An experimental and theoretical study is presented of the ${\mathrm{B}}^{+}$(1${\mathit{s}}^2$2pnl, n=4 and 5) doubly excited states. Experimentally, low-energy electron emission spectra of ${\mathrm{B}}^{+}$, excited by ${\mathrm{B}}^{+}$+He, ${\mathrm{B}}^{+}$+Ar, and ${\mathrm{B}}^{+}$+C-foil collisions, have been recorded with a zero-degree electron spectrometer setup. Several autoionizing states of ${\mathrm{B}}^{+}$ have been observed between 1 and 3 eV in the projectile emitter frame. The observed resonances, namely, (1${\mathit{s}}^2$2p4s) ${}_{}{}^3{}_{}{}^{}$P °, ${}_{}{}^1{}_{}{}^{}$P °, (1${\mathit{s}}^2$2p4p) ${}_{}{}^3{}_{}{}^{}$D${,}^1$D, (1${\mathit{s}}^2$2p4d) ${}_{}{}^3{}_{}{}^{}$F °, ${}_{}{}^1{}_{}{}^{}$F °, (1${\mathit{s}}^2$2p4d) ${}_{}{}^3{}_{}{}^{}$P °, and ${}_{}{}^1{}_{}{}^{}$P ° do not appear in any previous data. The observations also suggest that the population of triply excited states in B is not important in the excitation process. Theoretically, five-state (2s,2p,3s,3p,3d) close-coupling calculations using impact have been performed to determine positions, widths, and partial collision strengths for 22 of the lowest ${\mathrm{B}}^{+}$(1${\mathit{s}}^2$2pnl) resonance states. Excellent agreement between theoretical and experimental energy positions is obtained.

We analyze the modifications of the transport behavior of electrons in dense media due to the presence of a strong Coulomb field generated by an ion moving initially in close phase-space correlation with the electrons. These modifications play a profound role in convoy electron emission in ion-solid collisions. The transport behavior is studied within the framework of a classical phase-space master equation. The nonseparable master equation is solved numerically using test-particle discretization and Monte Carlo sampling. In the limit of vanishing Coulomb forces the master equation becomes separable and can be reduced to standard one-dimensional kinetic equations for free-electron transport that can be solved exactly. The comparison to free-electron transport is used to gauge both the reliability of test-particle discretization and the significance of Coulomb distortion of the distribution functions. Applications to convoy-electron emission are discussed.

An experimental and theoretical study of the electron emission, differential in emission angle and energy, is presented for ${\mathrm{He}}^{+}$ impact on several gaseous targets. Experimentally, data at 400-, 500-, and 750-keV/amu impact on He, Ne, Ar, Kr, and ${\mathrm{H}}_2$O are presented; theoretically, cross sections for He and Ar are calculated using the first-order Born approximation. By measuring the total electron emission, as well as electron-scattered projectile coincidences, contributions to the differential cross sections resulting from projectile ionization, from target ionization, and from simultaneous projectile-target ionization events are identified. Theoretical cross sections for each of these processes, as well as their total sum, are compared with the experimental data. From these comparisons, it is found that the present theoretical treatment does a reasonable job of describing the total projectile and target ionization cross sections, but is poor for the double-differential cross section. It also severely underestimates the simultaneous projectile-target ionization cross section.

Autoionization of He atoms following double excitation by electrons, protons, ${\mathrm{C}}^{\mathit{Q}+}$ (Q=4–6), and ${\mathrm{F}}^{\mathit{Q}+}$ (Q=7–9) ions has been studied. The electron-emission yields from the doubly excited 2${\mathit{s}}^2$${(}^1$S), 2s2p${(}^1$P), and 2${\mathit{p}}^2$${(}^1$P) states were measured at the reduced projectile energy of 1.5 MeV/nucleon for observation angles between 10° and 60°. The results indicate excitation to the 2${\mathit{s}}^2$${(}^1$S) and 2${\mathit{p}}^2$${(}^1$D) states increases as approximately ${\mathit{Q}}^3$, while excitation to the 2s2p${(}^1$P) state varies as approximately ${\mathit{Q}}^2$, where Q is the charge of the projectile. These charge dependences are significantly less than the ${\mathit{Q}}^4$ dependence expected in the independent-electron model, suggesting the interaction between the two target electrons is important in creating the doubly excited states.

