An analysis of the close-coupling method as it is traditionally formulated for electron-scattering problems shows that it can give rise to spurious resonances at intermediate energies when multiconfiguration target states are used. These resonances are associated with excited states of the target that are not explicitly included in the close-coupling expansion. We develop a modification of the close-coupling technique that allows one to isolate and systematically eliminate terms from the total wave function which are responsible for the spurious behavior. We have applied this technique to the study of electron-impact dissociation of ${\mathrm{F}}_2$ through its two lowest excited states. The complex Kohn variational calculations we have performed on this system incorporate accurate, multiconfiguration descriptions of the target wave functions.

Electron-impact dissociative ionization of tetrafluoromethane (${\mathrm{CF}}_4$) was studied with the use of a pulsed electron beam time-of-flight apparatus. The absolute partial ionization cross sections of ${\mathrm{CF}}_3^{+}$, ${\mathrm{CF}}_2^{+}$, ${\mathrm{CF}}_3^{2+}$, ${\mathrm{CF}}^{+}$, ${\mathrm{CF}}_2^{2+}$, ${\mathrm{F}}^{+}$, and ${\mathrm{C}}^{+}$ were measured from threshold up to 500 eV. The total ionization cross section was obtained by charge weighted summing of all the observed partial ionization cross sections. A total cross section for dissociation into neutral fragments was inferred from our total ionization cross section and the total dissociation cross section of Winters and Inokuti [Phys. Rev. A 25, 1420 (1982)]. The present results for the partial ionization cross sections are as much as 9% (${\mathrm{CF}}_3^{+}$) to 81% (${\mathrm{F}}^{+}$) higher than the previously published absolute measurements of Stephan, Deutsch, and Märk [J. Chem. Phys. 83, 5712 (1985)] at 80 eV, but are in agreement with their recently revised estimates for the singly charged ions. We also found that dissociative ionization was a dominant process for electron-impact energies above 30 eV, accounting for 85% of the total dissociation cross section at 80 eV.

We describe close-coupling calculations for photoionization and recombination using the R-matrix method. Photoionization of a large number of bound states of each atom or ion is considered, and cross sections are computed at a fine mesh of energies to delineate the extensive resonance structures. Photoionization of ground and excited configurations with n≤10 and l≤3 is considered. Partial photoionization cross sections into excited states of the residual ion are also obtained for the ground states of ${\mathrm{C}}^0$, ${\mathrm{N}}^{+}$, and ${\mathrm{O}}^{2+}$. An important type of autoionizing resonances due to the photoexcitation of the core ion is discussed. As the photoionization cross sections incorporate autoionization in detail, it is possible in principle to employ the Milne relation to obtain total, radiative plus dielectronic, recombination cross sections and rate coefficients. However, practical difficulties arise, and various approaches are described to overcome these. Comparison is made with the earlier results for photoionization cross sections and recombination coefficients. This work forms an extension of the Opacity Project.

It was shown some time ago by Brown and Goble [Phys. Rev. 173, 1505 (1968)] that the low-frequency approximation for single-photon bremsstrahlung derived originally by Low [Phys. Rev. 110, 974 (1958)] could be extended to the two-photon case. A further extension to the case of N-photon bremsstrahlung is obtained here. For definiteness the scattering system is taken to be that of a Dirac electron in the presence of a static potential, but greater generality is possible, since, as in the earlier treatments, the argument is based on the generally valid gauge-invariance requirement. The first two terms in the low-frequency expansion may be constructed from a knowledge of the on-shell field-free scattering amplitude. If, in addition, the physical single-photon bremsstrahlung amplitude is known to first order in the frequency, the first four terms in the expansion of the N-photon amplitude, for N>1, may be determined. Some remarks are included concerning the possible utility of the theorem in external-field problems and in the derivation of infrared radiative corrections to scattering processes.

