The polarization of photons emitted from radiative recombination of bare ions with free electrons is discussed in the nonrelativistic dipole approximation in the low-relative-energy limit. Within this approach an analytical expression is derived for the degree of polarization of photons from radiative recombination of ions with fixed-energy electrons for an arbitrary (n,l) state. For a relative-velocity distribution of the electron beam, characterized by the longitudinal ${\mathit{kT}}_{\mathrm{♯}}$ and transverse ${\mathit{kT}}_{\mathrm{\perp }}$ electron-beam temperatures, we found also analytical forms for the polarization rates. For a flattened distribution (${\mathit{kT}}_{\mathrm{♯}}$≪${\mathit{kT}}_{\mathrm{\perp }}$) the polarization rates for different (n,l) states peak at a direction, perpendicular to the ion-beam axis (in the moving frame), where the photons are completely linearly polarized for the s states. The results are discussed in the context of experiments aimed to observe the photons from radiative recombination in the electron coolers of storage rings.

The cross sections for the double ionization, ionization plus excitation, and double excitation of ${\mathrm{H}}_2$ by protons and electrons have been measured as a function of the angle between the internuclear axis of the target and the projectile-beam direction. Measurements are reported for angles ranging from 18° to 90° and projectile-beam energies of 0.75, 1.0, and 2.0 MeV/amu. Simple model calculations predict a different angular dependence depending on whether the final configuration of a particular reaction channel is or is not dipole allowed. Comparison is made between prediction and measurement. For both electron and proton projectiles, the sum of the yield in all channels follows the form corresponding to a dipole-allowed final state. Some individual channels do not.

A classical-path theory for broadening of atomic spectral lines by diatomic perturbers has been developed within the Anderson-Baranger-Royer formalism. This method allows more accurate treatment of nuclear motion than the quantum-mechanical and quasistatic methods in present use. Results from this theory are compared with the two more approximate methods for potentials based on the Mg+${\mathrm{H}}_2$ system. Physically reasonable behavior is found, with similarities and differences among results for the three methods following expected trends. The internal state of the perturber is found to effect the line shape. Features due to this phenomenon are discussed in terms of the shape of the difference potential surface.

Studies of the cross sections for electron-impact excitation of the valence states of carbon monoxide, i.e., the a ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }$, ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }$, a’ ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{}^{\mathrm{-}}{}_{}{}^{}$, d ${}_{}{}^3{}_{}{}^{}\mathrm{\Delta }$, I ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{\mathrm{-}}{}_{}{}^{}$, and D ${}_{}{}^1{}_{}{}^{}\mathrm{\Delta }$ states, have been carried out using the Schwinger multichannel variational method. Both differential and integral cross sections are obtained and compared with available experimental data. Reasonable agreement between the present results and experiment is seen for most of the states, and some differences are discussed. Estimates of the total cross section for electronic excitation of CO by low-energy electrons are provided.

The convergence of the close-coupling formalism is studied by expanding the target states in an orthogonal ${\mathit{L}}^2$ Laguerre basis. The theory is without approximation, and convergence is established by simply increasing the basis size. We present convergent elastic, 2s, and 2p differential cross sections, spin asymmetries, and angular-correlation parameters for the 2p excitation at 35, 54.4, and 100 eV. Integrated and total cross sections as well as T-matrix elements for the first five partial waves are also given.

The stopping cross section, the straggling parameter, and three higher moments over the energy-loss cross section have been evaluated for a moving point charge interacting with atomic hydrogen. The aim was to estimate the accuracy of predictions based on four commonly used models for computation of stopping parameters. The standard of reference was a straight evaluation of the Born approximation. All considered models, i.e., the dielectric theory, the binary-encounter model, the kinetic theory, and the harmonic-oscillator model, have been shown to be capable of providing reliable predictions within their respective regimes of feasibility, but distinct variations show up from lower to higher moments and from higher to lower projectile energy.