Using a powerful laser-based experimental method, the cross section for electron excitation of Na(3S) atoms to the Na(3P) state has been decomposed into partial components with respect to changes in the spin and angular momentum of the atomic electron, and these partial cross sections are further reduced to their differential character with respect to the electron scattering angle. Partial, differential cross sections are reported for electron collision energies from threshold 2.1 to 3.6 eV, and compared to available calculations.

The semiclassical impact-parameter method is applied to the processes of state changing and energy transfer in the collision of a highly excited Rydberg atom (n≥20) with the polar molecules HF and HCl. The relative motion of the molecule and atomic nucleus is taken to be rectilinear; the electron-molecule and ion core-molecule interactions are represented by cutoff dipole forms. Cross sections for transitions involving quantum numbers n and l of the atom and rotational quantum number j of the molecule are obtained for a range of collision energies and initial atomic and molecular states. Comparisons are made with the results of earlier classical studies and with the quantum-mechanical impulse approximation. Collision rates are calculated and compared with experimental values for l mixing and n and j changing. The agreement between experiment and theory is shown to be satisfactory, within the uncertainties of both the measurements and the theory. Cases of agreement and disagreement between various theories are examined. One finding of the present work is that the quantum-mechanical impulse approximation appears to significantly overestimate the values of various state-changing cross sections when the internal energy defect is small. The validity of the impulse approximation for collisions of Rydberg atoms with polar molecules is discussed.

Compton scattering of 320-keV γ rays through 45° and 115° by K-shell electrons of gold and holmium has been studied by the well-known coincidence technique. Double-differential cross sections with respect to outgoing photon energy and angle have been determined from 30 keV up to the kinematic limits. Target-Z-dependent false coincidences arising from bremsstrahlung emitted by photoelectrons were estimated both by theoretical analysis and by measurements carried out with gold targets of thicknesses differing by more than a factor of 4. The analysis of the bremsstrahlung process included the often neglected effects of the angular distribution of photoelectrons and of multiple scattering of electrons during the slowing-down process within the target. The bremsstrahlung generated false coincidences obscure the possible observation of the predicted infrared divergence in the low outgoing photon energy regime and lead to a significant contribution even in the quasi-Compton regime. The single-differential cross sections obtained after a correction for bremsstrahlung are in reasonable agreement with theoretical calculations.

We have studied projectile K x-ray satellite yields in collisions between 50-MeV ${\mathrm{Ar}}^{\mathit{Q}+}$ ions (Q=4,6,11,12,13) and C foils with various thicknesses (2–100 μg/${\mathrm{cm}}^2$). For foil thicknesses below about 10 μg/${\mathrm{cm}}^2$, Q dependence of the yields has been observed, reflecting the nonimmediate L-shell equilibrium. For this region, it has been shown that the mean K-hole formation cross section also varies with the target thickness and Q. The variation has been attributed to the varying chance of K-to-L excitation with the increase or decrease of L holes in solid. The projectile charge fractions after passage through the target have also been measured for the foil-thickness region 2–20 μg/${\mathrm{cm}}^2$ including the thicknesses for which charge-state equilibrium is not attained. By combining these data, the K-hole formation cross section specifying the number of L holes at the initial stage of a collision has been estimated. The results have been compared with theoretical predictions for cross sections of K-to-continuum ionization and K-to-L excitation.

