Ab initio theoretical calculations are presented for the cross section, angular distribution, and spin polarization of photoelectrons for the $5s$ subshell of xenon. Starting from an initial basis of nonrelativistic Hartree-Fock wave functions, the authors treat the effect of final-state spin-orbit interaction in the Breit-Pauli approximation exactly. Electrostatic correlations between the $5s$ and $5p$ subshells are also included. Good agreement is obtained with experiment in the case of the photoionization cross section, but only qualitative agreement with experiment and with other theoretical calculations is obtained in the case of the photoelectron angular distribution due to the difficulty of obtaining accurate values for forbidden transition intensities starting from a nonrelativistic basis set. Spin polarization of as much as 20% is predicted near the $5s$-subshell cross section minimum. Finally, some effects of the large discrepancies between Hartree-Fock and Dirac-Fock subshell binding energies and the corresponding experimental values are discussed.

Hyperfine-structure (hfs) measurements of the 592-nm $5d{\left[\frac{7}{2}\right]}_4^o-6p{\left[\frac{5}{2}\right]}_3$, the 612-nm $5d{\left[\frac{3}{2}\right]}_2^o-6p{\left[\frac{5}{2}\right]}_3$, and the 581-nm $5d{\left[\frac{3}{2}\right]}_2^o-6p{\left[\frac{3}{2}\right]}_1$ transitions in Cs II are performed with the beam-laser technique. The hyperfine coupling constants $A$ and $B$ are evaluated from the experimental data. A comparison with semiempirical calculations of the hfs constants is made. Radiative lifetimes of the levels $6p{\left[\frac{5}{2}\right]}_3$, $6p{\left[\frac{3}{2}\right]}_2$, and $6p{\left[\frac{3}{2}\right]}_1$ are measured with the cascade-free beam-laser technique. The measured lifetimes are compared with lifetimes obtained from semiempirical calculations using the Coulomb approximation.

Absolute cross sections for the emission of Balmer-$\alpha$ and Balmer-$\beta$ radiations from H + Ar collisions between about 25- and 2500-eV hydrogen-atom energy and the polarization of the emitted radiations are reported. For hydrogen-atom energies above 250 eV, the contributions to these radiations from decay of the long-lived $3s$ and $4s$ excited states of hydrogen are resolved. The emission cross sections reach maximum values in the 100-eV hydrogen-atom energy range, where they are larger than the ionization cross sections for this reacting pair. The experimental techniques used to make the measurements are described and the results are compared with the work of other investigators. The available data are considered on the basis of a suggested model for the interaction.

Absolute cross sections for the emission of Balmer-$\alpha$ and Balmer-$\beta$ radiations from ${\mathrm{H}}^{+}$ + Ar collisions between about 50- and 2500-eV proton energy and the polarizations of the emitted radiations are reported. For proton energies above 300 eV, the contributions to these radiations from decay of the long-lived $3s$ and $4s$ excited states of hydrogen are resolved. The Balmer-$\alpha$ emission cross section exhibits a maximum at about 1-keV proton energy. No such dominant feature is present at the $n=4\mathrm{}$ level of excitation. The experimental techniques used to make the measurements are described, and the results are compared with the work of other investigators and discussed on the basis of the interaction between these colliding species.

The authors report the first application of the Schwinger variational principle to electron-molecule scattering. Results for electron-${\mathrm{H}}_2$ scattering in the static-exchange approximation show that the Schwinger method can provide accurate solutions of the scattering problem with small descrete basis sets. The Schwinger variational expression is found to converge far more quickly with respect to the size of the basis than any other algebraic expansion technique considered to date. Results are also presented for hybrid trial scattering wave functions containing both continuum and discrete basis functions.

To examine experimental cross sections for charge transfer in collisions of partially stripped heavy ions with atomic hydrogen at low collision energies, unitarized-distorted-wave-approximation calculations are performed using a model in which the projectiles are replaced by bare nuclei of a given effective charge. The results show the presence of a strong oscillatory dependence of the cross sections on effective charge due to the crossings of diabatic potential curves in the low-energy region below 10 keV/amu. The considerable differences in the measured cross sections for impacts of ions of different elements (B, C, N, and O) observed by Bayfield et al. and Crandall et al. at low impact energies are attributed to this oscillatory behavior.

Differential cross sections for 15-145-keV proton impact excitation of atomic hydrogen to the $n=2\mathrm{}$ level have been determined for center-of-mass scattering angles 0 to 1.2 mrad. The cross sections were obtained from an analysis of the angular distribution of protons which had lost an energy corresponding to the excitation of atomic hydrogen to its $n=2\mathrm{}$ level. The differential cross sections obtained are in rather good agreement with available coupled-state calculations as well as the simpler Glauber-approximation calculations. However, at the larger scattering angles the cross sections obtained from the theoretical treatments appear to decrease more rapidly than the experimental data.

