Starting from a transfer-matrix formalism, we study the electronic structure of a model of a periodic two-dimensional array of small disks of electrons or ‘‘quantum dots’’ that is infinite in both the x and y directions and is in a transverse magnetic field. The disks are coupled via scattering matrices which allow electrons to be transmitted between them. Unlike tight-binding models, such systems must in general allow for more than one degree freedom per site. Despite this, the spectrum for this system, which is dependent on the transmission probability, has many properties in common with spectra that have been obtained for periodic tight-binding systems. In particular, the energy bands become split when the magnetic field takes rational values and the current amplitudes can obey a type of Harper’s equation. In certain limits, the spectrum generated by our model becomes equivalent to that of the standard two-dimensional tight-binding model.

Electric-field effects on the N=3 ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$(1) resonance of ${\mathrm{H}}^{\mathrm{-}}$ are investigated theoretically using a method of complex-coordinate rotation. Products of Slater orbitals are used to represent the two-electron wave functions with ${\mathit{l}}_{\max }$=5 employed for individual electrons. Block matrices with up to ${\mathit{L}}_{\max }$=6 are used. Convergence behavior for the resonance parameters is examined by using different values of ${\mathit{L}}_{\max }$ and ${\mathit{l}}_{\max }$. Results for the energy position and width of the N=3 ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$(1) resonance under the influence of external electric fields are compared with experimental observations.

Energy and charge dependence of total cross section for ionization of the excited states with n≤5 of hydrogen, in collisions with the bare ions up to ${\mathrm{O}}^{8+}$, has been investigated theoretically by using the continuum distorted wave–eikonal initial state (CDW-EIS) approximation. In contrast with the similar calculations of Fainstein et al. [J. Phys. B 23, 1481 (1990)], the present results show that the CDW-EIS approximation becomes identical to the Born approximation at high energies, even in the ionization of excited states. A comparison is made between the present results and other theoretical predictions.

Muon transfer and elastic scattering in low-energy collisions of t+dμ(1s) are studied by the hyperspherical coupled-channel method in which many closed channels are explicitly coupled to obtain converged cross sections. A hybrid procedure of the diabatic-by-sector and the traditional adiabatic-basis-expansion methods is developed for solving a larget set of coupled equations influenced by the nearly singular nature of couplings induced by numerous avoided crossings. It is shown that the adiabatic states belonging to the n=2 excited states in the separated-atom limit contribute significantly to the muon transfer. The discrepancy between the previous hyperspherical calculation of Fukuda et al. [Phys. Rev. A 41, 145 (1990)] and other existing theoretical calculations has been resolved by taking into account the couplings with these excited states.

Extensive calculations and measurements of barium autoionizing Stark spectra below the 5d ionization thresholds are presented. The theoretical description is based on a combination of R-matrix calculations in zero external field and a Harmin-Fano-type field-dependent frame transformation. In addition to verifying the basic validity of this approximation scheme, we examine the scaling of calculated resonance linewidths with energy. Under certain circumstances, the average linewidths are found to scale approximately as ${\nu }^{\mathrm{-}4}$, with ν the relevant effective quantum number. Extreme departures from this scaling are observed for individual resonances, however, which could be important for the theoretical description of other processes in an electric field, such as dielectronic recombination.

We have calculated the total cross section of the recombination of an electron with hydrogenlike lead in the vicinity of the KLL resonances. The effects, including both the radiative overlap of identical levels and coupling between the different channels of the process, are taken into account in the frames of QED. The numerical results show the existence of noticeable interference effects in the cross section and may be of interest for the near future storage-ring experimental studies of recombination.

The energy distributions for 625-keV ${\mathrm{H}}^{+}$ ions transmitted through thin Si single crystals are studied for detailed angular scans through the 〈110〉 and 〈100〉 axial as well as the {111} and {110} planar channels. Well-resolved structures in the distributions taken near the 〈110〉 axial direction are observed. The experimental energy-loss distributions are very well reproduced by a Monte Carlo simulation using the semiclassical approximation model for core electrons and the two-component free-electron-gas model for valence electrons. The best fit to the data is obtained if the model energy losses are scaled up for core electrons and down for valence electrons by several percent. The experimental distributions can also be reproduced by assuming the mean excitation energy for distant collisions of the ion with core electrons equal to 1.4 times the binding energy for a given shell. No significant differences between the distributions obtained using the solid-state and free-atom valence electron densities have been found. The evolution of the distributions for the 〈110〉 axial scan is discussed in terms of ion trajectories and the flux distribution. Also, the azimuthally averaged mean energy loss is studied as a function of tilt angle with respect to the 〈110〉 axis.

