Diagrammatic many-body perturbation theory is applied to the calculation of potential-energy curves for ${\mathrm{He}}_2$. Both model and shifted perturbative procedures are described. The shifted scheme produces a spurious $R$ dependence for the long-range interatomic potential in both second and third order. Variational-perturbative upper-bound methods produce energies that approach an incorrect asymptotic limit at large internuclear distances. In contrast, the model-perturbative scheme in both second and third order achieves a proper distance dependence for the ${\mathrm{He}}_2$ potential curve. Four basis sets are employed to illustrate basis-set effects.

We show that the existing and widely used results for an approximation to eikonal exchange amplitudes, which contain an indeterminate phase, are not valid. We derive a new analytic result, containing a well-defined phase, which can be used unambiguously with direct amplitudes to calculate full differential cross sections. The modulus of our result also differs from that of the previous formulations. Comparisons are made with the earlier $e^{-}$-H approximate exchange calculations and with exact eikonal exchange calculations.

We have made comprehensive measurements of the noble-gas-induced (He, Ne, Ar, Kr, and Xe) broadening cross sections of the Na $3S-nS$ ($3S-nD$) transitions, from $n=5\mathrm{} \mathrm{to} 38$ ($n=4\mathrm{} \mathrm{to} 40$). These measurements were made using the trilevel-echo technique and are an extension and refinement of our earlier work. We find that the cross sections increase as the geometric cross section at low values of $n$, and then, at a value of $n$ which depends on the particular noble gas, begin to decrease and eventually reach an asymptotic value at high $n$. The broadening cross sections for the nearly degenerate $D$ states are significantly larger than those for the $S$ states and clearly show the effect of $l$-changing collisions. We also compare our measured $S$-state broadening cross sections with the cross sections calculated using the theory of Omont and find good qualitative agreement.

Absolute differential, integral, and momentum-transfer cross sections for electrons elastically scattered from helium are reported for the impact energy range of 5 to 200 eV. Angular distributions for elastically scattered electrons are measured in a crossed-beam geometry using a collimated, differentially pumped atomic-beam source which requires no effective-path-length correction. Below the first inelastic threshold the angular distributions were placed on an absolute scale by use of a phase-shift analysis. Above this threshold, the angular distributions from 10° to 140° were fitted using the phase-shift technique, and the resulting integral cross sections were normalized to a semiempirically derived integral elastic cross section. Depending on the impact energy, the data are estimated to be accurate to within 5 to 9%.

The author presents results of quasiclassical close-coupling-approximation calculations for differential and integrated cross sections and multipole moments for electron-hydrogen impact excitation. These results are compared with calculations using other scattering-theory approximations and with experiment. Discrepancies between the various theoretical results and between theory and experiment are found. Possible sources for some of the discrepancies are mentioned.

The classical double-scattering model of Thomas for the capture of electrons from atoms by fast ions yields a cross section $\sigma$ which dominates over the single scattering contribution for sufficiently fast ions. The magnitude of the classical double-scattering $\sigma$ differs, however, from its quantum-mechanical (second-Born) analog by an order of magnitude. Further, a "fast ion" means an ion of some MeV, and at those energies the cross sections are very low. On the other hand, as noted by Bates, Cook, and Smith, the double-scattering cross section for the capture of atoms from molecules by fast ions dominates over the single-scattering contribution for incident ions of very much lower energy; roughly, one must have the velocity of the incident projectile much larger than a characteristic internal velocity of the particles in the target. It follows that we are in the asymptotic domain not at about 10 MeV but at about 100 eV. For the reaction ${\mathrm{H}}^{+}$ + C${\mathrm{H}}_4$→${\mathrm{H}}_2^{+}$ + C${\mathrm{H}}_3$ with incident proton energies of 70 to 150 eV, the peak in the angular distribution as determined experimentally is at almost precisely the value predicted by the classical model, but the theoretical total cross section is about 30 times too large. Using a quantum version of the classical model, which involves the same kinematics and therefore preserves the agreement with the angular distribution, we obtain somewhat better agreement with the experimental total cross section, by a factor of about 5. (To obtain very good agreement, one may have to perform a really accurate calculation of large-angle elastic scattering of protons and H atoms by C${\mathrm{H}}_3$, and take into account interference effects.) In the center-of-mass frame, for sufficiently high incident energy, the first of the two scatterings involves the scattering of ${\mathrm{H}}^{+}$ by H through an angle of very close to 90°, and it follows that the nuclei of the emergent ${\mathrm{H}}_2^{+}$ ion will almost all be in the singlet state. We have also calculated the cross section for the reaction ${\mathrm{D}}^{+}$ + C${\mathrm{H}}_4$→${\mathrm{}\left(\mathrm{HD}\right)\mathrm{}}^{+}$ + C${\mathrm{H}}_3$.