The electron-photon angular correlation function was measured between 80-eV electrons which excited the $2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ state of helium and 58.4-nm photons from the decay of that state for electron scattering angles ranging from 5° to 100°. The data have been analyzed to yield values of the ratio $\lambda$ of the differential cross section for exciting the $M_j=0\mathrm{}$ sublevel to the total differential cross section and the magnitude $\left|\chi \right|$ of the phase difference between the $M_j=0\mathrm{}$ and $M_j=1\mathrm{}$ excitation amplitudes. The data agree with all previous measurements within one standard deviation, with the exception of the large-angle values of $\lambda$ obtained by Hollywood, Crowe, and Williams. Possible causes of these discrepancies are discussed. The values of $\lambda$ and $\left|\chi \right|$ obtained in this work agree quite well with those given by the distorted-wave calculations of Madison over the entire angular range.

The spectrum of Kr viii has been observed between 180 and 2000 Å by using foil excitation of 2.5-3.5-MeV krypton ions. Twenty new transitions have been classified and eleven new excited-state energies have been determined within the $n=4-7\mathrm{}$ shells. The ionization potential is derived to be 1015 800±200 ${\mathrm{cm}}^{-1\mathrm{}}$. The excited-state energies and fine structures are compared with recent relativistic Hartree-Fock calculations. The $4p$-state lifetime has been measured by performing a simultaneous analysis of decay data for the $4p$ level and for its dominant cascade-repopulating levels. The $4p$ lifetime is found to be 30% shorter than previously measured values and is in excellent agreement with the result of a recent multiconfiguration Hartree-Fock calculation. The source of the discrepancy between this result and earlier measurements is discussed.

Energy losses are measured in ${\mathrm{Ne}}^{+}$ +${\mathrm{D}}_2$, ${\mathrm{Ne}}^{+}$ +${\mathrm{H}}_2$, and Ne + ${\mathrm{D}}_2$ collisions for beam energies $0.5\le E\le 3.5$ keV and scattering angles $\theta \le 5$ deg. Rotational and vibrational excitation of the target molecule is found, but the probability of direct electronic excitation is seen to be extremely small. The results indicate that the most probable laboratory energy loss $T_0$ for a projectile with mass $M_p$ scattered by a homonuclear binary molecule with atomic masses $M$ scales so that the quantity $f=\frac{T_{0}M}{\left(M_{p}E{\theta }^2\right)}$ is a function of the reduced scattering angle $\tau =E\theta$ only, as recently predicted theoretically by Sigmund. The function $f\left(\tau \right)$ is found to be the same for the ${\mathrm{Ne}}^{+}$ + ${\mathrm{D}}_2$ and ${\mathrm{Ne}}^{+}$ + ${\mathrm{H}}_2$ systems, but is strongly dependent on the charge state of the projectile.

An ab initio theoretical study is reported for dissociation of ${\mathrm{F}}_2$ via excitation of the lowest ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ electronic state by electron impact. Differential and integrated cross sections are given for impact energies from 5 to 40 eV. Our distorted-wave model predicts a resonancelike feature in the integrated cross section near threshold with a maximum value of about 0.4×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. When target polarization effects are included this feature disappears and the maximum cross section is reduced to about 0.15×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$.

Total cross sections are reported for electron transfer into the individual states $2s$, $2p_0$, and $2p_1$, and into all bound states of ${\mathrm{He}}^{+}$ in collisions of 1- to 70-keV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ ions with H atoms. Following Bates and McCarroll, plane-wave factors have been fully incorporated in a coupled-molecular-state approximation. All velocity-dependent matrix elements have been evaluated as power series in the nuclear velocity. The bases used include up to 10 states $1s\sigma , 2s\sigma , \dots , 3d\delta$, of the frozen He${\mathrm{H}}^{2+}$ molecule. Convergence of cross sections with respect to the size of the bound-state basis was studied: fewer basis functions are needed than when plane-wave factors are omitted. To test the convergence of the latter results with respect to the size of the bound-state basis, the two potentially important states $5g\sigma$ and $5g\pi$ have been added to the previously used 20-state ($1s\sigma , 2s\sigma , \dots , 4f\varphi$) basis, and were found to affect the cross section for transfer into all states by at most 6%. For transfer into all states, results with and without plane-wave factors converge at low energies, agreeing to within 5% at 1 keV. The agreement of the plane-wave-factor results with the experimental data for transfer into all states is excellent at ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ energies down to 8 keV, but poor at the lower energies, where the coupled-state results are believed to be more fully converged. For capture into the $2s$ state, the plane-wave-factor results lie above the upper bound of the experimental values by 25% at 20 keV.

The authors have measured the absolute total cross section for low-energy electrons scattered by sodium atoms in the $3{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$, $m_J=\pm \frac{3}{2}$ state using a modified atomic-beam-recoil technique. A single-mode tunable dye laser is used to excite these states selectively, and their populations are determined by using the atomic recoil in resonant photon interactions to separate excited from nonexcited beams spatially. Total cross sections have been measured at seven electron energies between 0.84 and 6.0 eV.

