We find that the cross section increases monotonically with energy near the lowest detachmnet thresholds, in contrast to the ${\mathrm{Se}}^{\mathrm{-}}$ photodetachment cross section studied earlier. Our calculations, which are based on ab initio methods to obtain multichannel scattering parameters, explain the near-threshold features in the ${\mathrm{O}}^{\mathrm{-}}$ photodetachment cross section observed in an earlier experiment. © 1996 The American Physical Society.

The linear polarization of resonance vacuum-ultraviolet radiation emitted by electron-impact excited Ne atoms has been measured in the energy range from threshold to 21 eV. An electron resolution of 80 meV has enabled the effects of negative-ion resonances on the polarization function to be examined in some detail. Predictions of the effects of negative-ion resonances on the observed polarizations have been made using a generalized Baranger-Gerjuoy theory. Excitation via electron exchange is shown to be the dominant process very close to threshold. © 1996 The American Physical Society.

We present a geometry for the investigation of the coplanar energy sharing (e,2e) collision processes at relativistic energies. In the coplanar constant ${\mathrm{\Theta }}_{1,2}$ geometry the angle between the detectors is held fixed, while both detectors are rotated around the normal to the scattering plane. This geometry is particularly suited for the investigation of the interaction between the heavy atom and the electrons in the incident and final channels. We present a systematic study of the K-shell ionization of gold at 300 keV using the relativistic distorted-wave Born approximation, investigate the relative importance of different spin channels and the variation of the triple differential cross section with the relative angle between the two outgoing electrons. Even though we are considering the unpolarized beam incident on unpolarized targets, we are able to demonstrate clearly the role of Pauli blocking. We identify those features in the cross section which are most sensitive to distortion and demonstrate their enhancement with the increasing nuclear charge. Finally, we discuss their dependence on impact energies. © 1996 The American Physical Society.

The influence of the photon bandwidth on the line shapes of the resonant Auger electron spectra has been studied. The observed features have been explained as due to the Auger resonant Raman effect, which has been extended to cover the cases of nonmonochromatic excitation. Changes in the line shapes and also several specific effects, connected with detuned excitation, have been demonstrated by experimental examples, matched with model calculations. The lifetime interference phenomenon, appearing in the decay of several close-lying resonance states to the same final state, is regarded together with the influence of the limited photon bandwidth. Special attention is paid to the problems of data handling. © 1996 The American Physical Society.

A general formulation utilizing three-body kinematics was developed to analyze the final-state momentum distributions of the electron, the recoil, and the projectile ion, for the ionization process in ion-atom collisions. Information on ionization dynamics can be identified and analyzed from the perspective of momentum distributions. The mechanism of electron capture into the projectile continuum was found to contribute a finite value at the kinematical threshold in the longitudinal recoil-ion momentum distribution. Detailed calculations using the continuum distorted wave–eikonal initial state approximation are compared with two recent measurements on momentum distributions of the recoiling ion and the ionized electron in single ionization of He by protons and by highly charged projectiles. © 1996 The American Physical Society.

We report absolute measurements of a rank four state multipole for He(3${\mathrm{}}^1$D) excitation by electron impact with a polarized first-photon–second-photon coincidence technique. The presented technique permits the extraction of total (scattering angle integrated) partial cross sections ${\mathrm{\sigma }}_{\mathit{m}}$ for the excitation of He(3${\mathrm{}}^1$${\mathit{D}}_{\mathit{m}}$) magnetic substates (m=0,±1,±2). The present results stringently test existing theories and, for example, indicate that most theoretical calculations underestimate the partial cross sections ${\mathrm{\sigma }}_0$ and ${\mathrm{\sigma }}_1$ and overestimate the ${\mathrm{\sigma }}_2$. The recent convergent close-coupling calculations of Fursa and Bray [Phys. Rev. A 52, 1279 (1995)] are in excellent agreement with all our measured data. © 1996 The American Physical Society.

A modified numerical scheme for the evaluation of Compton profiles in a weak laser field, based on an expression by Jain and Tzoar [Phys. Rev. A 18, 538 (1978)], is derived. The scheme is based on the reciprocal form factor and is computationally preferable to the original method. The scheme requires either a numerical Fourier transformation or the calculation of a one-dimensional integral, both of which are easily achieved. We focus on the numerical integration, which is readily performed with a Gauss-Chebyshev quadrature. Stable results may be obtained by avoiding numerical derivatives. Some numerical tests are performed to study the convergence properties of the old and modified methods. The modified scheme is applied to study some atomic and molecular systems. The dependence of the profile modulation on the atomic number Z throughout the periodic table is established. © 1996 The American Physical Society.

