A correlated two-electron continuum wave function satisfying the correct asymptotic boundary condition is used to calculate double-photoionization cross sections. It is built up as a product of three two-body Coulomb continua. As a highly correlated ground-state wave function is employed, the present results serve as a critical test of the two-electron continuum wave function on this double process. For all energies, the results exhibit a large discrepancy between the length and velocity forms. In the high-energy limit, the velocity form agrees quite well with recent synchrotron measurements and with previous calculations. At low photon energies, the approximation fails to account for the data in magnitude and shape.

The method of complex-coordinate rotation is used to investigate doubly excited shape resonances in ${\mathrm{H}}^{\mathrm{-}}$. We calculate resonance parameters for the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ shape resonance lying above the n=2 hydrogen threshold using Hylleraas-type wave functions. Comparisons are made with experimental measurements and with other theoretical calculations. In addition, we also report a calculation for ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ shape resonances lying above the n=3 hydrogen threshold.

The coherent production of relativistic fermionium by high-energy electrons in a crystal is calculated. It is shown that considerable enhancement of the cross section occurs at certain angles and energies.

We measured secondary-electron energy spectra of the H-like ions ${\mathrm{N}}^{6+}$, ${\mathrm{O}}^{7+}$, and ${\mathrm{Ne}}^{\mathit{q}+}$(q=3,7,8,9,10) incident on a solid Pt(110) surface at different angles (10°–90°) using impact energies of 10q and 15q keV. Clear evidence is found for subsurface electron emission of the KLL Auger electrons. The stepwise filling of both K-shell holes of the ${\mathrm{Ne}}^{10+}$ projectile is observed in the electron energy spectrum. The dynamics of the filling of the L and K shells in the solid is studied for the ${\mathrm{Ne}}^{10+}$ ion. It is found that hollow Ne atoms are formed inside the solid. With decreasing incident charge state of the Ne projectile the LMM Auger lines are shifted to lower energies due to the different screening of the nuclear charge, which is consistent with corresponding Hartree-Fock calculations.

Raman emission spectroscopy of molecular systems undergoing photodissociation in the presence of short laser pulses is studied using a time-dependent coherent-state analysis. The model accounts in a detailed way, but within a second-order perturbative approach, for dynamical events occurring on a large time scale: from early dynamics taking place during the pulse to late dynamics resulting in the population of vibrationally excited levels of the ground electronic state by stimulated overtone emission. Two observables are addressed: the frequency distribution of the spontaneously emitted photons (Raman spectrum) and the resulting overall vibrational distribution. The role played by the short duration of the pulse or its large bandwidth is analyzed and compared with a cw-laser experiment. When the pulse duration is large with respect to the molecular fragmentation time scale on the excited state but short enough for its bandwidth to cover the energy range of vibrationally excited levels of the ground electronic state, the populations of these levels are expected to increase in a noticable way by laser-stimulated emissions. The model implications are illustrated on van der Waals molecules (such as Hg..) which appear to be valuable candidates for a favorable compromise between the laser pulse duration (100 fs or reciprocally 160 ${\mathrm{cm}}^{\mathrm{-}1}$), the dynamical time scale on the excited state (typically 100 fs to 1 ps), and the dissociation energy of the ground electronic state (typically 50 to 500 ${\mathrm{cm}}^{\mathrm{-}1}$). The presence of quasibound levels, supported by the attractive part of the excited potential, gives additional and interesting information on the Raman spectrum which can be used as a long-range probe of the potential-energy surfaces taking part in the photodissociation process.

We have investigated the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$${\to }^3$${\mathit{P}}_1$ and ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$${\to }^3$${\mathit{P}}_0$ atomic fine-structure transitions in isotopically enriched samples of oxygen by means of tunable far-infrared spectroscopy. Accurate frequency measurements (a few parts in ${10}^8$) were made for all three stable isotopes and the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ hyperfine structure was resolved. The experimental values for the isotopic shift of ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ relative to ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ for the J=2→J=1 and J=1→J=0 fine-structure transitions are in agreement with theoretical calculations.

The ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{1\mathrm{-}}^3$${\mathit{P}}_2$ fine-structure splittings of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ have been measured by far-infrared laser magnetic-resonance spectroscopy. The signals for ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ were detected with a sample in its natural abundance (0.038%). The isotopic shifts from the corresponding interval in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ are +13.0 MHz for ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and +23.5 MHz for ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, in accord with theoretical expectations.

