The impact-parameter dependence of electron emission following slow ${\mathrm{Ar}}^{+}$+Ar collisions has been investigated experimentally. Coincidence measurements have been performed for incident energies of 150, 200, and 300 keV at electron emission angles of 40°–100° and projectile scattering angles of 0.3°–3.0°. An improved method of determining absolute doubly differential ionization yields from electron time-of-flight spectra has been applied. The experimental results are interpreted in terms of the molecular-orbital model of atomic collisions. It is shown that the well-known 4fσ promotion mechanism leading to L-shell vacancy production results in the emission of electrons to the continuum. A detailed analysis of the experimental data indicates that both promoted electrons are directly ionized out of the 4fσ level for small impact parameters. This is in good agreement with preliminary results of coupled-channel calculations and consistent with earlier inelastic energy loss measurements. From the total number of emitted electrons it was further possible to determine the number of highly excited and directly ionized M-shell electrons.

Electron-loss cross sections have been calculated for fully stripped ions with charge states +1 to +100 colliding with helium atoms in the energy range 100–1000 kev/amu. The classical-trajectory Monte Carlo method was used along with a model of the helium atom with two active electrons. The reactions studied were single and double ionization, single- and double-electron capture, and electron capture plus ionization. Cross sections for the single-electron-removal reactions agree well with experimental data. Double-electron-removal cross sections are in qualitative agreement with experimental results. The n-level distributions following electron capture show that the electron-capture–plus–ionization and double–electron-capture processes are peaked at a lower n level relative to single-electron capture.

Simple expressions are derived for all multipole, nonadiabatic, and finite-nuclear-mass corrections to the long-range effective potential due to two–Coulomb-photon exchange at threshold energy and also first order in energy correction for the scattering between a spinless point charged particle and a spinless Coulombic complex and between two spinless Coulombic complexes. All these corrections are treated on equal footing and the results are expressed, respectively, in terms of single-center and London-analog two-center atomic multipole spectral sums. An isomorphism between the point-particle–complex interaction and the complex-complex interaction is derived. All Hughes-Eckert–type mass-polarization terms are avoided by suitable choice of Jacobi coordinates.

The relative photoionization cross section of atomic oxygen for the production of singly charged ions has been remeasured in more detail and extended to cover the wavelength range 44.3–910.5 Å by use of synchrotron radiation. In addition, the contribution of multiple ionization to the cross sections has been measured, allowing total photoionization cross sections to be obtained below 250 Å. The results have been made absolute by normalization to our previously measured data. The use of synchrotron radiation has enabled measurements of the continuum cross section to be made between the numerous autoionizing resonances that occur near the ionization thresholds. This in turn has allowed a more critical comparison of the various theoretical estimates of the cross section to be made. The series of autoionizing resonances leading to the ${}_{}{}^4{}_{}{}^{}\mathrm{P}$ state of the oxygen ion have been observed for the first time in an ionization-type experiment, and their positions have been compared with both theory and previous photographic recordings.

Nonlinear variations affecting the widths and positions of laser-induced resonances in the molecular-dissociation process are discussed as a function of the electromagnetic field intensity. It is shown how the close-coupled equations of quantum scattering theory, taking into account multiphoton absorption and emission processes through the time-independent Floquet Hamiltonian, go beyond the widely used weak-field Fermi golden rule approximation. For the intermediate-field regime adiabatic electron-field channels can, in some cases, serve as a guide for the interpretation of nonlinearities, while for higher laser intensities very large field-induced mixings between molecular states predominate, requiring the introduction of an increasing number of Floquet blocks for converged calculations. The formalism is applied to the photodissociation of ${\mathrm{H}}_2^{+}$(1s${\mathrm{\sigma }}_{\mathrm{g}}$, v=0, J=1→2p${\sigma }_u$). Nonlinear effects appear for intensities larger than ${10}^{11}$ W/${\mathrm{cm}}^2$, for which experiments begin to be available.