The process of ionization in slow-atom-Rydberg-atom collisions is investigated. A resonant energy exchange mechanism is considered responsible for the electron transition into the continuum. Both symmetrical and asymmetrical systems are considered. The ionization probabilities and cross sections for a number of Rydberg states in the colliding systems ${\mathrm{H}}^{*}\mathrm{}\left(n\right)+\mathrm{H}$, ${\mathrm{Li}}^{*}\mathrm{}\left(n\right)+\mathrm{Li}$, ${\mathrm{H}}^{*}\mathrm{}\left(n\right)+\mathrm{Li}$ are calculated.

Previous measurements of the mobility of atomic argon ions in argon at room temperature have resulted in a single value for ${\mathrm{Ar}}^{+}$ mobility despite attempts to resolve the expected fine structure of the arrival ion currents. Therefore, to understand the momentum exchange that accompanies charge transfer during collisions of the two electronic states (${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$) of the argon ion with the parent atom, a crossed-atom-ion-beam experiment was undertaken to measure the kinetic energy and laboratory deflection angle of the product ions after charge transfer. The interaction region of the particles was maintained at essentially zero intensity of electric and magnetic fields. The product species was measured by means of a spherical energy analyzer, their identity being determined by a tandem mass analyzer. The ion source used consists of a low-pressure electron-emission sustained-ionization region followed by a magnetic sector for ${\mathrm{Ar}}^{+}$ selection. For laboratory-frame collision energies of 123 eV, two fixed principal scattering peaks are observed as a result of scanning the laboratory angle at low laboratory energy. They correspond to exothermic and endothermic processes having about 0.17-eV energy separation. The argon-ion fine-structure separation is about 0.178 eV. Under low resolution the two peaks appear unresolved and are seen as single peak at about 0° deflection from the neutral-atom beam. The peaks in the intensity distribution can be understood as resulting from an exothermic and endothermic process with scattering peaked at near 0° in the center-of-mass system. Adequate fits to the data require a broad background intensity peak, which would result from isothermic processes.

A novel, classical many-body model, previously introduced for nuclear collisions, has been extended to atomic and molecular structure, with the goal of providing a framework for atomic collisions. In addition to the usual kinetic and Coulomb potential terms, a momentum-dependent two-body potential acts between electron pairs of identical spin in order to approximate the Pauli constraint $r_{\mathrm{ij}}{p}_{\mathrm{ij}}\ge \hslash \xi p$, where $\xi$ is a dimensionless parameter here set equal to 2.767. A similar potential is introduced to simulate the Heisenberg constraint, $r_{\mathrm{iN}}{p}_{\mathrm{iN}}=h$, where $N$ refers to each nucleus. Because of these constraints, the atomic and molecular ground-state configurations are stable. The hydrogen ground state is given exactly. Calculations in ${\mathrm{H}}^{-}$, He, Li, Ne, and Ar reproduce total ground-state energies to better than 15%; this is considerably better than the Thomas-Fermi model, in which the errors are approximately 28% for neon and 23% for argon. The resulting electrostatic potential is in general intermediate between Thomas-Fermi and Hartree-Fock calculations. ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}_2$ molecules are overbound; in contrast, the Thomas-Fermi model does not bind neutral molecules.

Calculations of the ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}:{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ branching ratios and photoelectron angular distribution asymmetry parameters have been performed for the outer $p$ subshells of the noble gases. A pure $j-j$ coupled Dirac-Fock formalism was employed. The results are compared with experiment, where available, and the reasons for the various degrees of agreement are discussed. In particular, it is shown that the use of pure $j-j$ coupling leads to serious difficulties for low $Z$ where $\mathrm{LS}$ coupling predominates.

The effects of ion motion on the Stark profiles of the lines $\mathrm{H}\alpha$ and $\mathrm{H}\beta$ have been studied in a plasma jet for an electron density range from ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ up to 2×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The ion velocity was changed in the ratio 0.5 to 2 by using perturber ions of different masses. The authors have found an important effect in the central part of $\mathrm{H}\beta$ resulting in a smearing of the dip which becomes complete for $n_e\le 2\times {10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Good agreement with a previous experiment at higher electron densities is observed. Furthermore, a narrowing of the peak separation of $\mathrm{H}\beta$ has been observed for the greatest ion velocities at electron densities lower than 5×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. For $\mathrm{H}\alpha$, the importance of the Doppler broadening would have required more accurate gas and ion temperatures, nevertheless an increase in the total width of the line is noted when the ion motion becomes faster.

The $K{\alpha }_1^h$ and the $K{\alpha }_2^h$ hypersatellites of molybdenum were observed in a conventional x-ray spectrum analysis. The energy separation between the hypersatellites and the corresponding characteristic lines were measured and found to be in good agreement with the results obtained from Hartree-Fock calculations. The relative intensity ratio $\frac{K{\alpha }_1^h}{K{\alpha }_2^h}$ was computed and compared with the results of intermediate-coupling calculations.