In spite of decades of extensive studies of water, the experimental information of water clusters larger than the trimer is hardly available yet. To aid the better analysis of certain cluster properties in an experiment, we studied the small-water-cluster distribution (particularly for the minimum-energy structures) in the gas phase. Utilizing the thermodynamic information in the range from the water monomer to the octamer (except for the heptamer) by ab initio calculations, we investigated the mole fractions of the water clusters along the vapor pressure of the condensed phase. These mole fractions increase with increasing temperature or pressure, while the higher clusters increase still more. The entropy increment of the cyclic pentamer relative to the cyclic tetramer is particularly small; thus, the cyclic pentamer shows thermodynamically unusual characteristics. For the trimer, the cyclic structure is more stable than the linear structure at temperatures lower than ∼400 K, while above this temperature, the latter is more stable due to the entropy effect. Similar phenomena are also expected for the higher clusters. The mole fractions of the higher cyclic clusters are found to be very small in a vapor unless they are uncondensed with insufficient water molecules.

We present a theoretical analysis to explain the origin of different spatial distributions of laser-cooled sodium atoms in a magneto-optical trap. Such distributions, which include spherical clouds, rings, and rings with cores, are attributed to the radiation pressure due to misaligned Gaussian beams acting on each individual atom. The predictions of this theory are confirmed by experimental observations reported earlier. We were able to construct a diagram with several domains corresponding to the stable structures observed.

We present a semiclassical laser-cooling theory for an ion bound in a quadratic potential well, allowing for an arbitrary internal level scheme of the ion and an arbitrary polarization of the light field. The special case that the ion moves in a region much smaller than the wavelength of the exciting light (Lamb-Dicke limit), as well as an additional low-intensity limit, are worked out in detail. Explicit general expressions are given for the damping and diffusion tensors of the center-of-mass motion of the ion, and the light-induced renormalization of the mechanical oscillation frequencies is discussed. The formalism is implemented analytically using a computer program mathematica for an ion with a j=1/2→j=3/2 transition moving in a one-dimensional optical molasses consisting of two counterpropagating laser beams with perpendicular linear polarizations (lin ⊥ lin). For a weakly bound ion at negligible saturation, the greatest lower limit of excitation energy turns out to be (33/20${)}^{1/2}$-1/2≊0.78 vibrational quanta above the zero-point energy.

Strong-field effects in the light-induced collisional energy-transfer spectral profile were investigated for the europium-strontium binary mixture. When the laser amplitude is increased, the slope in the quasistatic wing changes, the spectral peak reaches saturation, and the spectral line shape loses asymmetry. These features are in fair to good agreement with predictions of existing theoretical models. However, no appreciable shift of the spectral peak was observed at intensities ranging from 23 MW/${\mathrm{cm}}^2$ up to 3.75 GW/${\mathrm{cm}}^2$, in contrast with the predictions of those models.

