The effect of a low-frequency laser field on photoionization and radiative recombination processes is studied theoretically, with particular attention paid to the combined influence of the electron-ion Coulomb interaction and the laser field on continuum states of the system. A procedure for representing the asymptotic states of the system, with both long-range interactions properly accounted for, is described. The transition rates are strongly affected by the existence of a Rydberg series of resonances. Methods developed for describing similar resonant processs that occur in the field-free case are generalized here to include the effects of the external laser field. A simple model, based on parametrizations of the type employed in quantum-defect theory, is used to study the modification of resonant structures in the cross section introduced by the external field. In the weak-field limit, a low-frequency approximation for bound-state Compton scattering is obtained, with resonance effects accounted for.

Measurements and theoretical analyses of total electron-capture cross sections for collisions of ${\mathrm{N}}^{4+}$ with ground-state hydrogen (deuterium) are reported in the energy range 1–300 eV/u. The present measurements have reduced relative uncertainty compared to previous absolute measurements at Oak Ridge National Laboratory [Huq et al., Phys. Rev. A 40, 1811 (1989)] and are used for detailed comparison with more recent coupled-channel molecular-orbital calculations [Shimakura et al., Phys. Rev. A 45, 267 (1992); Zygelman et al., ibid. 46, 3846 (1992)]. The most striking difference between the calculations was the increasing trend in the cross section for collision energies below 4 eV/u, as estimated by Zygelman et al. using only singlet states of the quasimolecule and the decreasing trend predicted by Shimakura et al. using both singlet and triplet states. The latter decreasing trend is consistent with the previous measurement. At 0.5 eV/u, the predicted singlet cross sections differ nearly by a factor of 2. Possible origins for this difference are explored. Strong structure is predicted in the cross section of Zygelman et al. and, to a lesser extent, in that of Shimakura et al. ; however, the phase of the structure is different in the two theories. Structures observed in the measurement at the higher collision energies are not in complete harmony with either prediction.

We investigate the problem of ion–solid-target collisions by means of a time-dependent coupled-channel description using ab initio relativistic four-component linear-combination-of-atomic-orbital molecular-orbital (LCAO-MO) wave functions as a basis. Large-impact-parameter calculations allow us to determine how the L and M shells of initially low ionized projectiles are fully stripped within a few atomic layers of the solid target. Once the inner-L-shell vacancies are created, our narrow-impact-parameter calculations give very good agreement with experimental results on K-shell vacancy production. As an example, we compare our results with measurements of 40.6-MeV ${\mathrm{Ar}}^{4+}$ and ${\mathrm{Ar}}^{12+}$ ions on Ca solid targets.

The double photoionization of an atom is considered in the near-threshold domain. The angular correlation pattern is analyzed for an arbitrary sharing of the excess energy by the electrons. The motion over the hyperspherical radius R is treated semiclassically which casts the problem in a nonstationary form. For motion over the hyperspherical angle α the set of classical trajectories is considered. They converge to the same (R→∞) limit which corresponds to a chosen energy-sharing ratio. For each α(R) trajectory the quantum wave packet (over the correlation angle ${\mathrm{\theta }}_{12}$) is propagated from the inner zone to the region of free-electron motion. At the border of the reaction zone a realistic boundary condition is imposed. The cross sections exhibit a weak dependence on the energy-sharing ratio.

The problem of electron scattering by a heavy hydrogenic ion is formulated relativistically by using a simplified version of hole theory in which the effects of virtual-pair creation and exchange of transverse photons are ignored. An effective-potential formalism is developed and a minimum principle for the effective potential is derived, which is valid in a range of energies below the ionization threshold, through a generalization of the projection-operator procedure developed some time ago in the context of the nonrelativistic scattering problem. A version of the minimum principle is shown to hold, even in cases in which an infinite Rydberg series of autoionizing resonances lies below the scattering energy. Concepts derived from quantum-defect theory are used, in conjuction with the effective-potential formalism, to describe the effect of these resonances on the scattering. Positive-energy projection operators appear naturally in the theory. Their presence ensures that ‘‘variational-collapse’’ difficulties are avoided, as required for the validity of the minimum principle. Some care is given to the analysis of different features of the scattering theory associated with the appearance of these projection operators.

