Integral and differential cross sections and electron-impact coherence parameters have been calculated for the 6$s^2$ ${}_{}{}^1{}_{}{}^{}\mathrm{p}$ ${}_{}{}^1{}_{}{}^{}\mathrm{P}$ transition in neutral barium using both the distorted-wave approximation and first-order many-body theory. Target-state wave functions have been calculated using Hartree-Fock orbitals along with configuration-interaction and intermediate-coupling mixing. Results are presented for electron-impact energies of 5, 10, 15, 20, 30, 40, 60, 80, and 100 eV. The cross sections are compared to available experimental and theoretical results.

A theory of ion-atom and ion-molecular collisions at high energies is formulated in which the projectile ions undergo simultaneous electron transfer and excitation (TE). A unified description of both resonant and nonresonant processes is presented. The resonant mode (RTE) is mediated by one of the target electrons, which excites the projectile and at the same time is captured by the projectile ion, thus forming a doubly excited intermediate state of the projectile ion. In the impulse approximation, this is related to the dielectronic recombination process in electron-ion collisions. The same doubly excited states can also be formed by one of the target electrons and the target core, interacting with the projectile ion in which the target core does the exciting and the electron is simply transferred. This is a nonresonant process (NTE), although the projectile is again left in a doubly excited state. We propose here still another mechanism for the formation of doubly excited states, UTE. The new process involves two target electrons in a nonresonant way, just as in the NTE, such that the electron that excites the projectile is left in the continuum, as viewed in the projectile rest frame. Some of the experimental evidence for these reactions is discussed.

The 3s-3p excitation of Na atoms by ${\mathrm{He}}^{+}$ ions in the impact energy range 0.6–10 keV has been studied both experimentally and theoretically. In the experiment, the total excitation cross section, the alignment of the Na 3p state, and the collision-induced change of spin polarization of the Na valence electron were investigated. For the latter purpose, the Na atoms were polarized by a hexapole state selector, and a magnetic field was applied to the collision region, resulting in a spin polarization of the Na ground-state atoms of $P_S$≊0.74. The spin polarization of the final 3p excited state was derived from the measured circular polarization of the fluorescence light. The experimental results are compared to published calculations in the Coulomb approach and to our own calculations in the molecular approach using a 12-state molecular expansion and employing a common translation factor. The molecular approach gives satisfying agreement with all experimental data. In particular, the measured increase of spin depolarization with decreasing collision energy is well reproduced. The depolarization originates from the exchange interaction between the 1s electron of ${\mathrm{He}}^{+}$ and the valence electron of Na during the collision.

Doubly differential cross sections have been measured as a function of the electron energy and observation angle for electron emission following collisions of 6-MeV/u ${\mathrm{U}}^{38+}$ and ${\mathrm{Th}}^{38+}$ on He and Ar. The electron-emission data show an enhancement at forward angles and a decrease at backward angles with respect to scaled-cross-section results based on the Born approximation. Comparison with classical-trajectory Monte Carlo calculations suggests that the deviation from the Born approximation can be explained by continuum-continuum couplings. By comparing with previously published data, we found that the forward enhancement as well as the backward decrease follow a q/$v_p$ (q,$v_p$ are the projectile charge and velocity) scaling.

The multiconfiguration Hartree-Fock method developed by Saha [Phys. Rev. A 39, 5048 (1989)] to study scattering of electrons from atoms has been applied to the low-energy elastic scattering of electrons from helium atoms. The short-range electron correlation and the long-range dynamical polarization of the target by the scattering electron, which are very important in these calculations, have been taken into account in an accurate ab initio manner through the configuration-interaction procedure. Detailed results for phase shifts, elastic differential, integral-elastic, and momentum-transfer cross sections for electrons elastically scattered from helium are reported for the low and intermediate energies ranging from 0.58 to 50 eV. The present results are compared with accurate experimental measurements and theoretical calculations. It is found that the present multiconfiguration self-consistent-field method produces high-quality results that show excellent agreement with experimental measurements and compare well with other accurate theoretical calculations.

The Fock-Tani Hamiltonian is found for systems containing two protons and one electron. It is shown that a post-prior symmetrical T-matrix element for $a^{+}$+($b^{+}$$c^{\mathrm{-}}$)→($a^{+}$$c^{-}$)+$b^{+}$ may be found from that of the simpler proton-proton-electron system if a and b are treated as isospin projections of a single type of nucleon. The Coulomb-exchange contribution to the inelastic (isospin flip) scattering of this system gives a first-order T matrix that is completely symmetrical with respect to post and prior interactions and orthogonalizations, a symmetry of the exact T matrix.

