High-temperature spin-exchange optical pumping was used to measure the density shifts of the hyperfine splitting of ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}$ in He, Ne, Ar, and ${\mathrm{N}}_2$. Measurements over a limited temperature range also yielded linear temperature dependences. The respective results for the fractional shift, $\frac{\partial \Delta \nu }{\partial \rho {\nu }_0}$ (in ${10}^{-9\mathrm{}}$ ${\mathrm{Torr}}^{-1\mathrm{}}$), are 40.6 ± 1.4, 26.2 ± 0.2, 14.2 ± 0.9, and 33.6 ± 2.6 and for the linear temperature dependence, $\frac{{\partial }^2\Delta \nu }{\partial T\partial \rho {\nu }_0}$ (in ${10}^{-12\mathrm{}}$ ${\mathrm{Torr}}^{-1\mathrm{}}$°${\mathrm{C}}^{-1\mathrm{}}$), are 13.3 ± 1.5, 5.0 ± 0.6, -6.4 ± 1.2, and -4.9 ± 0.8.

The dipole oscillator strengths ($f$) for some transitions within the ground complex of the magnesium isoelectronic sequence up to nobelium ($Z=102\mathrm{}$) are calculated using the relativistic wave functions obtained by the parametric-potential method. The length and velocity formulations are used for the transition operator. For the resonance lines, the trend along the sequence differs strongly from the predictions of the nonrelativistic $Z$-dependent theory of many-electron atoms. This behavior is explained in the framework of the relativistic $Z$-dependent theory, which introduces a double power-series expansion in $Z^{-1\mathrm{}}$ (for correlation) and $Z^2{\alpha }^2$ (for relativistic effects). The $f$ value for the first resonance transition increases for low values of $Z$, as the departure from the Russell-Saunders coupling becomes more important, and decreases at high values of $Z$ owing to the contraction of the orbitals toward the nucleus. The $f$ value for the second resonance transition does not fall off for large $Z$, since the frequency of the transition increases approximately as $Z^4$ for high values of $Z$, owing to the relativistic corrections (spin-orbit, Darwin, and $p^4$ terms). The length and velocity formulations are discussed, with particular emphasis on how relativistic contributions to the transition energies or to the transition matrix elements occur. Corrections coming from the finite size of the nucleus, the Breit interaction, and the Lamb shift are introduced; although they are significant, they do not alter the general shape of the curves giving the $Z$ dependence of the $f$ values. On the contrary, retardation effects remain unimportant over the entire range of $Z$.

A simple variation-perturbation method recently developed is applied to the calculation of electron affinities. In this method a common basis is used for both the ion and the neutral system. The resulting secular equations are of a very small dimension. The matrix elements entering these equations can be evaluated by diagrammatic methods. The formalism is compared with the one-body Green's function method. The electron affinities of Li, Na, F, Cl, and OH are calculated.

This work reported cross sections for emission of 3914-Å radiation from collisions of ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ projectiles with ${\mathrm{N}}_2$ target molecules. The 3914-Å photons are emitted in the decay of the $B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state of ${\mathrm{N}}_2^{+}$ to the $X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ ground state, the vibrational quantum number being zero for both excited and ground state. The energy range of the projectiles in these experiments was from 0.5 to 34.0 keV.

Target $K$-shell x-ray production cross sections, x-ray energy shifts, and $\frac{K\beta }{K\alpha }$ ratios have been measured for 7-35-MeV ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ ions on thin solid films of Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Comparisons of the data were made with theoretical predictions obtained from the binary-encounter approximation and the plane-wave born approximation (PWBA). These theories were found to overpredict the experimental data by a factor of 10 at the lower energies. However, at the lower energies, the PWBA, with modifications for increased target electron binding, Coulomb deflection, target electron polarization, and relativistic effects produced results in good agreement with the experimental data. At higher energies, a systematic deviation of the theoretical predictions from the experimental data was found which increases as the ratio of $\frac{Z_1}{Z_2}$ becomes larger. Corrections to the fluorescence yields attributable to multiple ionization processes and the addition of electron capture contributions to the previously indicated direct ionization theories produced results in excellent agreement with the experimental data with the exception of those for the light elements at the highest energies.

The stopping cross sections of ${\mathrm{H}}_2$O ice and vapor for $\alpha$ particles of energy 0.3-2.0 MeV have been measured, respectively, by a thick-target elastic-scattering method and with a differentially pumped gas-cell system. The stopping cross section of ${\mathrm{H}}_2$O vapor (probable error ≃ 1.1%) was found to be (4-12)% higher than that of ${\mathrm{H}}_2$O ice (probable error ≃ 4%). This difference is less than that found previously for protons [(10-14)%] in the same velocity interval. It is proposed that the physical-state effect in the stopping cross section of ${\mathrm{H}}_2$O is caused by changes in the modes of electronic excitation in the molecule caused by the state of aggregation.

