We present a ten-channel eikonal treatment of the $2{}_{}{}^{1,3}{}_{}{}^{}P$, $3{}_{}{}^{1,3}{}_{}{}^{}S$, $3{}_{}{}^{1,3}{}_{}{}^{}P$, and $3{}_{}{}^{1,3}{}_{}{}^{}D$ excitations of atomic helium, initially in the $2{}_{}{}^{1,3}{}_{}{}^{}S$ metastable states, by incident electrons with energy $E$ (eV) in the range $5\le E\le 100$. Integral and differential inelastic cross sections are obtained. Also, the angular-correlation parameters $\lambda$ and $\chi$, which respectively provide the relative population and relative phase of the collisionally excited $P$ magnetic substates, and the circular polarization fraction $\Pi$ of radiation emitted from these $P$ states, are determined as functions of scattering angle $\theta$ and $E$. No measurements exist to date. The principle of detailed balance is explicitly demonstrated for the $2{}_{}{}^1{}_{}{}^{}S-1\mathrm{}{}_{}{}^1{}_{}{}^{}S$ superelastic collision.

Thick-target yields and x-ray spectra have been measured for argon ions impinging on graphite targets at energies between 30 and 200 keV. The thick-target yields of carbon $K$-shell and argon $L$-shell x rays are analyzed on the basis of a simple model involving multiple collisions and molecular-orbital promotions. The analysis results in average argon $L$-shell fluorescence yields and total decay rates for argon ions traversing the solid. These values are found to be substantially lower than those appropriate to argon ions traversing a gas target. An explanation for the differences is proposed, and found to be corroborated by analysis of the observed x-ray spectra. An approximate determination of the argon $M$-shell vacancy distribution is obtained and compared with beam-foil charge equilibrium measurements.

Two experimental tests of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$-$\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ close-coupling collision calculations were performed with a highly mass-resolved ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ incident ion beam and an atomic hydrogen scattering target. The total single-electron-transfer cross section ${\sigma }_{21}\left(\mathrm{H}\right)$ was measured for ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ energies between 7 and 144 keV, with the results in better agreement with molecular-state calculations than with theory employing atomic states. Above 25 keV this cross section is 50% above values obtained by Shah and Gilbody for incident ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ with the same collision velocity. Also measured was the cross section ${\sigma }_{21}^{*}\left(\mathrm{H}\right)$ for single-electron transfer into the $2s$ state of ${\mathrm{He}}^{+}$ using an ion-photon time-coincidence technique over the ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$ energy range 7 to 70 keV. These data differ with the corresponding ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{++}{}_{}{}^{}$-$\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ data of Shah and Gilbody at the higher energies. For ${\sigma }_{21}^{*}\left(\mathrm{H}\right)$, the observed energy dependence as well as the magnitude at the lower energies are in marked disagreement with all close-coupling results.

Impact of 10- to 30-keV ${\mathrm{H}}^{+}$ or ${\mathrm{He}}^{+}$ ions on polycrystalline metal surfaces causes some projectiles to be backscattered in a neutral excited state. These projectiles subsequently radiatively decay, emitting Doppler-broadened spectral lines. By analysis of the spectral shape of these lines, we are able to determine the probability of radiationless deexcitation of the excited backscattered atoms. Quantitative measurements of spectral intensity indicate that less than 1% of all projectiles are backscattered in an excited state. The relative variation of total spectral line intensity with angle of projectile incidence and with projectile primary energy has been successfully predicted using a model which assumes that the probability for excited-state formation is independent of the scattered projectile's energy and direction. We also predict the variation in total spectral line intensity with target atomic number. Finally, we examine briefly the sputtering and excitation of Al under ${\mathrm{He}}^{+}$ impact.

We have investigated the elastic scattering of positronium atoms by hydrogen atoms below the excitation threshold at 5.1 eV, limiting the investigation to the spin-zero (singlet) state of the two electrons. Nonresonant $S$-wave phase shifts have been obtained from normalizable trial functions by extracting the center-of-mass wave function and examining its spatial behavior. An effective-range expansion is fitted to the phase shifts and the previously obtained positronium-hydride bound-state energy to yield a scattering length $a\cong 5.3a_0$ compared with Fraser's corrected exchange result, $a=7.28\mathrm{}{a}_0$ ($a_0$ is the Bohr radius). The stabilization method suggests that a resonance occurs at a scattering energy of 4.5 eV, about 0.6 eV below the excitation threshold. Using the complex rotation method, we have obtained evidence confirming the existence of the resonance and an estimate of its width of about 6 × ${10}^{-2\mathrm{}}$ eV.