Total and partial photoionization cross sections for the ground state of Na leaving the residual ${\mathrm{Na}}^{+}$ ions in the three (2${\mathit{p}}^6$ ${}_{}{}^1{}_{}{}^{}$S, 2${\mathit{p}}^5$3s ${}_{}{}^{3,1}{}_{}{}^{}$P °) and seven (2${\mathit{p}}^6$ ${}_{}{}^1{}_{}{}^{}$S, 2${\mathit{p}}^5$3s ${}_{}{}^{3,1}{}_{}{}^{}$P °, 2${\mathit{p}}^5$3p ${}_{}{}^3{}_{}{}^{}$S${,}^{3,1}$D${,}^1$P) lowest LS states are calculated using the R-matrix method. Additionally, a 12-state R-matrix calculation is performed leaving the residual ${\mathrm{Na}}^{+}$ ions in LS states of the 2${\mathit{p}}^6$, 2${\mathit{p}}^5$3s, and 2${\mathit{p}}^5$3p manifolds and 2${\mathit{p}}^5$3d ${(}^{3,1}$P °${,}^1$D°) to determine the cross-section magnitudes of the satellite levels 2${\mathit{p}}^5$3p ${}_{}{}^1{}_{}{}^{}$S and 2${\mathit{p}}^5$3d ${}_{}{}^{3,1}{}_{}{}^{}$P ° and to assess their influence on the total cross section.

Correlation and polarization effects are taken into account through the use of extensive configuration-interaction target wave functions. The results cover both the single- and multiple-channel photon energy regions (2–3.8 Ry). We find that the three-state total cross section is characterized by resonance structure dom- inated by an isolated 2${\mathit{p}}^5$3${\mathit{s}}^2$ ${}_{}{}^2{}_{}{}^{}$P ° resonance at 2.37 Ry (1.62 Mb) and a main resonance in the multiple-channel energy region at 2.84 Ry (21 Mb) due entirely to the 2${\mathit{p}}^5$3s ${}_{}{}^3{}_{}{}^{}$P ° level. The 2${\mathit{p}}^5$3s ${}_{}{}^{3,1}{}_{}{}^{}$P ° satellite lines have unusually large cross sections which dominate that of the main line throughout the energy range. The three-, seven-, and 12-state calculations predict essentially the same behavior for the total cross section. Various partial photoionization cross sections are presented to elucidate the predicted strong satellite enhancement of the single-channel total cross section. We conclude from comparison of the three-, seven-, and 12-state results that coupling effects are important in the multiple-channel region and that the three-state calculation predicts the total cross-section behavior reasonably well over a wide energy range.

We have produced highly spin-polarized atomic hydrogen by spin-exchange optical pumping. A tunable ring dye laser is used to polarize rubidium atoms by optical pumping. The cell containing the rubidium vapor is coated with paraffin in order to reduce spin relaxation due to wall collisions. Hydrogen gas is dissociated in an inductive discharge and flows continuously through the cell, in which the hydrogen atoms are polarized by spin-exchange collisions with the polarized rubidium atoms. Atomic-hydrogen polarization as high as 2〈${\mathit{J}}_{\mathit{z}}$${\mathrm{〉}}_{\mathrm{H}}$=0.72(6) has been observed, which is the highest polarization yet produced by this method. However, the rubidium polarization may be limited to this value due to radiation trapping at higher rubidium densities. The spin-relaxation rate of atomic hydrogen on a paraffin-coated cell is also measured and corresponds to about 7600 wall bounces between wall relaxation.

The S(μ) and L(μ) moments of the dipole oscillator strength distribution and the associated mean excitation energies ${\mathit{I}}_{\mathrm{\mu }}$ have been calculated for the helium atom using the polarization-propagator formalism. It is found that, if correlation is included, agreement with experimental and other theoretical values is good. The mean excitation energy for stopping is found to be ${\mathit{I}}_0$=42.41 eV.

The relative photoionization cross section of atomic nitrogen for the production of singly and doubly charged ions has been measured from 44.3 to 275 Å and from 520 to 852 Å. The results have been made absolute by normalization to one-half of the molecular nitrogen cross section at short wavelengths. The smoothed atomic nitrogen cross sections σ can be accurately represented, at short wavelengths, by the equation σ(Mb)=3.67×${10}^4$${\mathit{E}}^{\mathrm{-}2.3}$ as a function of the photon energy E (eV), thereby allowing the cross sections to be extrapolated to the nitrogen K edge at 31 Å.