The coincidence technique has been used to obtain the energy spectrum of $L$ Auger electrons ejected during close ${\mathrm{Ar}}^{+}$ + Ar collisions in which projectile ions with energy $T_0$ were scattered through an angle $\theta =14\mathrm{}°$ and were ionized to a known charge $m$. The energy of the incident ions ranged between 50 and 375 keV, corresponding to minimum internuclear separations $r_0$ from 0.18 to 0.05 Å, respectively. All spectra appear broad and nearly structureless, a fact which strongly suggests that a large number of different electronic configurations are populated in such events. An analysis based upon integrals over the appropriate coincidence counts spectra allows us to derive the $L$ Auger yield per scattering event. In gentler collisions with $0.13 \AA <r_0<0.18\mathrm{} \AA$ each encounter produces two Auger electrons. They are the result of $2p$ holes which are created when electrons make transitions out of the strongly promoted $4f\sigma$ molecular orbital. In closer events, $r_0<0.13\mathrm{}$ Å, the yield rises, and indeed, collisions are observed in which as many as four or five additional $L$ holes appear. We conclude that this increase is caused by $3d\pi \to 3d\delta$ rotational couplings. We further conclude that the bulk of the inelastic energy losses characteristic of these close collisions are due to $L$-vacancy creation.

We have measured absolute yields of $K$ x rays from the decay of heliumlike $\mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ and lithiumlike $\mathrm{}\left(1s2s2p\right)\mathrm{}{}_{}{}^4{}_{}{}^{}P_{\frac{5}{2}}^{}{}_{}{}^{}$ states produced by the passage of 30-50-MeV beams of Si, S, and Cl through thin foils of C, Al, Ti, and Ni. The x-ray yields saturate for sufficiently thick foils at values which are strongly dependent on the foil material. This behavior is attributed to the dependence of the saturation $K$-vacancy-bearing fraction of the beam on target atomic number. A model based on the factorization of inner- and outer-shell processes is described and used to discuss the data.

It is shown that the energies $E_{\mathrm{Nl}{l}^{\prime }}\mathrm{}\left(m\right)\mathrm{}$ of Feshbach resonances in Ne, Kr, Ar, and Xe are related to their respective ionization potentials via an equation of the type $E_{\mathrm{Nl}{l}^{\prime }}\mathrm{}\left(m\right)=A_{\mathrm{Nl}{l}^{\prime }}I\left(m\right)+B_{\mathrm{Nl}{l}^{\prime }}$, where $E_{\mathrm{Nl}{l}^{\prime }}\mathrm{}\left(m\right)\mathrm{}$ is the energy of the $N\mathrm{th}$ resonance with excited electron angular momenta $l$ and $l^{\prime }$ in species $m$, $I\left(m\right)\mathrm{}$ is the ionization potential of species $m$, and $A_{\mathrm{Nl}{l}^{\prime }}$ and $B_{\mathrm{Nl}{l}^{\prime }}$ are independent of atomic species $m$. It is shown how the above linear relation simplifies grouping of families of resonances with similar configurations in Ne, Kr, Ar, and Xe. The linear relation implies that for Feshbach resonances electron-electron interactions external to the ion core dominate over electron-ion-core interactions within the ion core.

We have measured the relative optical excitation functions of the 3776-Å ($7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\to 6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$), 2768-Å ($6{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\to 6{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$), 3529-Å ($6{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\to 6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$), and 3519-Å ($6{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\to 6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$) lines, and the polarization function of the 2768-Å line, using crossed beams of electrons and thallium atoms, for electron energies from thresholds to 1500 eV. The electron energy resolution was ∼0.3 eV for energies below 13 eV, and the atom beam was optically thin. By normalizing the resonance-line excitation functions to Born theory in the high-energy limit the $7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$, $6{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$, and $6{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ level-excitation cross sections are obtained. The $7{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$-level excitation function rises very rapidly immediately above threshold, while the $6{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ level rises much more slowly. The 2768-Å polarization function shows strong resonance at a few electron volts above the threshold. The $6{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ level excitation function peaks at lower energy as expected for a dipole-forbidden transition, but shows a small $E^{-1\mathrm{}}{log}_{10}E$ behavior at the high-energy limit.

Cross sections for electron capture from inner atomic shells by fully stripped ions, of velocities high compared to the electron velocities in the inner-shell orbits, are calculated in the second Born approximation. The theory of Drisko for electron capture by protons from hydrogen is generalized to projectiles and targets of arbitrary atomic numbers $Z_1$ and $Z_2$, respectively. For ions of low velocity, the effects of binding and Coulomb deflection are accounted for in a manner similar to that of Brandt and his co-workers in the theory for direct ionization to the continuum of the target atom. The results are in good agreement with experimental capture cross sections, whereas the Oppenheimer-Brinkman-Kramers approximation overestimates such cross sections. The contribution of electron capture to inner-shell ionization cross sections increases with increasing $\frac{Z_1}{Z_2}$ and brings them into agreement with experiment.