Absolute and relative cross sections were obtained for the associative ionization (AI) reaction ${\mathrm{He}}^{*}+{\mathrm{Ne}}^{*}\to \mathrm{He}{\mathrm{Ne}}^{+}+e$ by a merging-beams technique over a range of interaction energy $W$ from 0.01 to 10 eV. The He* represents a composite of $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ and $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$, and the Ne* a composite of $\mathrm{Ne}\left(3s{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)$ and $\mathrm{Ne}\left(3s{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}\right)$. This study represents the first direct evidence of AI for which both reactants are excited. Studies were also made of the possible AI reaction ${\mathrm{Ne}}^{*}+{\mathrm{Ar}}^{*}\to \mathrm{Ne}{\mathrm{Ar}}^{+}+e$ from thermal energy to 10 eV.

A comprehensive theory of electron scattering from atoms and ions at high energies is formulated, incorporating the coupling of the scattering system to the radiation field. In particular, the photocapture process is discussed in detail, which can proceed either by a direct radiative capture or by dielectronic recombination following a radiationless capture into excited states. The latter process involves a collisional excitation of the inner-shell electrons of the target, followed by radiative decay of that excited state. The required sum over excitation probabilities to all the allowed bound states is estimated using the semiclassical projection operators and the single-particle model for the target system. A preliminary estimate of the cross sections for these modes of capture is obtained for the case of a hot-electron plasma interacting with highly ionized impurity ions.

Elastic collision cross sections of ${\mathrm{He}}^{+}$ and ${\mathrm{H}}^{+}$ ions incident on Au, Ag, Cu, Bi, Ni, and In, of density equivalent to less than a monolayer of target material, have been measured in the energy range 50-100 keV by the measurement of particles backscattered through a laboratory angle of 150°. The results point to a Thomas-Fermi potential description of the collision process.

A three-body general-type rearrangement collision is considered where the initial and final bound states are described by hydrogenlike wave functions. The solution obtained is for the first Born approximation where the full interaction potential is taken into account. When the initial state is the ground state, it is shown that for $\frac{n}{\mu Z}\gg 1$, where $n$, $\mu$, and $Z$ are, respectively, the principal quantum number, the reduced mass, and the nuclear charge of the formed atom, the capture cross section at all incident energies and for capture into the $s$, $p$, and $d$ angular momentum states behaves as $\frac{C}{n^3}+O\left(\frac{1}{n^5}\right)$, where $C$ depends on masses and charges of the particles, the final angular momentum, and the incident energy. An analytic expression for $C$ is given. It is shown that for the low-lying levels the $\frac{1}{n^3}$ scaling law at all incident energies is only approximately satisfied. The only exception is for capture into the $s$ states according to the Oppenheimer-Brinkman-Kramers approximation. The case for the symmetric collisions is considered and it is shown that for high $n$ and high incident energy $E$, the cross section behaves as $\frac{1}{E^3}$. Zeros and minima in the differential cross sections are given in the limit of high $n$ for electron capture by protons from atomic hydrogen, and for positronium formation by proton-atomic hydrogen collisions.

A many-body treatment of pressure shifts associated with foreign-gas broadening is presented. The theory treats the case of electrically neutral molecules interacting through higher-order permanent moments. The line-shape analysis is developed using graphical finite-temperature perturbation theory. It is shown that the simplest application of the method of moments leads to unphysical results for the pressure shift. It is found that, if certain "quasiparticle" approximations are made before computing the spectral moments, physically sensible results obtain. Explicit expressions for the pressure shift are given for the case of dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions.