We studied the potential electron emission arising from the interaction of H-like ${\mathrm{Ne}}^{9+}$ ions with clean Al(110) and Pt(110) surfaces at different glancing angles (10 °–90°) with 90-keV impact energy. Projectile KLL Auger electrons are found to be emitted from inside the solid. Therefore, the measured electron energy distributions are influenced by solid-state effects. We present a deconvolution procedure to determine the scattering contribution to the measured spectra for Al targets. After a subtraction of the scattering contribution we compare the resulting primary emission spectra with atomic structure calculations. The calculations show that at the moment of KLL Auger electron emission the projectile L shell is completely filled for an Al target, whereas for a Pt target there were on the average only five electrons in the projectile L shell. This indicates a relatively faster L-shell filling process for Al targets.

We have demonstrated in our experiments that important additional information can be won even from a stationary neutron beam by registering the detection time. Time selection was performed by registering the time of arrival of neutrons with respect to reference events. It was thus possible to measure the time-dependent neutron-neutron pair correlation function of a statistical beam. The sequence of time-correlated pairs is more strongly correlated than the Poissonian distribution of the initial events. Using this method it is possible to enhance the contrast and the phase resolution.

We present experimental results on metastable hydrogen-beam Stern-Gerlach interferometry using either static magnetic fields or time-dependent magnetic fields. The shared experimental scheme is presented. The dispersive behavior of the induced phase shift in the static situation is evidenced. The time-dependent (pulsed) scheme has provided experimental evidence for an external wave-function phase shift in atom force-free dynamical evolution. This so-called scalar Bohm-Aharonov (SAB) effect is thoroughly investigated. As opposed to the static field case, the nondispersive behavior of the SAB effect is validated. Experiments that mix static and pulsed magnetic fields are performed to reach the provisional goal to test nontrivial phase additivity. In all cases, experimental results are shown to be consistent with theoretical predictions derived from a tentative model including the deterministic magnetic phase shifts as well as additional random shifts due to experimental imperfections.

We discuss the formulation of neutron-interferometry experiments in terms of density matrices of plane waves. It is shown that the shape of the interference pattern does not depend upon the off-diagonal terms. As special cases, a wave packet and an incoherent superposition of plane waves give rise to the same shape of the interference pattern, provided that the intensity of the plane waves composing the incoherent superposition are equal to the square modulus of the amplitude of each Fourier component of the wave packet.

We investigate the stability of a classical three-dimensional hydrogen atom subject to a periodic train of alternating δ-like pulses, or ‘‘kicks.’’ The intense-field stabilization effect, widely investigated for the case of monochromatically driven atoms, is numerically shown to occur in this purely classical model, and its dependence on various parameters is analyzed. Our results support the view that classical intense-field stabilization is determined by the stability of the motion in a direction transverse to the field polarization, and confirm previously established estimates for the onset of stabilization.

Photoassociation in laser-assisted collisions of ultracold alkali atoms has been used recently to perform high-precision spectroscopy of very-long-range states. Rovibrational spectra close to the Na(3 ${}_{}{}^2{}_{}{}^{}$S)+${\mathrm{Na}}^{\mathrm{*}}$(3 ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$) dissociation limit have been obtained by monitoring an enhanced ${\mathrm{Na}}_2^{+}$ ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.

We have studied the multielectron dissociative ionization of ${\mathrm{Cl}}_2$ with intense 130-fs laser pulses at 395, 610, and 790 nm, and 2-ps pulses at 610 nm. The kinetic-energy releases and the fragmentation branching ratios are found to be essentially independent of the laser wavelength and of the pulse duration. Moreover, for all fragmentation channels up to ${\mathrm{Cl}}_2^{8+}$→${\mathrm{Cl}}^{4+}$+${\mathrm{Cl}}^{4+}$, the measured energy releases are systematically 70% of those expected from a simple Coulomb explosion of the molecule at its neutral ground-state equilibrium internuclear distance. This constant-energy ratio is even more surprising in the picosecond regime, where the pulse duration largely exceeds the molecular ground-state vibrational period by a factor of 30 so that the molecular motions cannot be considered as frozen during interaction. The results are tentatively explained in terms of laser-induced stabilization of the transient molecular species via charge-resonance or charge-transfer couplings. In this picture, the electron removal occurs at the end of the laser pulse, leading only then to the explosion of the multicharged ions.