The dielectronic recombination rate coefficients for the ten initial states of the 1$s^2$2l2l’ configurations of the Be-like ions have been calculated for six ions with atomic numbers Z=30, 34, 36, 42, 47, and 54 in the isolated-resonance approximation. The detailed Auger and radiative rates for each autoionizing state were evaluated using the multiconfiguration Dirac-Fock model in intermediate coupling with configuration interaction. The present results are compared with the existing theoretical values and the predictions of the Burgess and the Burgess-Merts semiempirical formulas.

Time-evolution patterns for energy absorption in a driven Hénon-Heiles Hamiltonian with and without extra off-diagonal random-coupling terms are reported. The time dependence of the average number of photons absorbed is quite different for these two systems, being fluence dependent for the latter. Similarities are found between the dynamics of the driven system with random coupling and a four-mode molecular model which interacts with a quantum monochromatic field. Comparison is made between the presently found mechanism of excitation and the diffusive energy growth due to externally induced chaos found in the kicked-rotor or in microwave ionization of the hydrogen atom.

A simple analytic formula for the photodetachment cross section of ${\mathrm{H}}^{\mathrm{-}}$ in an electric field is derived. The formula becomes identical to a formula derived many years ago in the vanishing-field limit. Three special features appear in the cross section in the presence of an electric field: a quantum tunneling effect, a new threshold law, and oscillations. These appear when the photon energy is, respectively, less than, equal to, or greater than the binding energy of ${\mathrm{H}}^{\mathrm{-}}$. Enough information is provided in this paper so that one can calculate the cross section quickly.

This paper describes a new technique for obtaining level positions, linewidths, autoionizing times, and oscillator strengths of core-excited levels and transitions. A tunable laser is used to deplete the population of a radiating core-excited level, as other levels within the core-excited manifold are accessed. A saturation technique which allows the measurement of autoionizing times whose linewidth lies well beneath the combined Doppler-hyperfine profile is developed and demonstrated.

A study is presented of the photodetachment cross section of the ${\mathrm{H}}^{\mathrm{-}}$ ion near the one-electron threshold in electric fields ranging from approximately 5×${10}^{\mathrm{-}7}$ up to 2.4×${10}^{\mathrm{-}4}$ a.u. The lowest-field data, nominally at zero field, are consistent with the Wigner threshold law for p-wave processes. At larger field values, photodetachment using σ-polarized laser light displays the expected lowering of apparent threshold and evidence of tunneling. Using π-polarized light, the same features are seen with the additional feature of oscillations superimposed on the cross section. Three complementary explanations are presented for the oscillations.

We investigate the model dependence of the real part of the anomalous x-ray atomic scattering factor f’. Both the Dirac-Slater and the Dirac-Kohn-Sham potentials with theoretical and experimental binding energies are considered. The values of f’ are found to be sensitive to the models in the neighborhood where f’ changes sign. Deviations of the values of f’ between different models can be as large as 20% (or more) in this region. Comparisons between the theoretical and experimental results in this region will tell us which model is better for the calculations of f’.

The theory of two-frequency multiphoton ionization of hydrogen is reconsidered with particular emphasis on gauge aspects. The analysis is specialized to the case when a relatively weak high-frequency radiation field causes the ionization, while an intense low-frequency radiation field produces structured continuum states for the ionized electrons. Previous results, available in the literature, are recovered as limiting cases of the expressions derived in the present paper. Angular distributions, total cross sections as a function of the low-frequency field intensities, and photoelectron spectra are calculated. Several new features are found and compared with the available information.

Significant fluctuations, above the shot-noise limit, have been observed in the intensity of fluorescent light scattered from atoms excited by an intense noisy laser, where a central role is played by the nonlinearity of the atom-field interaction. By considering the variance of the atomic populations, we show that ‘‘noise spectroscopy’’ is sensitive to the field statistics of the laser. We specifically consider the cases of resonance fluorescence (analogous in the weak-field limit to analysis by a Fabry-Pérot interferometer), two-photon excitation, and double optical resonance for variable laser intensity and bandwidth. We show that large differences in the ‘‘noise’’ spectra can occur between lasers characterized by a phase-diffusion or a phase-jump model, although these models would give the same values for the mean atomic populations. We believe that observation of population fluctuations can become a useful method for characterizing laser noise.