We have calculated the contributions of excitation-autoionization and resonant-excitation-double-autoionization indirect-ionization mechanisms to electron-impact ionization of Li-like ${\mathrm{B}}^{2+}$, ${\mathrm{C}}^{3+}$, ${\mathrm{N}}^{4+}$, ${\mathrm{O}}^{5+}$, and ${\mathrm{F}}^{6+}$ ions. We considered twelve autoionizing states arising from the 1s2${\mathit{s}}^2$, 1s2s2p, 1s2${\mathit{p}}^2$, 1s2s3s, and 1s2s3p configurations. Rydberg series 1s2s2ln’l’ and 1s2s3ln’l’ of resonances, which arise due to the resonant-excitation-double-autoionization process, are included in our calculation. The calculated resonance structures in the cross sections agree very well with recent crossed-beam measurements.

A theory of multiple scattering, exhibiting differences between electrons and positrons in interactions with matter, is developed. Additional differences in stopping power and annihilation for positrons are briefly discussed. Experimental verification of these differences is reported for plastic scintillators and Si(Li) detectors, which are gradually changed from a full-energy (E) detector towards an energy-loss (ΔE) detector by a variation of the incident electron and positron energy.

Positronium formation into ground and n=2 levels has been studied in collisions of positrons with atomic hydrogen in the framework of an approximation called the boundary-corrected continuum-intermediate-state (BCCIS) approximation in the energy range of 0.08–2 keV. The conventional continuum-intermediate-state approximation does not satisfy the correct boundary condition. It has been shown that, with a suitable choice of the distorting potential, the boundary condition may be satisfied with a proper account of the intermediate continuum states. It has also been shown that the BCCIS approximation leads to the same transition amplitude as may be derived using the Vainshtein-Presnyakov-Sobelman approximation. The results obtained here are found to be in good agreement when compared with other theoretical results.

It is shown that the leading asymptotic term of the small-angle intensity scattered by any amorphous sample is determined by the parallelism among subsets of the sample interfaces. Its general expression is ${\mathrm{\Sigma }}_{\mathit{i}}$[${\mathit{A}}_{\mathit{i}}$cos(${\mathrm{\delta }}_{\mathit{i}}$h)+${\mathit{B}}_{\mathit{i}}$sin(${\mathrm{\delta }}_{\mathit{i}}$h)]/${\mathit{h}}^4$, where the ${\mathrm{\delta }}_{\mathit{l}}$’s denote the distances between parallel surfaces and the ${\mathit{scrA}}_{\mathit{l}}$’s and the ${\mathit{scrB}}_{\mathit{l}}$’s are appropriate geometrical averages of the corresponding Gaussian curvatures. Since each surface is parallel to itself with a relative null distance, in the former expression the well-known Porod contribution comes out from the term relevant to δ=0. The expression is specialized to the case of those three-component samples where one of the constituting phases has a constant thickness and lies in between the remaining two phases which have no common interface. Different approximations are considered and, in the most favorable cases, it appears that the average Gaussian, mean, and squared mean curvatures of the dividing film can be determined.

A first-principles calculation based on an expansion of the time-dependent electronic wave function in terms of atomic orbitals (coupled-channel method) has been applied to evaluate the impact-parameter dependence of the electronic energy loss and the fluctuation in energy loss of swift ions colliding on H and He atoms at energies of 10 to 500 keV/amu. The results have been compared with experimental data as well as with other existing models, e.g., the local-density approximation in an electron-gas target, the harmonic-oscillator target treatment, and the first-order plane-wave-Born approximation. Our results show a nearly exponential shape of the mean electronic energy loss for small impact parameters, in contrast to the Gaussian shapes obtained by Mikkelsen and Sigmund [Nucl. Instrum. Methods B 27, 266 (1987)].

The experimentally observed high-l-state population of ions excited in ion-solid interactions differs sharply from l-state populations produced in ion-atom collisions. We have studied the population dynamics of electronic excitation and transport within the framework of a classical transport theory for ${\mathrm{O}}^{2+}$ (2-MeV/u) ions traversing C foils. The resulting delayed-photon-emission intensities are found to be in very good agreement with experiment. Initial phase-space conditions have been obtained from both classical-trajectory Monte Carlo calculations and random initial distributions. We find evidence that the very-high-l-state populations produced in ion-solid collisions are the result of a diffusion to high-l states under the influence of multiple scattering in the bulk of the solid.

