We examine subshell branching ratios of partial photoionization cross sections, within a relativistic single-electron screened potential, as a function of energy and principal quantum number, for uranium, tin, and carbon. We focus on features beyond the simple kinetic-energy effects made visible by the shift in photon energy, resulting from the fine-structure splitting, which corresponds to the same shape-resonance energy of the outgoing continuum electron. At low energies, deviations from statistical $\frac{\mathrm{}(L+1\mathrm{})\mathrm{}}{L}$ ratios are amplified by the presence and by the separation of Cooper minima in dominant matrix elements. Effects are largest for outer $p$ shells of high-$Z$ elements because the separation of Cooper minima is largest. At higher energies in a given element all the $\mathrm{nL}$ ratios for a given $n$ tend to merge into a common curve.

The linear-algebraic method for electron-molecule collisions developed by Schneider and Collins is generalized to include separable expansions of the exchange kernel. These separable expansions are constructed using bound-state integral programs and spherical-harmonic projections which may be performed by analytic methods. The short-range nature of the exchange operator obviates the need for large basis sets in the separable expansion and allows for a very efficient construction of the required integrals. The method is applied to $e+{\mathrm{H}}_2$ and $e+\mathrm{LiH}$ scattering in the static-exchange approximation. Comparison of the separable expansion with more standard numerical techniques for exchange is excellent (<1%) even for strongly polar systems like LiH.

We have formulated and applied a separable approximation for treating the nonlocal exchange interactions that arise in electron-molecule collision problems. A separable representation of the exchange terms in the electron-molecule interaction potential is obtained by projection onto a finite set of Cartesian Gaussian functions and is combined with a single-center expansion of the direct potential terms that accurately treats the long-range, multipolar forces. An integral-equation method is used to obtain a solution of the set of coupled equations obtained in a body-frame formulation of the collision problem. The method is illustrated by application to low-energy $e^{-}-{\mathrm{H}}_2$ and $e^{-}-\mathrm{L}\mathrm{i}\mathrm{H}$ scattering in the static-exchange approximation.

Cross sections and rate coefficients for the direct electron-impact ionization of sodiumlike ions have been calculated in a distorted-wave Born exchange approximation. For higher charge states the cross section is dominated by inner-shell ionization of the $2p^6$ subshell. Analytic fits to the data allow rapid calculation of cross sections and rate coefficients over a wide range of incident electron energies. The importance of electron-impact excitation to autoionizing states is discussed.

Electron-impact-ionization cross sections and rates have been calculated for several berylliumlike ions in the $1s^{2}2s^2{}_{}{}^1{}_{}{}^{}S$ ground state and the $1s^{2}2s2p {}_{}{}^3{}_{}{}^{}P$ metastable state using a distorted-wave Born exchange approximation. The ground-state cross section showed only minor deviations from classical scaling along the isoelectronic sequence. The $2s2p {}_{}{}^3{}_{}{}^{}P$ cross section was more than twice that of the ground state at C III, decreasing to a factor of 1.35 greater at Ar XV. Dominant ground-state configuration interaction of the form $2s^2+2\mathrm{}{p}^2$ changed the total cross section by only a few percent. An analytic fit to the distorted-wave results is given, and this allows the calculation of nonrelativistic electron-impact-ionization cross sections and rates for berylliumlike ions. A comparison with available experimental data shows good agreement between measurements and distorted-wave calculations.