The neutral decay of doubly excited resonances (DER’s) of ${\mathrm{N}}_2$ has been observed by measuring the excitation spectrum of vacuum ultraviolet fluorescence emitted from photodissociation fragments in the 20–38-eV region. A number of DER’s with the [(1${\mathrm{\pi }}_{\mathit{u}}$${)}^{\mathrm{-}1}$(3${\mathrm{\sigma }}_{\mathit{g}}$${)}^{\mathrm{-}1}$(${\mathrm{\pi }}_{\mathit{g}}$${)}^1$:C ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$]ns${\mathrm{\sigma }}_{\mathit{g}}$ Rydberg series (n=3–10) have been revealed. Rydberg progressions onto the ${\mathrm{N}}_2^{+}$(D ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$) state have also been observed. Strongly dissociative DER’s correlating with satellites have been shown as broad peaks in the 23–33-eV region, which have appeared to have a correlation with the single-holed (X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$)${\mathrm{\sigma }}_{\mathit{u}}$ shape resonance.

The electron-spin-resonance spectrum was measured for pseudorotating ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_3^{}{}_{}{}^{}$ clusters by a matrix-free method, the molecular-beam magnetic resonance. Magnetic focusing was used for polarization and separation of the trimer beam. The spectrum indicates three equivalent lithium nuclei and gives the (average) isotropic spin population of ρ¯=0.231 and a g factor of g=2.002 36. The ρ¯ value is very close to previous results obtained for matrix-isolated clusters, and the g factor is nearly equal to the free-electron value.

Measurements of the fission of multiply charged Au clusters are presented and interpreted in the context of the liquid-drop model. A detailed description of the experimental apparatus is given, with particular attention paid to the cluster source, a liquid-metal ion source. It is found that upon excitation by either photons or collisions, doubly charged clusters in the size range n=12–18 atoms may either evaporate a neutral atom or undergo fission, with the tendency towards fission increasing as cluster size decreases. Empirically the relative rate for fission is found to depend on ${\mathit{z}}^2$/n. This result is shown to be consistent with the liquid-drop model developed for the fission of heavy nuclei [N. Bohr and J. A. Wheeler, Phys. Rev. 57, 426 (1939)]. The fission barrier height, calculated from the Coulomb and surface energies of a deformed, charged metallic droplet, decreases strongly with cluster size, in agreement with the experimental results.

The sublevel time-domain magnetic-resonance signal is calculated for ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ atoms subjected to static or coherently modulated light-shift fields. We predict and experimentally confirm that the observed light shift may be scaled down by coherent light-shift modulation. We submit that the inhomogeneous light-shift broadening may be removed by this purely optical form of coherent averaging, which is referred to as the ‘‘light-shift concertina.’’ Our expressions also indicate that the velocity-changing collisions of atoms in a static light-shift field can cause narrowing of the magnetic-resonance line shape in a manner analogous to the effect of electronic exchange or molecular motion.

We report measurements of the microwave ionization of Rydberg atoms as a function of static electric field, for static fields from zero to the classical ionization limit. The microwave-ionization threshold field shows clear structure as a function of the static field, in some static field regimes actually increasing as the static field is increased. The dependence of the microwave field can be understood by considering the dynamics of the levels in a temporally varying field.

An externally applied static electric field, by mixing opposite-parity states, can add electric-quadrupole and magnetic-dipole transition amplitudes to an allowed electric-dipole transition. The resulting interference among the amplitudes causes the absorption of resonance radiation to depend linearly on the static field and on the atomic polarization state. We present a calculation of this effect for the 254-nm line of Hg where the atomic polarization is specified by the ground-state nuclear spin, and the excited-state hyperfine structure is resolved. Possible experiments to measure this effect are pointed out.

We present a detailed theoretical investigation of the transmission spectrum of a probe beam interacting with atoms in a one-dimensional optical molasses obtained with linearly cross-polarized counterpropagating pump beams. The study is performed for a ${\mathit{J}}_{\mathit{g}}$=1/2→${\mathit{J}}_{\mathit{e}}$=3/2 atomic transition in the limit where the Hamiltonian part of the atom-field coupling is predominant over the relaxation part. We analyze the stimulated Raman transitions occurring between different vibrational levels of the atoms in the periodic potential created by the light shifts, and we show a dramatic lengthening of the damping time of coherences between such levels due to the Lamb-Dicke effect. Very narrow Rayleigh resonances with a shape sensitive to the probe polarization appear for a probe frequency close to the pump frequency. We interpret these resonances in terms of scattering of the pump waves on density and magnetization gratings, and show that they provide important information about the dynamics and localization of atoms at the bottom of the potential wells. Such information should also be accessed by phase-conjugation experiments. Finally, indications on the treatment of other atomic transitions are given.