The degree of polarization of ${\mathrm{Ly}}_{\mathrm{\alpha }}$ emission in the photodissociation of ${\mathrm{H}}_2$ leading to H(1s)+H(2p) fragments is calculated in the energy region between 100 and 4000 ${\mathrm{cm}}^{\mathrm{-}1}$ above threshold. The calculations are performed by integration of the time-independent close-coupled Schrödinger equations for the B${(}^1$Σ), B’${(}^1$Σ), and C${(}^1$Π) electronic states of ${\mathrm{H}}_2$, using existing ab initio potential energy curves, nonadiabatic couplings, and transition dipole moments. The coherence effects due to the simultaneous excitation of the continua are demonstrated and analyzed in detail. It is shown that these effects are amenable to experimental confirmation.

We consider the squeezing of the electromagnetic field resulting form the interaction of light in a coherent state with one- and two-photon Jaynes-Cummings models of the two-level atom. We also consider high-order squeezing in the sense of Hong and Mandel [Phys. Rev. Lett. 54, 323 (1985)] and also of Hillery [Phys. Rev. A 36, 3796 (1987)].

A single-mode dye-laser model incorporating pump white noise through gain-parameter fluctuations is analyzed. It includes a fluctuating saturation term. Intensity fluctuations, a first-order-like transition, and intensity correlation functions are calculated and discussed. It is found that effects previously claimed to be a consequence of the presence of colored noise in the context of loss-noise models can be also explained by white gain noise, except for the existence of two time regimes in the early decay of intensity correlation functions.

The nonlinear susceptibilities of a heterogeneous medium are calculated by using a generalization of the standard T-matrix approach. Results for the susceptibilities for degenerate and nondegenerate four-wave mixing and nonlinear absorption are obtained. These are evaluated explicitly for a medium with nonlinear spherical grains. The role of various resonances in the generation of the signals is discussed. In the special cases of low concentration of inhomogeneities, present results agree with those of Flytzanis and co-workers [Opt. Lett. 9, 344 (1984); 10, 511 (1985); J. Opt. Soc. Am. B 4, 5 (1987)].

A nonperturbative theoretical study of four-wave-mixing processes in four-level systems is presented. Using the density-matrix formalism, numerical results are obtained for two arbitrarily intense pump fields with the third field a weak probe. The fourth wave, radiated by the system, is obtained analytically for some special cases. We find, for different frequency detunings, Rabi frequencies, and relaxation rates that the line shapes exhibit: (i) dynamic Stark splitting and shifting, and merging of resonances; (ii) power broadening with two-photon antiresonance behavior resulting from a destructive interference between different contributions to the optical polarization; and (iii) extra resonances induced by field intensities and saturation processes.

The steady-state temperature of an ion plasma in a Penning trap, cooled by a laser beam perpendicular to the trap axis, has been calculated and measured. The rotation of the plasma, due to crossed E and B fields, strongly affects the minimum attainable temperature. This is because the velocity distribution of the ions, as seen by a laser beam intersecting the plasma at some distance from the axis of rotation, is skewed, and this leads to a change in the velocity distribution (and hence temperature) at which a steady state is attained. The calculated temperature is a function of the intensity, frequency, and position of the laser beam, and of the rotation frequency of the plasma. Temperatures of ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ plasmas were measured for a wide range of experimental parameters. The lowest and highest temperatures were approximately 40 mK and 2 K. The measured and calculated temperatures are in agreement.

The Gibbs-Bogoliubov inequality has been reexamined and put in a form more appropriate for obtaining an accurate Helmholtz free energy. Starting from this inequality, in conjunction with the classical one-component-plasma reference system, we present theoretical results of related thermodynamic quantities. By comparing the latter with available experiments, we find that (i) the calculated thermodynamic functions such as the Helmholtz free energy, the internal energy, the excess entropy, etc., are reasonably predicted and (ii) the variationally determined plasma parameters emerge naturally to values expected from liquid-structure experiments. In particular we notice that the magnitudes of the principal peaks of the liquid structure factors agree quite well with the freezing criterion of the classical one-component plasma as discussed by A. Ferraz and N. H. March [Solid State Commun. 36, 977 (1980)] in relation to liquid alkali metals.