Cross-section measurements for inelastic collisions of rare-gas ions with Na Rydberg atoms that change the orbital angular momentum $l$ are reported. ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, and ${\mathrm{Ar}}^{+}$ ions at 400-2000 eV collide with laser-excited $\mathrm{Na}\mathrm{}\left(\mathrm{nd}\right)\mathrm{}$ atoms in a crossed-beam experiment. Time-resolved field ionization is used as a selective detection method. The $l$-changing cross section is found to vary with $n$ as $n^{\beta }$, where $\beta =5.12\mathrm{}\pm 0.20$ when projectile and bound-electron velocities are comparable. The velocity dependence is given in the range $\tilde{v}=0.6-4.0\mathrm{}$, where reduced velocity $\tilde{v}$ is the ratio of ion and electronic orbital velocities. Evidence is offered that the induced transitions are predominantly dipole in character under single-collision conditions but that rapid equilibration of higher $l$'s occurs by successive ion impacts. Initial $\mathrm{Na}\mathrm{}\left(\mathrm{ns}\right)\mathrm{}$ states yield an $l$-changing cross section at least an order of magnitude smaller than $d$ states and appear to go most readily to $\mathrm{}(n-1\mathrm{})l\ge 2$ states in endothermic nondipole-allowed $\Delta n=1\mathrm{}$ processes. The cross-section magnitude for ${\mathrm{Ne}}^{+}+\mathrm{Na}\mathrm{}\left(28d\right)\mathrm{}\to {\mathrm{Ne}}^{+}+\mathrm{Na}(28,l\ge 3)$ at 1000 eV is $\approx 5\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^2$, in agreement with existing theories. This exceeds the geometric cross section $n^4\pi a_0^2$ by a factor of ${10}^3$ and indicates that passage of an ion closer to the Rydberg atom than 30 orbital radii $n^2{a}_0$ suffices to induce $l$ change in the velocity-matching region.

Absolute electron-impact optical-excitation functions of 26 transitions of the sharp, principal, diffuse, fundamental, and $\mathrm{nP}-4S$ series of sodium have been measured in the impact-energy range 0-150 eV. The determination of the target-atom number density was made by measuring the attenuation of sodium resonance radiation from a fluorescence cell upon passage through the collision chamber. Direct-excitation cross sections of 14 states ($4S$, $5S$, $6S$, $7S$, $3P$, $4P$, $5P$, $6P$, $3D$, $4D$, $5D$, $6D$, $6F$, and $7F$) have been determined from the measured optical-excitation cross sections with the aid of radiative-transition probabilities taken from the literature. These direct-cross-section results are compared with theoretical calculations based upon the Born approximation and the multistate-close-coupling approximation. Measurements were also made of the polarization of several sharp, principal, and diffuse series-resolved doublets, and from these results the direct-excitation functions of the separate orbital magnetic sublevels of the $3P$ state have been determined.

A Monte Carlo program to calculate the distributions of separation and relative velocity resulting from the dissociation of fast molecular ions traversing thin foils, including effects of Coulomb explosion, multiple scattering, and differential energy loss, has been developed. Here it is used to explore the effect of the single-scattering cross section employed on the distributions which result. It is demonstrated that different single-scattering distributions—based upon different assumptions concerning atomic screening—give rise to differing separation distributions even though they result in essentially identical multiple-scattering distributions. Such effects must be taken into account in models of subsequent molecular or atomic recombination of the exiting particles.

Ab initio molecular-orbital methods are utilized to obtain potential-energy curves and coupling-matrix elements to assess the cross section for the charge transfer ${\mathrm{S}}^{2+}\left({}_{}{}^3{}_{}{}^{}P\right)+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to {\mathrm{S}}^{+}+{\mathrm{H}}^{+}$ at electron-volt collision energies. Results of quantal, coupled-channel calculations confirm the indications from the positions of the potential-energy curves that the cross section is quite small—less than about ${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$ at 1 eV.

Laser-fluorescence spectroscopy and the laser-rf double-resonance method are combined to classify lines in the Pr I absorption spectrum. The double-resonance technique is found to be particularly valuable for this purpose since, by focusing on the lower level of a transition, it leads in many cases to unique line assignments. Precise (∼ 1 ppm) measurements are reported for the hyperfine-structure (hfs) constants of 15 low levels, and 0.5% values for 34 higher levels. Classifications are given for many lines in the region $5746<\mathrm{}\lambda <6148\mathrm{}$ Å. Preliminary values are derived from the double-resonance data for the single-electron hfs integrals $a_{4f}$, $a_{5d}$, $a_{6s}$, $b_{4f}$, and $b_{5d}$ for the low-electron configurations of Pr I.