An extended Wannier-ridge model is put forward that reproduces essentially all available experimental data on the relative angular distribution of the electrons emitted in near-threshold double photoionization. Under the condition of equal-energy sharing by the electrons, the model is applicable to unexpectedly high values of the energy excess above the threshold. Depending on the character of the correlations in the initial state, complicated angular distribution patterns appear with the secondary structure.

The recoil longitudinal momentum and Q value for electron capture with and without accompanying target ionization were measured for high-energy ${\mathrm{O}}^{7+}$-${\mathrm{F}}^{8+}$+He collisions using coincident recoil momentum spectroscopy. Electron capture into the projectile K shell and L-and-higher shells is separated in the recoil longitudinal momentum. The spectra for capture show that capture to the L and higher shells dominates at energies from 0.6 to 3.7 MeV/u while capture to the K shell increases with increasing collision energy. The relative contribution of capture to the K shell is consistent with theoretical expectation. Transfer ionization is compared with single capture within the independent-electron approximation. The results show that transfer ionization can be interpreted as being due to independent processes of capture and ionization.

Three-photon resonance enhanced four-photon ionization of xenon shows preferential formation of ${\mathrm{Xe}}^{+}$${(}^2$${\mathit{P}}_{1/2}$) when intermediate ns[3/2${]}_1$ Rydberg states are excited in the three-photon step. Here, the nl[K${]}_{\mathit{J}}$ notation refers to ${\mathit{j}}_{\mathit{c}}$l coupling, in which the angular momentum of the ion core, ${\mathit{j}}_{\mathit{c}}$, is coupled to the orbital angular momentum of the Rydberg electron l to give K, which is then coupled to the Rydberg electron’s spin to give J. The primes following l denote states belonging to the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ core. Multichannel-quantum-defect theory identifies these intermediates as nearly pure Rydberg states belonging to the ${\mathrm{Xe}}^{+}$${(}^2$${\mathit{P}}_{3/2}$) core. An equivalent behavior is found for the 7s’[1/2${]}_1$ intermediate state which preferentially produces ${\mathrm{Xe}}^{+}$${(}^2$${\mathit{P}}_{3/2}$). On the other hand, for nd and nd’ resonances we observe final-state distributions which closely mirror the core character of the intermediate state. The suppression of the parent core channel for s-type Rydberg states is shown to be due to a Cooper minimum in the ns→ɛp transitio n amplitudes. As a consequence, minor components of the resonance state wave functions play a major role in the selection of the final ionization channel, in particular via the much larger amplitudes of nd’→ɛf’ transitions.

A fivefold differential cross section for electron-impact double ionization of Ne, Ar, Kr, and Xe is calculated for an incident energy of about 5.5 keV and low momentum transfer. The calculation is based on the first Born approximation. Correlation is taken into account both in the initial state and in the final state. The results obtained are in reasonably good agreement with the available experimental data. Many other typical situations are investigated in order to identify the best choices of the kinematical parameters for future experiments.

Integral cross sections have been calculated for the dipole-allowed transitions from X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ to the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ and the C ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{+}{}_{}{}^{}$ states in electron–hydrogen-molecule scattering at 60 eV. Experimental data and a universal function [A. Z. Msezane and I. A. Sakmar, Phys. Rev. A 49, 2405 (1994)], which can extrapolate the generalized oscillator strength through the nonphysical region to ${\mathit{K}}^2$ (momentum transfer squared) =0, have been used in the calculation. A better agreement between theoretical results and experimental data has been achieved by using the universal function in the range of small scattering angles (θ<10°). The results show that the extrapolation used by experimentalists underestimated the differential cross sections (DCS’s) at small scattering angles. Our calculation suggests that the universal function can be used to extrapolate the DCS for the electron-molecule scattering down to 0° at high energy.