An analytical expression for the radial integral occurring in bound-continuum matrix elements is derived for hydrogenic atoms in arbitrary states n,l. In the limit where the dipole approximation is valid, the solution is used to calculate the ionization absorption cross section for hydrogenic atoms in the 1s, 2s, 2p, 3s, 3p, and 3d states. Simple analytical expressions are presented for each of the nine bound-continuum integrals with n<4.

The mean energy loss by slow electrons moving in a gas and rotationally exciting the molecules is proportional to the rotational stopping cross section (RSCS) defined by multiplying the cross section for a rotational transition Γ→Γ’ by the transition energy and by summing the product over Γ’. The RSCS for linear and symmetric-top polar molecules are studied in the adiabatic-rotation approximation at scattering angles θ larger than a critical value ${\mathrm{\theta }}_{\mathit{c}}$ and in the first-Born approximation for θ<${\mathrm{\theta }}_{\mathit{c}}$. Formulas independent of ${\mathrm{\theta }}_{\mathit{c}}$ are derived for the RSCS for any Γ as the sum of the RSCS for the ground rotational state Γ=0 and a term that depends on Γ in a simple analytic form and is proportional to the dipole moment D squared. For a vanishing D, this term vanishes and the formulas reduce to a previously proven theorem for nonpolar molecules that the RSCS is independent of Γ. Useful expressions are also derived for the average of the RSCS over the Boltzmann distribution of Γ.

The impact-parameter dependence of the K-shell ionization cross section has been measured for protons and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ ions incident on thin-foil targets of Ti, Ni, and Cu at selected energies in the region from 0.4 to 2.0 MeV/u. Comparison is made of these experimentally determined values with the results of a recently developed theoretical model in which the electron binding energy and initial-state wave function are allowed to respond in a time-dependent fashion to the field of the projectile as a function of its position along a hyperbolic Coulomb trajectory. Generally good agreement between the theoretical and experimental values is obtained. A comparison between the results of this theoretical model and two other published theoretical results and with experimental data for 0.5-MeV p in Cu is also shown. Remarkably close agreement is seen, particularly between the predictions of the rather disparate theoretical models. Measurements for a single system, 2.0-MeV p in Sm, involving ionization from the L shell, are also reported.

The production of the saddle-point electrons in slow H+${\mathrm{H}}^{+}$ collisions is discussed in the framework of the quantum adiabatic approach. An analytical expression for the angular distribution of the ejected electrons is obtained.

Properties that depend on the anisotropy of the electron momentum distribution of ${\mathrm{N}}_2$ are not easy to calculate accurately. Even rather elaborate work [Phys. Rev. A 36, 5111 (1987)] concluded that computations using still better one-particle basis sets were necessary. This paper reports improved configuration-interaction calculations that use a better basis set and also take explicit account of molecular vibration. Most of our results have converged to an accuracy of 0.5% or better. However, it turns out unexpectedly that the coefficients in the MacLaurin expansion of the isotropic Compton profile need to be verified by further theoretical and/or experimental work.

Multiphoton free-free transitions within a full second-order treatment of the scattering potential are considered with two specific aims: (i) to test some widely used approximations against exact calculations; (ii) to present differential cross sections for the laser-assisted positron scattering, in the regime of intermediate field intensities (up to ${10}^{13}$ W/${\mathrm{cm}}^2$). In the case of a field linearly polarized along the incident particle momentum, the results clearly discriminate among the approximations, setting interesting limitations to each of them as functions of the scattering potential features and of the field parameters. Each approach shows good performance for particular scattering angle ranges, pointing to the need for improved techniques to work out fully reliable and easy-to-handle approximations.

Photoionization cross sections (σ) and photoelectron angular distribution parameters (β) across the (ns’,nd’) autoionization resonances for Ar, Kr, and Xe have been measured with photon resolution widths as low as 0.023 Å by means of synchrotron-based photoelectron spectroscopy. The experimental results are compared with those obtained by other experimental techniques and theoretical results. The enhanced resolution allows a redetermination of the width of the ns’ resonances.

An explicit coordinate realization of the algebraic Hamiltonian for scattering with SO(2,3) dynamical symmetry is derived. The resulting expression for the scattering potential turns out to be dependent on energy and angular momentum.