The energy, wave function, oscillator strength, and various radial integrals of the ${}_{}{}^2{}_{}{}^{}D_{}^e{}_{}{}^{}$ resonances of the Li atom are calculated using a saddle-point technique. In the elastic-scattering region, the calculated positions agree excellently with those of accurate experimental measurements for all observed resonances. However, in the energy region between the ${\mathrm{Li}}^{+}$ $1s2s {}_{}{}^3{}_{}{}^{}S$ and $1s2s {}_{}{}^1{}_{}{}^{}S$ thresholds, the agreement with the optical-absorption experiment is comparatively poor. This is reasonable in view of the asymmetry observed in the line shapes, which implies a possible difference between the maximum line intensity and resonance position. A more definitive comparison can only be made if a full scattering calculation is carried out or if the widths and line-profile parameters are measured in the experiment. A tentative assignment is also suggested for the lines observed by Rassi et al. in this energy region. If these assignments are correct, it would mean the present calculation agrees more closely with the scattering experiment than with the optical-absorption experiment between the lowest-two inelastic-scattering thresholds.

Numerical results from a calculation of the positron bremsstrahlung spectrum in a partial-wave expansion are presented for $Z=8\mathrm{}$, 92 at photon energies 10, 50, and 500 keV. Data are presented both for a point-Coulomb potential and for a screened Hartree-Slater central potential. Comparison is made with the corresponding electron bremsstrahlung. In the point-Coulomb case the nonrelativistic Sommerfeld formula or the Elwert factor provides a quantitative prediction of the ratio of the two spectra for $Z=8\mathrm{}$, qualitative for $Z=92\mathrm{}$; similar accuracy is achieved in the screened case with a ratio of screened normalization. The positron spectrum is strongly suppressed for low-energy positrons, which cannot penetrate toward the repulsive central potential. For high-energy positrons or long-distance interactions the positron spectrum approaches the electron spectrum.

The ground-state hyperfine constant $A$ and the nuclear-to-electronic $g$-factor ratio $\frac{g_I}{g_J}$ of ${}_{}{}^{25}{}_{}{}^{}\mathrm{Mg}_{}^{+}{}_{}{}^{}$ have been measured by a laser optical-pumping double-resonance technique. The ions were stored in a Penning trap at a magnetic field of about 1 T. The results are $A=-596.254\mathrm{} 376\left(54\right)\mathrm{}$ MHz and $\frac{g_I}{g_J}=9.299\mathrm{} 484\left(75\right)\mathrm{}\times {10}^{-5\mathrm{}}$. The magnetic field at the ions was stabilized by servoing it to an ($\Delta m_I=0\mathrm{}$, $\Delta m_J=\pm 1$) electronic Zeeman transition. Other hyperfine ($\Delta m_I=\pm 1$, $\Delta m_J=0\mathrm{}$) transitions were detected while the field was thus stabilized. The derivative of the $(m_I,m_J)=(\frac{-3\mathrm{}}{2}, \frac{1}{2}) \mathrm{to} (\frac{-1\mathrm{}}{2}, \frac{1}{2})$ transition frequency with respect to magnetic field $B_0$ goes to zero at $B_0\simeq 1.24$ T. The corresponding resonance was observed at this field with linewidths as small as 0.012 Hz ($Q=2.4\mathrm{}\times {10}^{10}$) by implementing the Ramsey interference method with two coherent rf pulses separated in time by up to 41.4 s.

With the use of synchrotron radiation, the $\beta$ parameters of the Xe $5p$ components have been measured between 20 and 105 eV in a regime in which both relativistic and many-body effects are important. The experimental $\beta$ values of ${}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ are found to be in good accord with the relativistic random-phase-approximation (RRPA) theory, but only in fair agreement with the Dirac-Fock prediction. Similarly, the branching ratio $\sigma \frac{\left({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)}{\sigma \left({}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)}$ agrees well with the RRPA. However, the Dirac-Fock model accounts satisfactorily for the difference in the $\beta$ values of the components indicating similar many-body effects acting on both components. A brief description is given of the new gas-phase angle-resolved electron-spectrometry system used in this work.

