We report the observation of a two-step radiative collisional ionization between ${\mathrm{Na}}^{*}\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$ and $\mathrm{Ba}\left({6s}^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ in which an optical photon is absorbed simultaneously with the collision, ${\mathrm{Na}}^{*}\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)+\mathrm{Ba}\left({6s}^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\hslash \omega \to {\mathrm{Ba}}^{*}\left({6p}^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\mathrm{Na}\left(3s {}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ and with subsequent photoionization of the excited barium, ${\mathrm{Ba}}^{*}\left({6p}^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)+\hslash \omega \to {\mathrm{Ba}}^{+}+e$. We observe peak ion intensity close to the difference frequency between ${\mathrm{Ba}}^{*}\left({6p}^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ and ${\mathrm{Na}}^{*}\left(3p {}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\right)$ as well as a quadratic increase of ion intensity with laser power. These results confirm, complement, and extend the interpretation of an earlier observation.

Atomic inner-shell processes involving excitation, ionization, and charge transfer have been investigated for MeV/amu Ar+Xe collisions. The individual processes were isolated and identified by detecting coincidences between argon $K$ x rays and the outgoing projectile charge states of interest. Measurements were made for 160- and 180-MeV argon ions with incident charge states $q=14,15, \mathrm{and} 17$ colliding with Xe target atoms under single-collision conditions. Cross sections for argon $K$ x rays coincident with the outgoing projectile charge states $q-1\mathrm{}$, $q+1\mathrm{}$, and $q+2\mathrm{}$ were measured, as well as cross sections for total $K\alpha$ x-ray production and total single-electron capture. The data indicate that nearly 50% of the x rays emitted by the projectile are associated with some kind of charge-changing event. For $K$ x rays coincident with single-electron capture it is expected that the recently discovered process of resonant transfer and excitation contributes to the measured cross section.

The rotational excitation cross sections for $e^{-}-{\mathrm{H}}_2$ and $e^{+}-{\mathrm{H}}_2$ scattering have been calculated at low energies using a distorted-wave approximation. A newly proposed model exchange potential has been used in the case of $e^{-}-{\mathrm{H}}_2$ scattering to examine its suitability in predicting the rotational excitation cross sections. The results obtained are found to be in qualitative agreement with the experimental as well as other theoretical results.

Cross sections are calculated for the charge transfer of ${\mathrm{N}}^{3+}$ ions in collisions with atomic hydrogen at impact energies up to 5 keV/amu. The calculated cross sections exceed the values measured between 860 and 5080 eV/amu by about 20%. The charge-transfer process results in the emission of extreme-ultraviolet radiation at 45 nm whose detection would serve as a measure of the detailed mechanism by which charge transfer proceeds.

A new procedure for matching hyperspherical coordinate (HS) wave functions onto close-coupling (CC) wave functions for two-electron systems at a hyperradius $R=R_0$ is shown to give agreement with accurate ${}_{}{}^1{}_{}{}^{}S$ phase shifts for $e+\mathrm{H}$ up to the ${2s}^2$ resonance. The procedure uses far fewer basis functions than are required in either a HS or a CC calculation alone. The strengths of each method are thus effectively combined to obtain wave functions describing highly correlated two-electron motion near the nucleus ($R<\mathrm{}{R}_0$) and largely independent electron motion away from the nucleus ($R>\mathrm{}{R}_0$).

Previous results are extended to obtain the ionization potential of an arbitrary atom in an arbitrarily polarized single-mode steady-state laser. The results come from energy conservation and ideas associated with the ponderomotive potential. They are used to discuss the width of the energy distribution of electrons obtained from multiphoton ionization.

Measurements of the fine-structure splitting of the ${4s}^2{4p}^{2}P$ ground-state term of the gallium isoelectronic sequence using low-inductance sparks, laser-produced plasmas, and tokamak plasma sources are presented. The observations are in excellent agreement with semiempirical predictions using screening parametrizations, and permit a refinement of these predictions.

The continuous oscillator strengths and photoionization cross section of He and ${\mathrm{Li}}^{+}$ are calculated by use of the polarized-orbital method. Matrix elements involving distorted-target wave functions have been included. The results are found to be in excellent agreement with the experimental values.

A simple semiempirical scaling procedure for predicting the intensity ratios of $\mathrm{np}-1s$ transitions in multiply ionized atoms is presented.

Period-doubling bifurcations into a chaotic state are found in a quantum statistical model of multiphoton excitation of a molecule when the laser field is modulated sinusoidally. This chaotic state characterizes a strange attractor coexisting with a periodic one. Interesting dynamical behavior associated with these attractors is described and practical implications of this molecular property discussed.

The properties of the photon-counting distribution, for arbitrary intervals, produced by a mixture of coherent radiation and the radiation scattered by a strongly pumped two-level atom are calculated. Such properties reflect in an important way on the squeezing characteristics and the anomalous correlators of the scattered radiation.

Experimental evidence is reported of an oscillatory dependence of an effective viscosity on the Knudsen number for nitrogen and argon gases in the transition regime, which in our experimental setting corresponds to the range of pressures from 1 μm to ∼20 μm column of Hg at $T=293\mathrm{}$ K. These results support that the damped oscillatory dependence of the stress on the Knudsen number, previously reported for helium, is a property of gases in the transition regime.

A recent refinement by Büttiker, Harris, and Landauer [Phys. Rev. B 28, 1268 (1983)] of Kramers's theory of activated escape at low damping is extended to the non-Markovian case with memory damping.

A recently published addition theorem for the bound-state hydrogenlike atomic orbitals [T. Yamaguchi, Phys. Rev. A 27, 646 (1983)] is critically analyzed. It is shown that the convergence of the expansion derived by Yamaguchi is not guaranteed since the continuum states of the hydrogen spectrum are not included properly in the expansion. Alternative complete orthonormal sets of functions for which the problems with the continuum states do not occur are suggested. Overlap integrals of bound-state hydrogenlike functions which serve as Fourier coefficients in the addition theorem are also investigated. It is demonstrated that in all cases explicit expressions for the overlap integrals can be derived.

A quantum-mechanical version is presented of a recent classical study of conditions under which perpendicular dc electric and uniform magnetic fields give rise to runaway electrons along the electric field direction. The quantum-mechanical calculation reproduces the classical result.

Extension of a previous classical analysis, to include the presence of negative ions in a multigrid extraction gap of a negative-ion source, yields the same results as those of a recent quantum-mechanical calculation.

Following a proposal by Asher Peres we have carried out a neutron interferometer experiment searching for quaternion noncommutative contributions to the neutron-nuclear scattering amplitude. Using slabs of Ti and Al we find that the phase shift experienced by one of the beams in our interferometer is invariant under the interchange of the two slabs to an accuracy of 1 part in 30 000.

Interaction potentials for $\mathrm{Ne}({2p}^{5}3s,{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{})+\mathrm{Ne}$ have been determined from differential cross sections measured at thermal energies. The potentials are of shorter range and the gerade-ungerade splitting is inverted compared with nearly all previous determinations. It is shown how the sign of the gerade-ungerade splitting can be obtained directly from the data. An upper limit for the cross section for fine-structure-changing collisions is obtained.

For asymptotically high incident velocities we provide simple, heuristic, almost classical, derivations of the cross section for forward charge transfer, and of the ratio of the cross section for capture to the elastic-scattering cross section for the projectile scattered through an angle close to $\frac{\pi }{3}$.