We present detailed experimental results for the photoionization processes of barium atoms in the ground state, from 16 to 180 eV photon energy. Using electron spectroscopy, we have measured partial photoionization cross sections for the 6s, 5p, 5s, and 4d subshells, including satellite to main-line branching ratios and 5p and 4d Auger spectra. These results reveal, in particular, significantly lower values of the 4d photoionization cross section than previously predicted by a number of theoretical calculations, or deduced from photoabsorption data. They are, however, in better agreement with two most recent, yet unpublished, calculations. A particular care has been brought to the regions of 5p and 4d discrete electron excitations, where the excitation functions of the main ${\mathrm{Ba}}^{+}$ exit channels, and also of the ${\mathrm{Ba}}^{2+}$ decay channel in the case of 5p excitations, have been studied. Finally, we have measured the oscillator strength for excitation of some isolated 5p resonances and for the whole 5p subshell.

Rydberg states in lithium have been excited from the ground state by absorption of π-polarized photons in the presence of parallel electric F and magnetic B fields. The magnetic field strength (B=3.11 T) is high enough to reach the inter-n-mixing regime, and a small electric field strength (F≃25 V/cm) is sufficient for the lowest state of the n=30 manifold to penetrate into the high-energy part of the n=29 manifold. The first anticrossing is thoroughly studied. Away from the anticrossing the spectrum exhibits two lines, but at the anticrossing a third component appears in the structure in the same energy range. This provides an experimental confirmation for the existence of pairs of almost-degenerate states with opposite parities in the high-energy part of the diamagnetic manifold of lithium. The two components of this doublet behave differently at the anticrossing. The experimental data have been interpreted in a global treatment in which core effects and intermanifold couplings are simultaneously taken into account. The different behavior of low- and high-lying states of the multiplet with respect to these two interactions is analyzed in detail.

Starting from the Hartree-Slater independent electron model, we perform configuration interaction (CI) calculations for both the initial state and the final state in atomic photoionization processes. Our CI K-shell photoionization cross section for Ne displays an enhancement near the threshold due to the dynamical screening effect of the K vacancy, which is in good agreement with experiments. Furthermore, the K-shell ratio function of the CI Ne K shell to the CI ${\mathrm{Ne}}^{+}$ K-shell photoionization cross sections shows no oscillatory behavior, in contrast to the ratio in the Hartree-Slater independent electron calculation.

Momentum distributions and spectroscopic factors are obtained in a high-resolution study of argon at 500, 1000, and 1500 eV by electron-momentum spectroscopy. The shapes and relative magnitudes of the 1500-eV cross sections are in excellent agreement with the results of a distorted-wave impulse approximation calculation. The final states belonging to the ${}_{}{}^2{}_{}{}^{}\mathrm{S}_{}^{\mathrm{e}}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{}^{\mathrm{o}}{}_{}{}^{}$ manifolds are identified and their spectroscopic factors (pole strengths) are obtained. These are found to be independent of energy and momentum in the range 0.1–1.9 a.u. within experimental error, although some momentum dependence is observed for the spectroscopic factor leading to the 4s ${}_{}{}^2{}_{}{}^{}S$ ion state due to initial-state correlations. The first momentum profiles for excited states belonging to the ${}_{}{}^2{}_{}{}^{}\mathrm{P}_{}^{\mathrm{o}}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{D}_{}^{\mathrm{e}}{}_{}{}^{}$ manifolds are obtained. The latter are entirely due to initial-state correlations. Comparison is made with several many-body calculations. The data show the importance of core quadrupole ${(}^1$D) excitations in describing electron correlations in both the initial Ar ground state and in the final ionic states.

The relativistic formulation is given for the angular distribution of the electrons produced in the photoionization by linearly polarized photons and the distribution of photons of a particular polarization produced in the inverse process of radiative recombination. The electrons are treated as moving in a self-consistent Hartree-Slater central potential. Results are given for the radiative recombination onto heliumlike nickel and neonlike barium for electrons in the energy range below 100 keV.

We report results of large-scale nonperturbative (Floquet) calculations of rates for harmonic generation and total ionization of H(1s) by fields whose intensity ranges from about ${10}^{12}$ to 3×${10}^{13}$ W/${\mathrm{cm}}^2$ and whose wavelength ranges from 265 to 1064 nm. At long wavelengths and moderate to high intensities, perturbation theory yields estimates of total ionization rates that are orders of magnitude too large, and estimates of harmonic-generation rates that exhibit a qualitatively incorrect behavior with respect to order. We have studied the influence of resonances on the ionization rates, and we illustrate the resonance enhancement of the ionization yield for a realistic pulse. Finally, we address several formal aspects of Floquet theory, including the convergence of the induced dipole moment, gauge invariance, and the normalization of the wave function.