Dye-laser photodetachment of the electronic and vibrational ground state of N$\mathrm{O}_2^{}{}_{}{}^{-}$ is accomplished in a mass-identified drifted tube. Photodetached electrons are detected. The absolute-magnitude photodetachment spectrum is determined in the visible range of 5400 to 4350 Å and the ultraviolet range between 3550 and 2600 Å. The size of the cross section varies by more than 3 orders of magnitude when the size of the lowest vibrational transition is compared with that of the total cross section at ultraviolet energies. The electron affinity of N$\mathrm{O}_2^{}{}_{}{}^{-}$ is determined to be $E_A\left({\mathrm{NO}}_2^{-}\right)=2.275\mathrm{}\pm 0.025$ eV. A Franck-Condon factor analysis of the absolute-magnitude spectrum yields, for the first time, structural constants of N$\mathrm{O}_2^{}{}_{}{}^{-}$ in the free gas phase. They are $r=1.15\mathrm{}\pm 0.02$ Å and $\theta ={119.5}^{\circ }\pm {1.0}^{\circ }$. The solar detachment rate of N$\mathrm{O}_2^{}{}_{}{}^{-}$ is 0.042±0.010 ${\sec }^{-1\mathrm{}}$. A peroxy isomer of N$\mathrm{O}_2^{}{}_{}{}^{-}$ is not detected.

Broadening and shifts of the Xe I line at $\lambda =3.51\mathrm{}$ μm are measured at two temperatures and for various perturbers, He, Ne, Ar, and Xe, by use of a laser linear-absorption experiment. A theoretical interpretation is proposed which involves the quantum theory of collisions. The long-range part of the interaction potential is described by the successive orders of the multipole expansion of the interaction between excited-state atoms and ground-state perturbers; the short-range part is described by an extension of a Fermi-type potential, and a cutoff function is used for the intermediate case. For all perturbers, calculated broadenings and shifts are shown to be sensitive to the position of the potential well, but not to its depth.

Output characteristics of a traveling-wave monomode laser with an intracavity three-level absorber are investigated. The laser frequency is assumed to be symmetrically detuned from the two ground-state sublevels for a $\Lambda$-type transition scheme. It is shown that bistability and hysteresis in the laser output, characteristics of a first-order phase transition, may arise much before the absorber saturates. For this reason, bistability may be observed even if the gain medium saturates faster than the absorber, in contrast with the case of a two-level saturable absorber. The physical mechanism for bistable behavior is coherent population trapping which accumulates all absorbing atoms in the ground-state sublevels with a sharp decrease in absorption.

We demonstrate the generation of a coherent signal by an incoherent process. In a first step, population is transferred from the ground state to two excited states by a collisional redistribution process. Then, in a second step, a coherent Raman signal is generated from these two populated excited states. At high pressure, it is predicted that a Raman-type resonance between two equally populated levels should be observed. Information about inelastic-collision cross sections and dephasing coefficients in the excited states is obtained.

We present a semiclassical treatment of two-photon laser-induced radiative collisions. We treat the two-photon—one-collision and two-photon—two-collision cases. We find a new intensity-induced collisional shift which makes the two-photon line shape highly sensitive to the intensity, and may make the line shape symmetric.

Previously published papers have noted the existence of two-level periodicities in idealized theoretical models of monochromatic excitation of multilevel systems. The present paper provides a theoretical derivation of such two-level dynamics from the more general $N$-level rotating-wave-approximation Schrödinger equation and applies the resulting formulas to different extremes of excitation in a nondegenerate system of sequential (ladderlike) excitation: first, when intermediate levels have much larger cumulative detunings than Rabi frequencies, and second, when intermediate Rabi frequencies are much larger than detunings. In each case one has simple analytic expressions for the effective two-level Rabi frequencies and detunings.