The contribution from electron scattering to the width and shift of the ${\mathrm{H}}_{\mathrm{\alpha }}$ and ${\mathrm{Ly}}_{\mathrm{\beta }}$ lines of neutral hydrogen is evaluated. Elements of the S matrix for electron-atom scattering calculations are obtained from close-coupling calculations including pseudostates, either of the variational or the optical potential type. The convergence of sums over angular-momentum variables is discussed. Results are compared with some other calculations and with experiment where possible.

The autoionizing (${\mathit{X}}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$) 8f, 9f, and 10f, v=1 states of ${\mathrm{N}}_2$ have been studied by using double-resonance ionization spectroscopy via numerous rotational levels of the ${\mathit{a}}^1$${\mathrm{\Pi }}_{\mathit{g}}$, v’=5 state. These nf states lie in a complex region of the spectrum that also contains transitions to high vibrational levels of the b’ ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ valence state and Rydberg states that converge to the ${\mathit{X}}^2$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$, ${\mathit{A}}^2$${\mathrm{\Pi }}_{\mathit{u}}$, and ${\mathit{B}}^2$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ states of the ion. A long-range interaction model, which is based on the interaction of the Rydberg electron with the polarizability and quadrupole moment of the ${\mathrm{N}}_2^{+}$ ion core, has been used to intepret the f-state structure and to identify the extent of perturbations due to interactions with nearby states. A generalized least-squares fit of the model to the energies of the unperturbed 8f state yields an effective quadrupole moment of (3.2±0.1)${\mathit{ea}}_0^2$ and an isotropic polarizability of (16.1±0.4)${\mathit{a}}_0^3$ for the ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$,${\mathit{v}}^{+}$=1 state of the ion. In addition, a new state that is observed to perturb the 9f, v=1 state is tentatively identified as the previously unobserved ( ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$)3dπ ${}_{}{}^1{}_{}{}^{}\mathrm{\Delta }_{\mathrm{u}}^{}{}_{}{}^{}$, v=2 state. The results presented here complete the interpretation of the single-photon absorption spectrum of ${\mathrm{N}}_2$ from the first ionization potential at ∼125 667 to ∼126 850 ${\mathrm{cm}}^{\mathrm{-}1}$.

Relativistic Dirac-Fock calculations of the Kα satellite lines for several elements having atomic numbers between 19 and 32 are presented along with the results from nonrelativistic Hartree-Fock calculations. Relative intensities and energies are compared with experimental results and a previous calculation. Generally good agreement is found with some deviations.

The far-infrared spectra of HCl in Ar at T=162.5 K and at densities of 100, 200, 300, 400, and 480 amagat have been studied by applying a non-Markovian theory in which the theoretical dipolar absorption coefficient is obtained by a superposition of basic resonances (Lorentzian lines in the Markovian limit) modified by nonadditivity effects due to the existence of cross correlations between different rotational transitions. In terms of only two phenomenological parameters (the square interaction strength and its correlation time) obtained by fit to the experimental spectra, we have analyzed, qualitatively and quantitatively, the influence of the memory as well as of the nonadditivity effects on the system under study: while the former are not very significant, it is shown that the latter have great importance in calculating the theoretical dipolar absorption coefficient, even for the Ar-lowest density of 100 amagat. As a consequence, strong deviations from the additive superposition of Lorentzian lines are observed, especially in the minima between lines, which mainly consist in an enhancement of the absorption (constructive interference) in the low-frequency side and in a reduction of the absorption (destructive interference) in the high-frequency side.

Ten fine-structure intervals separating n=10, L=4, 5, 6, and 7 Rydberg states of ${\mathrm{H}}_2$, all with (ν=0, R=1) ${\mathrm{H}}_2^{+}$ cores, have been measured using fast-beam microwave-optical techniques, determining the relative positions of all these states to a precision of about ±0.3 MHz. These experimental results are compared with predictions of the expected structure, which use the polarization model to relate the Rydberg fine structure to properties of the ${\mathrm{H}}_2^{+}$ ion, yielding experimental determinations of certain electric properties of the (0,1) state of ${\mathrm{H}}_2^{+}$: quadrupole moment, Q=1.642 95(30)${\mathit{ea}}_0^2$, and dipole polarizabilities, ${\mathrm{\alpha }}_{\mathit{S}}$=3.1770(34)${\mathit{a}}_0^3$, ${\mathrm{\alpha }}_{\mathit{T}}$=4.015(36)${\mathit{a}}_0^3$.