We present a microscopic optical-Bloch-equation model for N two-level atoms being driven by a monochromatic radiation field. The model is obtained from the equations of motion that result from the microscopic Hamiltonian by introducing at a certain stage an Ansatz that accounts for spontaneous emission and bath dephasing in a unitary manner. The equations of motion are a numerically tractable set of coupled c-number differential equations with a discrete, stochastic term deduced from the quantum-mechanical equations of motion. We apply these equations to various model problems and show that they correctly describe such varied optical phenomena as superradiance, photon echoes, and absorptive bistability. In particular, they yield an interesting microscopic picture of the dynamical phase transition that occurs in the latter example. We conclude that this unified approach has advantages that make it useful for studying a number of problems of current interest.

In this paper we derive the Fokker-Planck equation that describes the motion of a charged particle, trapped in a static harmonic potential and cooled by radiation pressure forces exerted by a laser beam. This equation is then discussed in both the classical and quantum regimes. The model adopted here is a simple one, in which one-dimensional motion and a two-level atom are assumed, and the time evolution of the system is described in the quasiclassical limit, in which the quantum nature of the photon exchanged between the particle and the field plays no role. We report relevant graphs for the loss of kinetic energy suffered by the particle in the cooling process and estimates of the lowest kinetic temperature that can be reached in these experiments.

The effects of optical guiding on sideband instabilities in a Raman free-electron laser (FEL) are studied numerically and experimentally. An axisymmetric two-dimensional (2D) computer code that includes the effects of space charge and diffraction in an overmoded waveguide is developed to simulate sideband growth in the Columbia University FEL, which generates radiation of millimeter wavelength. It is found in both the simulation and the experiment that the effect of refractive optical guiding, which slows down the radiation group velocity, shifts the sidebands away from the signal carrier. We also find numerically that refractive optical guiding enhances the filling factor of the electron beam and perturbs the electron distribution, and thereby increases the sideband growth rate. We show that the sideband growth rate can be depressed by tuning the FEL so that the real part of the effective index of refraction associated with the electron beam decreases. The effect of wiggler tapering on the sideband growth is also studied with the 2D code. A significant reduction in the sideband growth rate in an efficiency-enhanced wiggler has been demonstrated and is qualitatively consistent with experimental measurements.

The electrostatic field generated by a charged dielectric sphere on a conducting half plane has been investigated both experimentally and theoretically by means of electron holography in a transmission electron microscope. By this technique it is possible to obtain, on the reconstructed image, two-dimensional representations of the in-plane projected potential distribution around the charged sphere. We will show how this example, dealt with in the linear system theory, gives the key to the interpretation of more general plane distributions of potential, for instance, that generated by the reverse-biased p-n junctions.

Sum and difference squeezing are both higher-order, two-mode squeezing effects. Sum squeezing is turned into normal squeezing by sum-frequency generation. The operators that are used to define it form a representation of the su(1,1) Lie algebra. Difference squeezing can generate normal squeezing through difference-frequency generation. The operators that characterize it form a representation of the su(2) Lie algebra. Both are nonclassical effects. For uncorrelated modes the presence of squeezing in at least one of the modes is a necessary condition for the existence of either sum or difference squeezing. If the modes are correlated this is no longer true. Both sum and difference squeezing can be generated by combining a squeezed mode with one in a coherent state. Sum squeezing is also produced by a parametric amplifier. In order to better illustrate some of these effects a pictorial representation of sum squeezing is presented.

We present an approximation of a non-Markovian solution for the time evolution of the mean number of quanta in an anharmonic oscillator linearly and stochastically coupled to a thermal bath. A colored Gaussian bath is explicitly considered, and a quantum treatment of the bath correlation functions, based on the use of a relation between their real and imaginary parts, is applied. The behavior of the oscillator is characterized by a reduced set of (five) parameters referring to the oscillator, the bath, and their mutual interaction. A comparison is made between the prediction of the approach for the mean stationary number of quanta and its exact value, which enables us to analyze its accuracy. When the Markovian limit holds, an explicit expression for a parameter measuring the time scale for the relaxation of the oscillator is obtained in terms of the characteristic parameters of the model. Except for very high temperatures, the anharmonicity has a small influence on this relaxation time. A numerical study of the time dependence of the mean number of quanta is presented on the basis of the reported analytical solution. We find that the anharmonicity is not a significant parameter in the appearance nor in the increase of non-Markovian effects.

We present a simple tool to calculate numerically all dynamic and spectral properties of an arbitrarily driven radiatively damped two-level system. In particular, we present resonance fluorescence spectra for a bichromatically amplitude-modulated driving discussed recently in connection with quantum chaos.

Above-threshold ionization (ATI) by a smooth laser pulse is investigated in terms of the essential-state approach. Time evolution of ATI spectra is demonstrated. It is shown that the size of the electron peaks grows with the interaction time, and the peaks’ bell-shaped envelope in the spectra moves to higher electron energies during the rise of the laser pulse.