An approximately relativistic theory of bound states which ensures Poincaré invariance of the atomic system to relative order (v/c${)}^2$ is used to derive the Zeeman interaction Hamiltonian correct to order ${\mathrm{\alpha }}^3$ and to all orders in ${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$ for an arbitrary three-body system. This approach is distinctly different from methods used in the past. The terms of order ${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$, ${\mathrm{\alpha }}^2$, and ${\mathrm{\alpha }}^3$ agree with previous results. The terms of order ${\mathrm{\alpha }}^2$${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$ and smaller do not agree with previous results. The terms of order ${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$, ${\mathrm{\alpha }}^2$, ${\mathrm{\alpha }}^3$, and ${\mathrm{\alpha }}^2$${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$ are used to calculate the g factors for n=2 ${\mathrm{He}}^4$ and ${\mathrm{He}}^3$. The corrections of order ${\mathrm{\alpha }}^3$ and ${\mathrm{\alpha }}^2$${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$ have not previously been calculated for the P-state g factors. Our results for the ${\mathrm{He}}^4$ 2 ${\mathrm{S}}_1^3$ ${\mathrm{g}}_{\mathrm{J}}$ factor [${\mathrm{g}}_{\mathrm{e}}$(1-40.9158×${10}^{\mathrm{-}6}$)] can be compared with the result of Grotch and Hegstrom [Phys. Rev. A 8, 1166 (1973)] [${\mathrm{g}}_{\mathrm{e}}$(1-40.9157×${10}^{\mathrm{-}6}$)]. We calculate the ratio of this g factor to the g factor of 1 ${\mathrm{S}}_{1/2}^2$ hydrogen to be 1-23.211×${10}^{\mathrm{-}6}$. This agrees with the result of Grotch and Hegstrom [Phys. Rev. A 8, 1166 (1973)], 1-23.211× ${10}^{\mathrm{-}6}$, and the best experimental result [G. M. Keiser, H. G. Robinson, and C. E. Johnson, Phys. Rev. A 16, 822 (1977)] 1-23.214(50)×${10}^{\mathrm{-}6}$. We also find that including the mass polarization in the 2 ${\mathrm{S}}_1^3$ wave function contributes a negligible amount, 1.4×${10}^{\mathrm{-}10}$, to the ${\mathrm{He}}^4$ 2 ${\mathrm{S}}_1^3$ ${\mathrm{g}}_{\mathrm{J}}$ factor. We use 125-term variational wave functions in a configuration-interaction basis and explicitly examine the effect of configuration mixing on the g factors. We find the ${\mathrm{He}}^4$ 2 ${\mathrm{P}}_{\mathrm{J}}^3$ ${\mathrm{g}}_{\mathrm{s}}^{\prime }$ factor to order ${\mathrm{\alpha }}^2$${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$ to be ${\mathrm{g}}_{\mathrm{e}}$-80.4010×${10}^{\mathrm{-}6}$. This agrees (to order ${\mathrm{\alpha }}^2$) with the results of Lewis and Hughes [Phys. Rev. A 8, 2845 (1973)]. We also calculate radiative corrections of order ${\mathrm{\alpha }}^3$ that contribute to the 2 ${\mathrm{P}}_{\mathrm{J}}^3$ ${\mathrm{g}}_{\mathrm{L}}^{\prime }$ factor ${\mathrm{g}}_{\mathrm{L}}^{\prime }$=1-${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$+8.838×${10}^{\mathrm{-}6}$) and find that they are too small (1.79×${10}^{\mathrm{-}7}$) to resolve a long-standing discrepancy between theory and experiment for the 2 ${\mathrm{P}}_{\mathrm{J}}^3$ ${\mathrm{g}}_{\mathrm{L}}^{\prime }$ factor. These radiative corrections have not previously been calculated. Our value for the ${\mathrm{He}}^4$ ${\mathrm{g}}_{\mathrm{L}}^{\prime }$ factor is 1.36 standard deviations above the best experimental value. This respresents a significant improvement over the results of Lewis and Hughes which is 1.97 standard deviations above the best experimental value. The ${\mathrm{He}}^4$ 2 ${\mathrm{P}}_{\mathrm{J}}^3$ ${\mathrm{g}}_{\mathrm{X}}$ factor is found to be -5.344×${10}^{\mathrm{-}6}$. We have also calculated the ${\mathrm{He}}^4$ 2 ${\mathrm{P}}_1^1$ ${\mathrm{g}}_{\mathrm{L}}^{\prime }$ factor (${\mathrm{g}}_{\mathrm{L}}^{\prime }$=1-${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$-15.771×${10}^{\mathrm{-}6}$). This g factor has not previously been calculated. Finally, we have to introduce a g factor for the helium 2 ${\mathrm{P}}^3$ states in the |${\mathrm{Jm}}_{\mathrm{J}}$LS〉 representation: ${\mathrm{g}}_{\mathrm{y}}$=1.08×${10}^{\mathrm{-}8}$ for ${\mathrm{He}}^4$ and ${\mathrm{g}}_{\mathrm{y}}$=1.06×${10}^{\mathrm{-}8}$ for ${\mathrm{He}}^3$. This g factor comes from terms of the order ${\mathrm{\alpha }}^3$ and α(${\mathrm{m}}_{\mathrm{e}}$/${\mathrm{m}}_{\mathrm{N}}$).

We study the changes appearing in Gaussian momentum distributions of quantum-mechanical wave packets after a traversal of a linear level crossing. The numerical results are discussed in terms of the semiclassical Landau-Zener theory. We observe a momentum filtering effect in nondissipative crossings. The results obtained from studies of dissipative level crossings suggest that one can obtain substantial changes in momentum distributions with these systems. The Landau-Zener theory provides qualitative understanding of the processes taking place in the dissipative level crossing.