We study the dynamics of multiphoton-absorption chirped adiabatic passage (CHIRAP) in atoms, and show that the dynamics of the excited-state fine-structure transitions can be nonadiabatic even though the optical transfer to the highly excited electronic states is fully adiabatic. Hence, care must be taken to ensure that the dynamics is selective if multiphoton-absorption CHIRAP is to be used as a pump- ing mechanism to populate very-high-lying states that can serve as the pumping levels for uv lasers. We investigate the dynamics as a function of chirp rate and field intensity in the 5s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$→5p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$→5d ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$,5d ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{5/2}$ rubidium system, and show that selectivity can be maintained despite the near degeneracy of the ${}_{}{}^2{}_{}{}^{}$D manifold.

Whereas mechanical rotation of a transparent solid induces rotation of the plane of linear polarization of a light beam, for a dilute vapor this mechanical Faraday effect must disappear. By studying the structure of the collision operator that is familiar in the unified theory of spectral line broadening, we demonstrate that a nonzero effect arises for a vapor of absorbing atoms embedded in a buffer gas. The size of the effect is easily compared with the magnetic Faraday effect, when the angular velocity is replaced with the Larmor precession frequency. Then, the mechanical effect is smaller by a factor of the order of the collision frequency multiplied with the duration of a collision. Furthermore, the mechanical effect is a linear combination of the frequency derivative of a (collision-broadened) dispersion curve and the derivative of an absorption curve.

We have measured the transitions from the bound 3snk to the autoionizing 3pnk Mg Stark states in static and microwave electric fields. Due to the small separation of the ${\mathrm{Mg}}^{+}$ 3${\mathit{p}}_{1/2}$ and 3${\mathit{p}}_{3/2}$ states there is substantial configuration interaction between Stark states converging to the two limits, and the ${\mathit{n}}^{\mathrm{-}4}$ scaling of the autoionization widths expected from the simplest one-limit picture is not observed.

We have studied the transition probabilities and the photofragment energy and angular distributions in a resonant two-photon transition to the continuum with an elliptically polarized radiation field. The general formalism applies both to ionization of atoms within the framework of a single-electron model and the dissociation of diatomic molecules with the involvement of Σ states only. Detailed calculations for an initial J=0 state give the observable quantities as functions of detuning, intensity, and the polarization parameter for several combinations of the system parameters such as bound-bound and bound-continuum couplings and the Fano q parameter. For sufficiently intense fields the Raman-like coupling between the two coherently excited magnetic sublevels of the resonant J=1 state (one of which can be decoupled from the ground state by a linear transformation) causes a significant enhancement of the transition probability near the Fano minimum obtained for linearly or circularly polarized fields. The evolution of the photofragment kinetic-energy distribution changes with the ellipticity parameter of the radiation. The angular distribution becomes intensity dependent for general ellipticity parameters; the shapes of the resultant distribution in different space-fixed planes are discussed.

We report the discovery of an integrable three-dimensional Hamiltonian system with a velocity-dependent potential. A two-dimensional restriction of that system is (for low frequencies) a good approximation of the motion (in the polarization plane) of the hydrogen atom in circularly polarized microwave fields. An additional integral of motion of the integrable two-dimensional system (being approximate invariant for the hydrogen atom in a circular field) is used in the calculation of the classical ionization field threshold. The result is consistent with all available experimental observations.

The theory of light-induced collisional energy transfer (LICET) in the Eu-Sr system is extended to include all the degenerate M states that occur in the separated-atom basis states. Previous treatments were restricted to only the M=0 substates. Numerical solutions are obtained for the resulting nine-state dynamic coupled system to determine transitions probabilities on a given straight-line path. Intense laser fields of linear and circular polarization are treated. It is found that the full M-state treatment results in dependences on the trajectory orientation angles which are much more complex than in the M=0 treatment. The exact and quasisymmetries involving these orientation angles are described in simple group-theoretic terms. The resulting spectral line shapes (LICET transition cross sections as a function of laser detuning) are seen to have structures that are understood in terms of the coupling matrix elements, as well as appreciable Stark shifts. These shifts, which appear to be an essential outcome of the theory, are not yet seen in measurements of the LICET line shape.