The stopping po$\stackrel{\mathrm{´}}{\mathrm{w}}$er of ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}$, ${}_{}{}^{109}{}_{}{}^{}\mathrm{Ag}$, and ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$ in nickel and gold was measured as a preliminary test of a new technique for measuring energy loss based on the $\gamma$-ray Doppler shift. The analysis of the data was based on the theories of Lindhard, Scharff, and Schiott for nuclear and electronic stopping. The results are compared with the semiempirical predictions of Northcliffe and Schilling and the Lindhard-Scharff-Schiott theory.

Qualitative arguments suggest that diffraction effects might be important for the interpretation of the experiments of Shih et al. We derive a tractable quantum theory to describe the scattering of neutral atoms at long range by van der Waals forces from solid cylinders. The theory is based on an angular-momentum expansion and evaluates the scattered intensity close to the cylinder in the geometrical shadow region. The formal quantum theory indicates that one is far from the conventional classical limit, in the sense that WKB and stationary-phase approximations are not valid. Yet an essentially exact evaluation appears to agree with the classical prediction, at least over the range of deflections that have been experimentally studied. This surprising classical-quantum correspondence is clarified by a Regge-pole analysis. Finally we also set limits on the extent of specular reflection from the cylinder surfaces. The discrepancy between theory and experiment remains unresolved.

The angular and the energy distributions of fast ionized recoils produced from copper irradiation by noble-gas ions have been investigated. The distributions have been found to be strongly determined by regular arrangement of the target atoms. The results obtained are discussed in terms of the blocking-effect model.

The density model that was successfully used to obtain electronic stopping cross sections for slow-moving heavy ions traversing solid media was applied to estimate the well-known $Z_2$ oscillations in these cross sections. Good agreement with experimental results was generally found. Some of the possible sources of discrepancy are discussed. Relations between the results of Compton-profile measurements which depend on the bulk properties of the solid and the electronic stopping in thin foil targets assist in resolving ambiguities in the $Z_2$ dependence of the experimental results. It is concluded that the electronic stopping cross sections are not a universal function of $E$, $Z_1$, and $Z_2$. They depend strongly on the method of the preparation of the target foil and on its history.

The continuum-distorted-wave (CDW) method is used to determine total electron-capture cross sections $Q=\Sigma {\mathrm{nl}}^{}\sigma \mathrm{}\left[\mathrm{nl}\right]\mathrm{}$ for proton and $\alpha$-particle projectiles incident on ${\mathrm{Li}}^{+}$ in energy ranges of $100\le E\le 3000$ keV and $100\le E\le 10000$ keV, respectively. A configuration-interaction (CI) wave function is used to describe the ${\mathrm{Li}}^{+}$ electrons; for comparison, capture cross sections for ${\mathrm{H}}^{-}$ and He are examined. For each system the percentage change in $Q$, $\Delta (\mathrm{HF}\to \mathrm{CI})$, is given for progression from a Hartree-Fock (HF) to a CI description of the target electrons. The main emphasis in this work is devoted to a rationalization of the trends observed in the ordering of $Q$ for these three targets. This was achieved by an analysis of the CDW expression for an individual capture cross section $\sigma [nl, n^{\prime }{l}^{\prime }]$, where $\mathrm{nl}$ and $n^{\prime }{l}^{\prime }$ are the states of the "active" (captured) and "passive" electrons, respectively.