Larmor’s theorem expresses the analogy between the motion of a charged particle in a homogeneous magnetic field and the motion in a system in uniform rotation. This analogy suggests that effects of a magnetic field may have a rotational counterpart. We discuss the differences and the analogies between the magnetic Faraday effect and the analogous effect induced by a rotation of the sample. A central issue is to what extent the macroscopic rotation of a material system is communicated to the wave functions of the electrons in the system.

The Volkov wave function describing the motion of a charged particle in a laser field serves as a modified plane wave in the formulation of external-field collision theory and is widely used in applications. Exact solutions are unavailable for those cases in which the target, as well as the projectile, carries a net charge. It is shown that this difficulty may be overcome through the adoption of a variational formulation of the theory in configuration space. The essential feature of this procedure is the specification of boundary conditions to be satisfied by the trial functions whereby the combined effect of the Coulomb potential and the external field is accounted for with sufficient accuracy. Wave packets constructed from such trial functions satisfy the physical requirement that they follow classical trajectories at asymptotic times. The formalism is applied to the problem of potential scattering in a low-frequency external field and leads to an approximate transition amplitude that serves as a generalization of the Kroll-Watson approximation, reducing to it for potentials having no long-range Coulomb tail. In addition, a relatively simple Coulomb generalization of the cross-section sum rule is obtained. As a second application a low-frequency approximation is derived for the amplitude for laser-assisted electron-impact ionization. It is based on a choice of trial functions that accounts for the effect on the asymptotic motion of these particles of the long-range final-state Coulomb interactions among pairs of charged particles, in the presence of the laser field. The applicability of this approach to multiphoton-ionization processes is discussed briefly.

Quantum interference between an electric-quadrupole transition and a Stark–induced-dipole-transition amplitude leads to a net orientation of an atomic excited state. When the barium 6${\mathit{s}}^2$ 1S-6s5d ${}_{}{}^1{}_{}{}^{}$D transition is excited in the presence of an electric field, we find the induced orientation of the 6s5d ${}_{}{}^1{}_{}{}^{}$D state to be (2.86±0.59)×${10}^{\mathrm{-}6}$(V/cm${)}^{\mathrm{-}1}$.

A theoretical analysis is given of the interaction of a laser field consisting of two orthogonal, linearly polarized, low-power standing waves, with an ensemble of J=1/2 ground-state atoms. The atoms are assumed to move with velocities below the Doppler limit of laser cooling. The two-dimensional light-induced atomic force is calculated for the specific case when the field polarization directions differ by π/2. The effective optical-pumping time is shown to increase strongly near the nodes of the laser field, leading to a force that can be much larger than that in the one-dimensional case. The force is not isotropic, and can lead to the bunching of the atoms in velocity space.

By using stochastic theory, we analyze the non-Markovian temporal behavior in four types of photon echoes, including the two-pulse echo, stimulated echo, accumulated echo, and heterodyne-detected accumulated echo. In particular, the origin of the exp(-${\mathit{ct}}^3$) dependence of the two-pulse echo in the limit of slow modulation is discussed in detail. It is also pointed out that, at the slow-modulation limit, the temporal behavior of the stimulated echo changes from exp(-${\mathit{ct}}^3$) dependence to exp(-${\mathit{ct}}^2$) dependence as the waiting time (the time interval between the second and the third excitation pulses) is varied.

We report an alternative approach to describe radiation-matter interaction that extends conventional perturbation theory to accommodate arbitrary field strength by introducing the concept of self-regulated partitioning of the interaction Hamiltonian. The treatment gives a clear physical insight into field-induced splitting of the states, called here partially dressed states, and also accounts for the transition of energy between the states. In general these states are characterized by the strength and form of the field-matter interaction and determined by comparison with density-matrix analysis. Using these states as base states, this theory allows for a perturbation-expansion analysis for arbitrary interaction strength. This treatment provides an analytic basis for describing multilevel radiation interactions, and by way of example we consider its application to third-harmonic generation.

The buildup and decay of the sound-wave gratings in stimulated-Brillouin-scattering cells were calculated taking into account the spatial dependence in the propagation direction of the focused laser beams. The incident intensities were varied up to the saturation values. The influence of the different parameters such as focal length, cell length, pulse duration, and phonon lifetime is discussed.