We estimate the vacuum polarization (VP) correction to the Coulomb interaction in collisions of muonic atoms. It is shown that the VP effect, amplified by the low-lying virtual state ${\mathrm{\varepsilon }}_{\mathrm{\vartheta }}$∼10 eV, is of the order of ∼1–2 % in the S-wave cross sections for pμ+p collisions as ɛ≤${\mathrm{\varepsilon }}_{\mathrm{\vartheta }}$. The VP amplitude becomes comparable to the anomalously small pure Coulomb amplitude for the singlet tμ+t scattering as ɛ→0 and near the Ramsauer-Townsend minima in the dμ+p and tμ+p scattering.

We have implemented a multichannel configuration-interaction complete-active-space (MCCI CAS) approximation to study electron-correlation effects in molecular photoionization. This approach includes both target relaxation and correlation due to coupling between different asymptotic scattering channels. The method employs a complete-active-space configuration-interaction wave function to represent the bound target states where the configuration interaction is computed using a single-center expansion to evaluate all integrals. The scattering equations are then solved using the Schwinger variational method. We present results of a detailed MCCI-CAS-Schwinger study of the photoionization of molecular nitrogen in the photon energy region of 19–26 eV, including nine coupled electronic channels. The results are in good agreement with the available experimental data. In particular, we obtain theoretical cross sections including interactions with the ionization channel leading to the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$ state of ${\mathrm{N}}_2^{+}$. The theoretical results are found to have a resonance structure similar to that observed in the experimental photoelectron asymmetry parameters in the (3${\mathrm{\sigma }}_{\mathit{g}}$${)}^{\mathrm{-}1}$ ionization channel near 22 eV.

Triple- (TDCS) and double- (DDCS) differential cross sections have been calculated for single ionization in electron-helium collisions for asymmetric geometry at intermediate and medium high energies. The TDCS and DDCS results have been presented for different kinematical situations and have been compared with the corresponding experiments. In the present prescription, the final-state wave function involves the correlation between the two continuum electrons and satisfies the three-body asymptotic boundary condition (for asymmetric geometry), which is an important criterion for reliable ionization cross sections. The sensitivity of the ionization cross sections (particularly of the TDCS) with respect to the choice of the bound-state wave function of the He atom has also been studied, using two different forms of wave function of the He atom. The binary-to-recoil peak intensity ratio against momentum transfer in TDCS is found to be in closer agreement with the experiment for the simple Hylleraas wave function than for the Hartree-Fock wave function. The DDCS results are found to be in good agreement with the experimental data of Müller-Fiedler et al. [J. Phys. B 19, 1211 (1986)] for lower ejected energy (${\mathit{E}}_2$), while for higher ${\mathit{E}}_2$ the results are closer to the measurements of Shyn et al. [Phys. Rev. A 19, 557 (1979)] and Avaldi et al. [Nuovo Cimento D 9, 97 (1987)].

Absolute differential cross sections for superelastic scattering have been measured for the sodium resonant transition in the incident electron energy range 3–20 eV. Results are obtained with no calibration being needed and are related to time-reversed inelastic scattering with unpolarized particles. Comparisons with distorted-wave, close-coupling, and convergent-close-coupling calculations are presented.

The single and double K-shell vacancy fractions for swift O and Si ions inside Fe and Gd foils have been measured using the probe layer technique. The measurements are carried out at velocities varying from 7.5 to 13.5${\mathit{v}}_0$ for Si ions and at a velocity of 7.8${\mathit{v}}_0$ for oxygen ions (${\mathit{v}}_0$=αc where α is the fine-structure constant and c is the speed of light). It is shown that all such available data for light ions fall on a smooth curve when plotted against the reduced velocity of the ion. These values are used along with the existing transient magnetic field data to derive the electron spin polarization acquired by the ions traveling inside ferromagnetic hosts. The degree of polarization is shown to decrease with the atomic number of the ions. This observation is, however, in disagreement with recent theoretical calculations.