This paper presents a theoretical study of $e^{-}$-CO scattering at collision energies from 1 to 7 eV in the static-exchange approximation. The $T$-matrix discrete-basis-set approach to electron-molecule scattering introduced by Rescigno, McCurdy, and McKoy is used together with the variational correction method of Fliflet and McKoy. The authors show the behavior of the ${}_{}{}^2{}_{}{}^{}\Sigma$ and ${}_{}{}^2{}_{}{}^{}\Pi$ channel eigenphases and extract the width and position of the ${}_{}{}^2{}_{}{}^{}\mathrm{II}$ shape resonance. Comparison is made with other theoretical and semiempirical results and with the corresponding resonance parameters for $e^{-}$-${\mathrm{N}}_2$ scattering. The momentum-transfer cross section is calculated and compared with the experimental data of Land and with other theoretical results.

A two-state theory is used to obtain the quenching cross sections of metastable helium ions in collision with noble gases (He, Ne, Ar, Kr, and Xe). The potentials used in the present calculation are based on information derived from the experimental scattering data by Smith et al. The results for the quenching cross sections are found to be in reasonable agreement with the recent data of Prior and Wang.

The laser-rf double-resonance technique has been applied to achieve high-precision measurements of the $N$ dependence of the spin-rotation and hyperfine interaction in the ${}_{}{}^2{}_{}{}^{}\Sigma (v^{\prime \prime }=0)$ ground state of CaF in a molecular beam. The effective spin-rotation-interaction constant is shown at small $N$ to depart markedly from the asymptotic value found at large $N$, and the average hfs splitting of the $J^{\prime \prime }=N+\frac{1}{2}$ and $J^{\prime \prime }=N-\frac{1}{2}$ levels is shown to exhibit a maximum for $N=11-12\mathrm{}$. These newly observed effects, though consistent with the theory, have not been explicitly predicted previously.

A theory describing the combined effects of stimulated Raman scattering and continuum self-phase modulation is developed. As may be expected, the effects are not simply additive. Calculations are presented which determine the interaction of these effects in various limits.

Hyperfine structures in the first excited $D$ levels of ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$, ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$, ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$, and ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ have been measured by cascade-fluorescence spectroscopy. We find that the magnetic-dipole coupling constants $A$ (in MHz) are $\left|A\right({}_{}{}^{39}{}_{}{}^{}\mathrm{K},3\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}\left)\right|<\mathrm{}1.8\mathrm{}$, $\left|A\right({}_{}{}^{39}{}_{}{}^{}\mathrm{K},3\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\left)\right|<\mathrm{}2.2\mathrm{}$, $A({}_{}{}^{85}{}_{}{}^{}\mathrm{Rb},4\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{})=+7.3\mathrm{}\left(5\right)\mathrm{}$, $A({}_{}{}^{85}{}_{}{}^{}\mathrm{Rb},4\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{})=-5.2\mathrm{}\left(3\right)\mathrm{}$, $A({}_{}{}^{87}{}_{}{}^{}\mathrm{Rb},4\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{})=+25.1\mathrm{}\left(9\right)\mathrm{}$, $A({}_{}{}^{87}{}_{}{}^{}\mathrm{Rb},4\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{})=-16.9\mathrm{}\left(6\right)\mathrm{}$, and $A({}_{}{}^{133}{}_{}{}^{}\mathrm{Cs},5\mathrm{}{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{})=-22.1\mathrm{}\left(5\right)\mathrm{}$.