We report an experimental investigation of the Cs ${\mathit{D}}_2$ line-shape changes due to laser-induced optical pumping and collisions in a buffer gas. A model dealing with the population of the ground-state hyperfine levels, the pressure broadening in the presence of Ne, and the diffusion of the atoms outside the laser beam describes well the complex line shapes.

Research on optimal control of quantum systems has been severely restricted by the lack of experimentally feasible control pulses. Here, to overcome this obstacle, optimal control is considered with the help of chirped pulses. Simulated annealing is used as the optimizing procedure. The examples treated are pulsed population inversion between electronic levels, and optimization of vibronic excitation in the presence of another electronic level. In the problem of population inversion effective potentials of displaced harmonic oscillators are used. For optimizing vibronic excitation the CsI model potential is used.

The harmonic-generation (HG) spectra of the ${\mathrm{H}}_2^{+}$ molecular ion generated by short, intense, linearly polarized laser pulses are calculated nonperturbatively by solving a three-dimensional time-dependent Schrödinger equation. It is found that while ${\mathrm{H}}_2^{+}$ radiates in a similar manner to an atomic system such as H or ${\mathrm{He}}^{+}$ at high frequencies, the molecular ion in high-vibrational states or at large internuclear distances behaves analogously to a two-level system in an intense, relatively-long-wavelength laser field. It will be shown that symmetric molecular ions should in general produce more efficient harmonic generation than atoms, due to the presence of molecular charge-resonance (CR) states, which have no atomic analogues. Two HG plateaus, one molecular due to CR transitions and one atomiclike, appear for a long-wavelength excitation. Each plateau has its own maximum harmonic-number cutoff, which can be rationalized in terms of the maximum energy which can be acquired by an electron in the laser field.

The photodissociation of homonuclear ions exposed to continuous-wave (cw) lasers gives rise to asymptotically diverging couplings when approached in the electric-field gauge. Quantization of the laser-induced resonances becomes possible by complex rotation, leading to accurate energies, widths, and wave functions. A pseudoflux is endowed in the closed-channel component when calculated in the diabatic frame and for high laser intensities. A complex energy flux analysis performed in the adiabatic representation is developed, replacing the standard Siegert analysis which is no longer adequate for the extraction of the width from the asymptotic amplitude of the open-channel function. At the limit of strong radiative couplings the remaining nonadiabatic couplings asymptotically vanish. In the special case of constant potential couplings, the width is given by the asymptotic amplitude of the open-channel function through a modified Siegert analysis. Application is made to ${\mathrm{H}}_2^{+}$, in laser frequency and intensity regimes where a single-photon description is sufficient.

A nonperturbative time-independent method of calculating accurately partial widths of resonances formed in fragmentation processes is presented. It is based on a complex-energy probability-flux analysis performed in the adiabatic representation and turns out to be applicable whatever the strength or the spatial dependence of the interchannel couplings is. The method is applied to the photodissociation resonances of ${\mathrm{H}}_2^{+}$ exposed to an intense continuous-wave (cw) laser field. The photon channel pathways followed during the fragmentation are determined in terms of partial fluxes. Multiphoton absorption rates and branching ratios are extracted and compared with other recent studies. In the limit of weak laser intensities the formulas for the partial widths reduce to those resulting from the standard Siegert form for the open-channel components of the resonance wave functions.

We partition the perturbation phase space in the three-element discrete nonlinear Schrödinger equation into symmetric and antisymmetric subspaces. We then show that chaotic motion in the neighborhood of symmetric trajectories is confined to the antisymmetric space. Chaos occurs in the system at arbitrarily low levels of nonlinearity, in agreement with previous calculations. We call this phenomenon ‘‘microchaos.’’

The effect of amplitude-phase multistability in the Dicke model inside a coherently driven cavity is discussed for the case of large Rabi frequencies and a small rate of atomic decay, compared with the decay of the cavity mode. The quantum and semiclassical interpretations of the predicted effect are given on the basis of the Tavis-Cummings model and a proposed model of weakly coupled harmonic oscillators. The analytical solution is obtained for a two-atom system. The possibility of observing amplitude-phase multistability is also considered.