We have measured $K$-shell ionization cross sections by electron impact for 19 elements with atomic number in the range $Z=13-92\mathrm{}$, $L$- and $M$-shell ionization cross sections for nine and two heavier elements, respectively, in the energy range 70-270 MeV. The results on the $K$-shell ionization were compared with the predictions from the revised Kolbenstvedt theory and the plane-wave Born approximation calculation by Davidović and Moiseiwitsch, and good agreements were obtained. The $L$- and $M$-shell ionization cross sections were also compared with the predictions from the Kolbenstvedt theory using photoeffect cross sections for these shells and good agreements were obtained, although there is some ambiguity in the photoeffect cross section used in the estimation. The $K$-shell ionization cross sections, together with other experimental results, were represented in a good scaled form based on the revised Kolbenstvedt theory.

High-resolution measurements of ion-excited $K\alpha$ x-ray spectra have been performed for a variety of Al, Si, S, and Cl compounds. An examination of the $K\alpha$ satellite intensity distribution revealed significant variations from one compound to another. This effect has been attributed to alteration of the $K$ and/or $L$ vacancy transfer rates due to the influence of chemical environment. The systematic trend displayed by the data tends to suggest that interatomic processes are important in the deexcitation of these highly ionized systems.

We have measured high-resolution electron decay-in-flight spectra of highly excited carbon ions, produced by 2-MeV ${\mathrm{C}}^{+}$ beams emerging from carbon foils. An almost complete identification for the resolved spectral lines is suggested for the prompt and time-delayed electron emission spectra. To assign the observed structures excitation energies for various core-excited states have been calculated using the generalized Brillouin theorem-multiconfiguration method of Schwarz and Chang. Experimental term energies for Coulomb and metastable autoionizing states in two-, three-, and four-electron carbon ions are tabulated and compared to calculated transition energies. Because of line blending most of the experimentally deduced term energies are uncertain to ±0.4%. Some spectral features in the high-energy portion of the prompt spectrum can be attributed to decays of doubly excited He-like carbon ions.

We report the first application of Baker's second-order eikonal approximation (SOEA) to atomic collisions and compare the predictions of SOEA with those of Glauber's first-order eikonal approximation (FOEA). The SOEA takes into account amplitude modulation, in addition to phase modulation of the incoming plane wave. In the case of elastic scattering of electrons from argon and krypton, it is found that the second-order correction term influences to a large extent the angular distribution of scattered electrons. In the intermediate-and high-energy range it is seen that the SOEA gives much better agreement than the FOEA with the staticfield calculation of Fink and Yates.

A perturbation theoretic calculation of the quenching of fast metastable ions by collision with rare-gas atoms is developed. The quenching is caused by transitions resulting from the interaction with the electrostatic potential of the target atom. A comparison with experimental data is given.

$L$-subshell ionization cross sections for Au, Tl, Pb, Bi, Th, and U by proton impact have been determined over the projectile range 0.5-3.5 MeV. The measured x-ray production cross sections of the total $L$ shell and of some well resolved lines or groups of lines are consistent with those obtained by different authors in the same regions of bombarding energies and atomic numbers. Ionization cross sections were obtained by using the above results and experimental values for the relative radiative transition probabilities, fluorescence yields, and Coster-Kronig factors. The branching ratios of radiative transitions were measured and are in good agreement with the widely used theoretical calculations of Scofield, Rosner, and Bhalla. The values of fluorescence and Coster-Kronig yields were taken from previously published experiments performed in our laboratories. The influence of these experimental data on the shape of cross section versus proton energy curves is discussed. Comparisons of the experimentally determined $L$-subshell ionization cross sections are made with calculations in the plane-wave Born (PWBA), semiclassical, and binary-encounter approximations. The large effect of binding-energy, trajectory, and relativistic corrections on the PWBA calculations invalidates quantitative conclusions regarding agreement between experimental and theoretical values. Semiclassical arguments are presented, however, to explain some general aspects of the ionization cross-section curves.

The branching ratio for the formation of bound positronium atoms in nuclear pair formation is calculated. It ranges from ∼${10}^{-8\mathrm{}}$ in light nuclei to ∼${10}^{-6\mathrm{}}$ in heavier nuclei. This mechanism yields $n{}_{}{}^3{}_{}{}^{}S$ positronium atoms formed in vacuum, perturbed only by the nuclear Coulomb field, and should provide a reasonably copious source for experiments on the vacuum eigenstates.