A theoretical study is presented of the effects of collisional perturbations on the time-resolved resonant and near-resonant photon scattering from a molecular system. The photon counting rate is formulated in terms of tetradic Green's functions in Liouville space. The collisional and radiative damping effects are handled by an effective-Liouvillian formalism. The formal expressions are disentangled by invoking a series of successive approximations, assuming short correlation times of the thermal bath, neglecting relaxation processes in the ground electronic state, considering rotational relaxation as providing the dominant damping mechanism in the excited electronic manifold, specializing to weak electromagnetic fields, neglecting some off-resonance processes and disregarding collision-induced radiative processes. The photon counting rate is expressed as a triple convolution of the photon counting rate for the free molecule with the Doppler Gaussian profile and with a Lorentz profile. The latter incorporates purely collisional effects due to intrastate cross relaxations and proper $T_2$ processes. The theory accounts well for the recent experimental data of Williams, Rousseau, and Dworetsky on collisionally perturbed ${\mathrm{I}}_2$ molecules.

Radiative and Auger transition probabilities to the $1s$ state have been calculated for the individual multiplets of $\mathrm{Ne}{\mathrm{}\left(1s\right)\mathrm{}}^1{\mathrm{}\left(2s\right)\mathrm{}}^j{\mathrm{}\left(2p\right)\mathrm{}}^k$ configurations. From multiplet fluorescence yields, the effective fluorescence yields ${\overline{\omega }}^i$ for Ne $K{L}^i$ charge states were derived. For statistical distribution of multiplet states, we find ${\overline{\omega }}^i=0.0159, 0.0176, 0.0199, 0.0248, 0.0390, 0.0862, \mathrm{and} 0.229$ for $i=0, \dots , 6$. These theoretical fluorescence yields agree well with experimental results from collisions of 30-MeV ${\mathrm{O}}^{5+}$ ions with Ne.

Krypton gas was excited by high-current relativistic electron bursts, and spectral and temporal characteristics of the uv continuum peaked at 1460 Å were measured as functions of pressure. Gas pressure ranged from 0.14 to 68 atm. The spectral full width at half-maximum of 90±6 Å was virtually independent of pressure from 3.4 to 68 atm. Exponential time constants for the buildup and decay of the uv intensity obeyed a $P^{-1.5\mathrm{}}$ pressure dependence for $P<\mathrm{}4\mathrm{}$ atm. Three pressure-independent intensity decay constants of 9±1.5, 32±6, and 350±70 nsec were obtained for $P>\mathrm{}6\mathrm{}$ atm. The efficiency for conversion of electron kinetic energy to uv energy was (4±1.6)%.

Using the 108- and 84-μm ${\mathrm{D}}_2$O laser lines, new laser-magnetic-resonance (LMR) spectra of the oxygen molecule in its ground state ($X{{}_{}{}^3{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{-}, v=0\mathrm{}$) are observed and analyzed. The corresponding transitions are $n=15\mathrm{}\to 17$ and $n=19\mathrm{}\to 21$, respectively. Bauer, Kamper, and Lustig's values of the $g$ factors are consistent with our results, but Hendrie and Kusch's values are not. Combining all LMR results with other results, we obtain rotational parameters $B_0=43.1004608\mathrm{}\left(75\right)\mathrm{}$ GHz and $B_1=-0.14520\mathrm{}\left(20\right)\mathrm{}$ MHz.

Spectral lines from ions in the nitrogen isoelectronic sequence due to transitions of the type, $2s2p^4{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2},\frac{5}{2}}^{}{}_{}{}^{}-2\mathrm{}{p}^5{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$, and $2s2p^4{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}-2\mathrm{}{p}^5{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$, are identified in the 100-Å region in laser-produced plasma spectra of titanium (Ti xvi) through cobalt (Co xxi). It is proposed that the intensities of these lines relative to the intensities of lines of the same ions due to transitions of the type $2s^{2}2p^3-2s2\mathrm{}{p}^4$ are sensitive to electron density in the range from ≈ ${10}^{18}$ to ≈ ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Calculations are performed for Fe xx, and a similar calculation is performed for a density-sensitive line ratio of Fe xix, i.e., $\frac{(2s2\mathrm{}{p}^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-2\mathrm{}{p}^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{})}{(2\mathrm{}{s}^{2}2p^4{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-2s2\mathrm{}{p}^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{})}$. This line ratio is also sensitive to electron density between about ${10}^{18}$ and ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The $2s2p^5{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}-2\mathrm{}{p}^6{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ line is newly indentified in Cr xvii, Co xx, and Ni xxi.