Various experimental results for the band oscillator strength of the (0,0) ultraviolet transitions of OH are examined in the light of a recent theory. A value of (1.09±0.04) × ${10}^{-3\mathrm{}}$ and 0.79±0.03 ${\mathrm{\AA }}^{-1\mathrm{}}$ is confirmed for the band oscillator strength and its rotational constant, respectively. New experimental results which verify the spin-independent nature of both are also presented.

The photoionization cross sections for the $3p$ and $3s$ subshells of the ground state of atomic chlorine have been calculated using many-body perturbation theory. We find significant electron-electron correlation effects for both cross sections. Resonances due to the $3p{}_{}{}^4{}_{}{}^{}nd$, $3p{}_{}{}^4{}_{}{}^{}ns$, and $3s3p{}_{}{}^5{}_{}{}^{}np$ configurations, and relaxation effects in the $3p$ subshell cross sections were also calculated.

Two collision-induced Raman spectra of gaseous neon are obtained, one with the linear polarization of the incident beam parallel to the direction of observation, and the other perpendicular. Absolute intensities are determined relative to nitrogen lines of known intensities. The comparison reveals the presence of a polarized contribution previously seen only in one other gas, helium. In addition, a depolarized component is obtained which agrees with and supplements an earlier measurement. For a direct comparison with experiment, collision-induced Raman spectra are also computed from wave mechanics, using models of the invariants (trace and anisotropy) of the neon diatom polarizability as input. For the depolarized spectrum an empirical model of the anisotropy is obtained which differs from the classical point-dipole expression by only 10% at the internuclear separation of 2.69 Å, the root of the interatomic potential. Similar attempts are made to develop an empirical trace model that is consistent with both the polarized spectrum and an earlier measurement of the second virial dielectric coefficient.

A 40-cm curved muscovite mica crystal spectrograph of the transmission (Cauchois) type has been used to identify six new high-frequency $K\alpha$ satellites of Fe at 1.9230, 1.9176, 1.9062, 1.8939, 1.8810, and 1.8698 Å by means of electron excitation. The first four satellites have been assigned to transitions ${\mathrm{}\left(1s\right)\mathrm{}}^{-1\mathrm{}}{\mathrm{}\left(2p\right)\mathrm{}}^{-n}\to {\mathrm{}\left(2p\right)\mathrm{}}^{-n-1\mathrm{}}$, for $n=2-5\mathrm{}$. Relativistic Dirac-Fock (RDF) calculations for the energies of initial and final configurations give excellent agreement with the experimentally observed energies. However, the last two lines have been only tentatively assigned to transitions ${\mathrm{}\left(1s\right)\mathrm{}}^{-1\mathrm{}}{\mathrm{}\left(2s\right)\mathrm{}}^{-n}{\mathrm{}\left(2p\right)\mathrm{}}^{-5\mathrm{}}\to {\mathrm{}\left(2s\right)\mathrm{}}^{-n}{\mathrm{}\left(2p\right)\mathrm{}}^{-6\mathrm{}}$, for $n=1\mathrm{} \mathrm{and} 2$ because RDF calculations for these configurations were were not available. This is the first time that more than two vacancies have been observed following electron bombardment of a target with $Z>\mathrm{}19\mathrm{}$. The possibility of such multiple ionization has already been predicted by the well-known shake-off theory. Previously reported results for high-frequency Fe $K\alpha$ satellites have also been recorded. By using the theoretically calculated energies given by Gianturco, the components of the transition $K^{-1\mathrm{}}{L}^{-1\mathrm{}}\to L^{-2\mathrm{}}$ assigned by Ramberg and Kennard to these satellites under a pure $L-S$ coupling scheme have been confirmed. The satellite ${{\alpha }_3}^{\prime \prime }$, which does not form a component of the earlier group, has been assigned for the first time to the transition ${}_{}{}^3{}_{}{}^{}P_{1,2}^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ allowed in the intermediate-coupling scheme.