Projectile Kα satellite and hypersatellite x-ray yields were measured in collisions of Ar ions with energies of 43, 52, 65, 80, 95, and 300 MeV with targets of C, Mg, Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Nb, Mo, Sn, Nd, Tb, Ho, Ta, and W with atomic number ${\mathit{Z}}_{\mathit{t}}$=6–74. The intensity ratio of hypersatellite to satellite lines is reported as functions of the target atomic number ${\mathit{Z}}_{\mathit{t}}$ and incident energy ${\mathit{E}}_{\mathit{p}}$. The vacancy-sharing model was found to predict the ${\mathit{Z}}_{\mathit{t}}$ dependence of the intensity ratio at 43 MeV in near symmetric collisions. At energies above 80 MeV, the direct Coulomb processes, exspecially the electron-capture processes, determine the ${\mathit{Z}}_{\mathit{t}}$ dependence. Average L-vacancy probabilities are reported for projectiles at energies of 43–95 MeV that acquire single and double K vacancies in the targets.

The scattering of electrons by molecules is considered. The structure of the molecule is described in terms of the vibron model, which is an algebraic approach to the description of rotational and vibrational molecular excitations. The dynamics of electron-molecule collision is discussed in terms of coupled-channel calculations, where we assume an isocentrifugal approximation. In the sudden limit, where the excitation energies of the molecule are neglected, the coupled-channel system decouples into eigenchannels. The scattering amplitudes of the eigenchannels, characterized by 1/${\mathit{r}}^2$ potentials, are obtained analytically. The resulting probabilities of excitation of the molecular states and the limiting values of the quasielastic cross sections are evaluated in the U(3) and the O(4) limits. A comparison of the results for the differential cross sections with experimental data and other calculations is made for LiF and KI.

X-ray elastic and inelastic scattering factors for He through Ne ground states are computed from configuration-interaction wave functions constructed from Slater-type orbitals. The formalism for computing these scattering quantities from multideterminantal wave functions and open-shell systems has been specifically addressed. The reliability of the present tabulations has been checked with the energies through sum rules. The effect of core, intershell, and valence electron correlations is analyzed.

The rotational excitation of ${\mathrm{N}}_2$ and ${\mathrm{Cl}}_2$ molecules on electron impact is investigated over a wide range of incident electron energies (0.01–1000 eV). Two different excitation mechanisms are reported, the importance of which depends on the impact energy. At low electron energies only a few rotational quanta are exchanged, and the differential cross section decreases exponentially with Δj. At high electron energies the excitation spectrum shows a rotational rainbow, i.e., the differential cross secton has a maximum at a relatively high Δj. The location of this maximum depends on electron energy E and scattering angle θ. For intermediate energies there is an interplay of these two mechanisms. It can be seen that the contributions of both mechanisms can roughly be added to yield the cross section for the observed process.

We measured the destruction cross sections of fast ${\mathrm{H}}_2$ molecules (3.0≤v≤7.0 a.u.) in helium, neon, and argon targets. We also measured, complementing previously published data, the ${\mathrm{H}}_2^{+}$ destruction cross sections in neon for 3.0≤v≤7.0 a.u. and in helium and argon for v=3.0 a.u. The ${\mathrm{H}}_2$ beam was obtained from fast ${\mathit{H}}_3$ molecules dissociated in an auxiliary target. These ${\mathrm{H}}_2$ and ${\mathrm{H}}_2^{+}$ destruction cross sections were compared with the previous ones for ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_3^{+}$ ions and also with the H electron-loss cross section, and a simple description is able to explain quantitatively the observed trends for these four sets of experiments, giving also information about the main destruction channels for the ${\mathrm{H}}_2$ and ${\mathrm{H}}_2^{+}$ molecules.