We have measured the mean lives of two levels in Rb ii by exciting them selectively in fast beams of metastable ${\mathrm{Rb}}^{+}$ ions with radiation from an argon-ion laser, and observing their time-resolved decay downstream. The results are for $5p\frac{3}{2}{\left[\frac{1}{2}\right]}_1$, 8.04 ± 0.08 nsec, and for $5p\frac{1}{2}{\left[\frac{1}{2}\right]}_1$, 6.9 ± 0.2 nsec. We describe some of the systematic errors that must be minimized in order that the accuracies obtained by this method approach or surpass those obtained by other means. Our results show clearly that advanced calculational methods are necessary in order to take full advantage of the experimental accuracy.

Observations of W vii were carried out in the spectral range 100 to 2000 Å utilizing normal- and grazing-incidence spectrographs. The light source was a sliding spark operated at peak currents of 1000 to 3000 A. Eighty-five energy levels of $4f^{13}\mathrm{nl}$ and $5p^5\mathrm{nl}$ configurations were deduced for $nl=5d, 6s, 6p, 6d, \mathrm{and} 7s$. These are based on the classification of 310 lines in the range 130 to 1436 Å. Calculations of all observed configurations were carried out with configuration interaction (CI) for the purpose of fitting radial integrals and confirming the analysis. The strong CI between $p^5\mathrm{nl}$ and $f^{13}\mathrm{nl}$ configurations where $nl=5d, 6p, 6d$ provided well-defined CI integrals in good agreement with Hartree-Fock values. A value of 984 100 ± 500 ${\mathrm{cm}}^{-1\mathrm{}}$ was derived for the ionization energy from the $4f^{13}\mathrm{ns}$ series. The resonance lines of Hf v for $5p^6-5\mathrm{}{p}^{5}5d$, $6s$ are given. A revised value for the the ionization energy of W vi is given.

The difficulty in attempting to prove that a given set of particles cannot form a bound state is the absence of a margin of error; the possibility of a bound state of arbitrarily small binding energy must be ruled out. At the sacrifice of rigor, one can hope to bypass the difficulty by studying the ground-state energy $E\left(\lambda \right)$ associated with $H\left(\lambda \right)\equiv H_{\mathrm{true}}+{\lambda }_v$, where $H_{\mathrm{true}}$ is the true Hamiltonian, $v$ is an artificial attractive potential, and $\lambda >0\mathrm{}$. $E\left(\lambda \right)$ can be estimated via a Rayleigh-Ritz calculation. If $H_{\mathrm{true}}$ falls just short of being able to support a bound state, $H\left(\lambda \right)$ for $\lambda$ "not too small" will support a bound state of some significant binding. A margin of error is thereby created; the inability to find a bound state for $\lambda$ "not too small" suggests not only that $H\left(\lambda \right)$ can support at best a weakly bound state but that $H_{\mathrm{true}}$ cannot support a bound state at all. To give the argument real substance, we study $E\left(\lambda \right)$ in the neighborhood of $\lambda ={\lambda }_0$, the (unknown) smallest value of $\lambda$ for which $H\left(\lambda \right)$ can support a bound state. A comparison of $E\left(\lambda \right)$ determined numerically with the form of $E\left(\lambda \right)$ obtained with the use of a crude bound-state wave function in the Feynman theorem gives a rough self-consistency check. One thereby obtains a believable lower bound on the energy of a possible bound state of $H_{\mathrm{true}}$ or a believable argument that no such bound state exists. The method is applied to the triplet state of ${\mathrm{H}}^{-}$.

The linked-cluster many-body perturbation theory is utilized to obtain an accurate value of the electric field gradient for the lowest ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ state of atomic boron. This diagrammatic method permits us to examine the relative importance of angular-polarization and dynamic correlation effects on the electric field gradient. Also, a detailed comparison with another many-body theory of contributions classified by Hartree-Fock labels is made. The resulting quadrupole shielding factor $R=0.0989\mathrm{}\pm 0.0099$ yields a field gradient $q=0.279\mathrm{}57\pm 0.00276$ a.u. Finally, we obtain from the quadrupole coupling constants the nuclear quadrupole moments of ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ as (8.543±0.104)×${10}^{-2\mathrm{}}$ and (4.099±0.047)×${10}^{-2\mathrm{}}$ b, respectively.

A scattered-wave formalism based on the one-electron Dirac equation for a muffin-tin potential is presented. It is shown that the resulting relativistic secular equations reduce to their nonrelativistic counterparts in the infinite-rest-energy limit.