Statistical properties of the energy levels derived from the (3d+4s${)}^5$4p spectrum of Cr i have been investigated. The hypothesis that the density distribution of the energy levels is of the Gram-Charlier type has been rejected by a ${\mathrm{\chi }}^2$ test. On the other hand, in sequences of the energy levels corresponding to fixed values of the total angular momentum J, the energy-level densities are shown to be Gaussian. In the fixed-J sequences, spacings between neighboring levels are Poisson-like. The low-order energy moments have been used to approximate the (3d+4s${)}^5$4p energy-level spectrum. The secular behavior and fluctuations of this spectrum are discussed.

The photoionization cross section of atomic hydrogen in an electric field can be understood as the sum of resonance contributions. Each contribution is proportional to the imaginary part of the square of a complex transition-dipole matrix element divided by an energy denominator and has a naturally asymmetric line shape. The main features of experiments, including asymmetry of the lines, blue shift of maxima with respect to calculated resonances near zero energy, ‘‘field-induced’’ structure above zero energy, and increasing ‘‘background’’ that begins above the classical saddle-point energy, are reproduced via variational calculations of the resonance energies and wave functions in parabolic coordinates.

The effect of multiple scattering (long-range, small-angle elastic Coulomb collisions) on the radiation field associated with transition radiation from a nonstationary plasmalike medium is addressed with the help of a simple model. The model assumes a relativistic charged particle traversing a single medium that is rapidly and abruptly changing its electromagnetic properties. An averaged diffusion tensor is calculated relativistically to effectively approximate a diminished formation zone in the medium, the effect of which on the differential radiation intensity and angular distribution of transition radiation is studied. It is shown here that multiple scattering is more than a fine-tuning effect, in that it has a marked enhancement effect on the differential yield over a wide and significant frequency range of the emitted photons as well as an appreciable shifting and slowing down of the rapid fall of the maximum intensity angular location over an equally significant angular range. It is suggested that one study the effect in an implosion-explosion process of a laser-prepared medium. Implications to transition radiation from a periodic medium and the more general process of transition scattering in nonlinear media are briefly discussed.

The method of zero-range potential is used to model negative atomic and quasimolecular ions in a circularly polarized laser field. The range of the laser-field parameters is extended in comparison with the previous calculations of the complex quasienergies of an atomic particle. A closed-form expression that defines the adiabatic electronic complex quasienergies of a two-state quasimolecular system is derived and solved numerically. Both laser-induced widths and shifts of the terms are discussed for a wide region of the laser-field parameters and internuclear distances, using ${\mathrm{H}}_2^{\mathrm{-}}$ and ${\mathrm{OH}}^{\mathrm{-}}$ ions as examples. The effect of suppression of the ionization rate of the excited state of a negative quasimolecular ion at small internuclear distances due to the presence of a laser field, as well as deepening of the ground-term potential well, was found.

A cryogenic technique has been used to produce polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ targets of up to relative density p=12 atm (≊3×${10}^{20}$ ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms/${\mathrm{cm}}^3$; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790–800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5${\mathit{S}}_{1/2}$→5${\mathit{P}}_{1/2}$ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure, which indicates the importance of Rb${\mathrm{-}}^3$He${\mathrm{-}}^3$He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure, light intensity, and cell geometry. The Rb${\mathrm{-}}^3$He spin-exchange cross section, 〈${\mathrm{\sigma }}_{\mathrm{SE}}$v〉=6.1×${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, was found to be independent of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure up to p=12.1 atm. Maximum ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ polarizations of 72–79% were observed with cells of 17 ${\mathrm{cm}}^3$ volume that contained ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at p=6–9 atm.