Two simple, one-dimensional models of a molecule are exploited to show how energy can be transferred from the electronic excitation to the vibrational modes or vice versa, when the molecule is subject to short, high-intensity pulses of laser radiation. The Born-Oppenheimer approximation is assumed to be valid even in these extreme conditions, and the nuclear motion is described in terms of wave packets. The time-dependent dynamics of these systems reveals that electronic transitions are not bound to occur at the end points of the nuclear oscillation, as they would according to the Franck-Condon principle. Moreover, the interaction with the field determines a sudden jump of the nuclear kinetic energy across the laser pulse. In the process, energy can be absorbed from, or released to, the nuclear kinetic degrees of freedom, depending on the delay of the pulse from the time in which the nuclear oscillation reaches its maximum elongation. This kind of intramolecular energy transfer occurs when the separation of the electronic levels varies with the nuclear coordinate, as is usually the case in real molecules and exemplified by the second molecular model. In the first model, on the other hand, the electronic level separation is independent of the nuclear coordinate, and the molecular kinetic energy shows continuity across the laser pulse. © 1996 The American Physical Society.

The cross sections for low-energy scattering of muonic hydrogen on hydrogen molecules are calculated in a semiclassical approach. The molecular binding is described with the ‘‘mass-tensor approach’’ and the electronic screening corrections are calculated in the Glauber approximation. This rather simple computational scheme is applied to the scattering of dμ on ${\mathrm{H}}_2$ and ${\mathrm{T}}_2$ and to the scattering of pμ and tμ on ${\mathit{D}}_2$. The results obtained are compared to the values reported by Adamczak et al. [Muon Catalyzed Fusion 7, 309 (1992)]. © 1996 The American Physical Society.

The dissociative attachment process in low-energy collisions [${\mathrm{Na}}_2$(${\mathit{v}}^{\mathrm{″}}$,${\mathit{j}}^{\mathrm{″}}$)+${\mathit{e}}^{\mathrm{-}}$→ Na + ${\mathrm{Na}}^{\mathrm{-}}$, with electron energies <1.0 eV] is studied as a function of vibrational excitation over a broad range of excited levels. We employ a crossed electron-molecule beams arrangement and two optical methods for preparing vibrationally excited states: Franck-Condon pumping (FCP) as well as the efficient and highly state-specific technique of stimulated Raman scattering with adiabatic passage (STIRAP). Both methods are capable of preparing vibrational levels with energies up to about 60% of the ${\mathrm{Na}}_2$${\mathit{X}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ binding energy (${\mathit{v}}^{\mathrm{″}}$≤30). Analysis of the FCP data shows an increase of more than three orders of magnitude in the state-dependent dissociative attachment rate as a function of the vibrational level up to ${\mathit{v}}^{\mathrm{″}}$=12. For ${\mathit{v}}^{\mathrm{″}}$≳12, which is close to the exoergic threshold, no further increase with ${\mathit{v}}^{\mathrm{″}}$ is observed. The dissociative attachment rates measured for single rovibronic states using the STIRAP technique confirm the validity of the conclusions drawn from our analysis of the FCP measurements. The potential curve of the ${\mathrm{Na}}_2^{\mathrm{-}}$${\mathit{A}}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ state, which is essential for the dissociative attachment process, has been determined from variational valence shell multireference configuration interaction calculations in its bound region but by imposing Feshbach projection in its resonance region. The crossing of this potential with that of the ${\mathrm{Na}}_2$(X) state is found between ${\mathit{v}}^{\mathrm{″}}$=11 and ${\mathit{v}}^{\mathrm{″}}$=12, in full agreement with the conclusions from analysis of the experimental data. The variation of the theoretical enhancement factors with the electron energy, calculated in the framework of the traditional resonance theory, are also reported. After convolution with the electron flux distribution, good agreement with the experimental enhancement factors is found. © 1996 The American Physical Society.

In a previous paper it has been shown that the longitudinal recoil-ion momentum (RIM) distribution for the single ionization of atoms by charged ions exhibits a finite value at the kinematic threshold. This structure can be ascribed to the divergence of the electron capture to the continuum peak in the ionization cross section. We generalize this result for the case of a neutral projectile. In particular we show that a sharp peak of the RIM distribution at the threshold can occur for given final-state electron-projectile systems. © 1996 The American Physical Society.

We present results for low- and medium-energy elastic and capture cross sections for positronium-atom–alkali-ion scattering using the coupled static close-coupling approximation. © 1996 The American Physical Society.

Angular distributions for the electrons produced by the decay of the 4d→6p excitations in xenon have been measured using an undulator radiation source. The resolution employed in the experiment has allowed us to derive separate β values for features which had not been isolated previously, as well as to determine β values for previously unreported lines. Comparison is made between existing experimental and theoretical results. © 1996 The American Physical Society.

A detailed experimental and theoretical study of the interaction of very-low-energy electrons with the polar target HF is reported. This interaction is investigated experimentally by measuring rate constants for ionization and state changing in collisions between K(np) Rydberg atoms with 90≲n≲400 and HF. The data are analyzed using the results of rotational close-coupling calculations that include possible effects associated with dipole-supported real or virtual states. A comparison of theory and experiment suggests that low-energy-electron–HF scattering is strongly influenced by the presence of a dipole-supported virtual state with an energy, for J=0, of ∼1–1.5 meV. The present work points to the importance of dipole-supported states in electron–polar molecule scattering. © 1996 The American Physical Society.