Observations of qualitatively different coherent transient phenomena associated with bichromatic optical excitation are reported. The effects demonstrated include the control of atomic dynamics through a variation of the initial relative phase of the driving fields and the polarization of population within atom-field dressed states. The dynamics observed are fundamentally more complex than those characteristic of monochromatically driven atoms.

A heterodyne spectroscopic scheme, using only one laser tuned to the atomic wavelength, is applied to the measurement of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(F=1/2-1/2 component)${\mathrm{-}}^4$He separation for the 2 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$–3 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_0$ transition at 389 nm. The experimental result is 45 394 425(137) kHz and the isotope shift deduced in the absence of hyperfine structure is 42 184 180(139) kHz. A new value of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclear charge radius is obtained, 〈${\mathit{r}}^2$${\mathrm{〉}}^{1/2}$=1.923(36) fm, in a regime of low energy and momentum transfer.

We use the wave function from the Schwinger multichannel method to calculate ${\mathit{Z}}_{\mathrm{eff}}$—an annihilation parameter of positron-atom (or molecule) scattering. These ${\mathit{Z}}_{\mathrm{eff}}$ are evaluated from the (N+1)-particle wave function and are calculated in a fully ab initio way (in a quantum chemistry sense). Our first applications are for positron scattering against He atoms and ${\mathrm{H}}_2$ molecules. Our results for both systems are in excellent agreement with experimental data at thermal energies.

Dissociative recombination of ${\mathrm{HeD}}^{+}$ was studied using a storage ring and compared with the results on ${\mathrm{HeH}}^{+}$. A strong peak at around 20-eV electron energy was observed in addition to a peak at nearly zero energy. Their shapes are very similar to corresponding ones on ${\mathrm{HeH}}^{+}$. The relative strengths of these peaks, however, are quite different between ${\mathrm{HeH}}^{+}$ and ${\mathrm{HeD}}^{+}$, which is clear evidence of isotope effect. By mixing ${\mathrm{O}}_2$ gas into the helium hydride ion source, it was confirmed that the peaks originate from the ground state of molecular ions. The dissociation energy for the low-energy peak was about 1 eV. This implies that the dissociation of the low-energy component occurs primarily to He+${\mathrm{H}}^{\mathrm{*}}$(n=2), although the possibility of some branching to n=1 cannot be excluded.

We have measured the final atomic excited-state alignment following the quantum-state-selected photodissociation of ${\mathrm{K}}_2$ through quasibound rotational resonance states using cw all-optical triple resonance spectroscopy. This process constitutes the half-collision analog of ${\mathrm{K}}^{\mathrm{*}}$+K scattering through a precisely quantum-state-selected orbiting resonance. The final atomic-state alignment depends sensitively on the parity of the excited continuum state. The experimental results show excellent agreement with theoretical calculations.

We study the influence of a solid on the Auger electrons emitted by highly charged ions incident on the solid surface. It is shown that the Auger peak is shifted in energy due to the interactions of the electrons in the solid target. We analyze how this line shift can be used to distinguish between electron emission before and after the projectile enters the surface. We present specific results for the cases of broad and narrow Auger lines. The solid-state-induced shift of the projectile KLL Auger peak is calculated for ${\mathrm{N}}^{6+}$(10 keV)→Ni and ${\mathrm{Ne}}^{9+}$(90 keV)→Cu. Good agreement is obtained with the results of the corresponding experiments.

We present results of molecular-dynamics simulations of ZnO clusters. At certain magic numbers of atoms, the clusters spontaneously form into spheroids reminiscent of fullerenes. Molecular-orbital calculations confirm that the spheroids are stable. We suggest some experiments that might be profitable.

We describe the results of our solutions of the time-dependent Schrödinger equation for Rydberg states of atomic hydrogen exposed to a short pulse of ‘‘half-cycle’’ long-wavelength radiation. By ‘‘short’’ we mean that the duration of the pulse is much less than the Kepler period of the Rydberg state being irradiated. Comparison is made with the results of experiments which have been reported recently [R. Jones, D. You, and P. Bucksbaum, Phys. Rev. Lett. 70, 1236 (1993)].

We describe the transient four-wave-mixing (FWM) response in systems where decay of the optically induced coherence depends on the excitation level of the system. Using modified optical Bloch equations, we show that excitation-induced dephasing due to phase-interrupting excited-state interactions qualitatively modifies the temporal behavior and polarization dependence of the FWM signal.

Applying a result of Vogel and Risken [Phys. Rev. A 40, 2847 (1989)] to quantum-oscillator states with random phase, we found surprisingly simple integral relations between the Wigner function and the quadrature distribution. In particular, we have shown that the balance of increasing and decreasing sections of the quadrature distribution decides the sign of the Wigner function.

The process of degenerate four-wave mixing is studied experimentally in dense sodium vapor in a rare-gas atmosphere. The influence of trapped fluorescence light on the ground-state orientation is shown to be responsible for the observed strong reduction of saturation phenomena with increased sodium density. The interpretation is based on a simple model and is supported by results obtained by suppressing the fluorescence.

Single and double K-shell vacancy production and K-K electron-transfer cross sections have been measured in the limit of zero target thickness for Fe and Ni induced by 1.25–4.70 MeV/amu ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ ions. The fluorescence yield ${\mathrm{\omega }}_{\mathit{k}}$ for Fe and Ni at x-ray emission was determined from the measured energy shifts of target K x rays, the intensity ratios of Kβ and Kα and using the statistical scaling procedure of Larkins [J. Phys. B 4, L29 (1971)]. The single-electron-transfer cross sections are compared with the atomic-orbital-expansion model (AO) of Fritsch and Lin [Phys. Rep. 202, 1 (1991)], and the semiempirical perturbed-stationary-state (PSS) approach of Lapicki and McDaniel [Phys. Rev. A 22, 1896 (1980)]. The AO calculations show excellent agreement with the single K-K transfer cross sections. This model also explains the double K-K transfer data quite well although it underestimates slightly the measured values. The direct K-shell ionization cross section data are compared with the ECPSSR (perturbed-stationary-state theory with energy-loss, Coulomb deflection, and relativistic corrections) calculations.

We apply the close-coupling method in terms of the hyperspherical coordinates to the two-electron system ${\mathrm{H}}^{\mathrm{-}}$. A two-dimensional matching procedure is used to connect the close-coupling wave function to an independent-electron wave function in the asymptotic region. The latter is described as the wave function of a detached electron moving in a dipole potential field of the neutral hydrogen atom. The total phbotodetachment cross sections and the partial cross sections for the production of the H atoms in different states n are calculated up to the n=4 hydrogenic threshold. The results obtained in the length and acceleration forms agree within about three significant figures. The magnitude and shape of the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ shape resonance just above the n=2 threshold are found to depend sensitively on the initial-state wave function. This appears to be one of the reasons for a disparity among the cross sections in the literature. The present result improves greatly on the existing computational results. The relative magnitudes of the partial cross sections for the production of H (n=1,2,3) atoms below the n=4 threshold are substantially different from the only previous results of the eigenchannel R-matrix calculations [H. R. Sadeghpour, C. H. Greene, and M. Cavagnero, Phys. Rev. A 45, 1587 (1992)]. The present results on both partial and total cross sections are in excellent agreement with experiments.

We present a general nonperturbative calculation of the photoionization cross section for highly asymmetrical two-electron systems. The calculations were performed using the eigenchannel R-matrix procedure together with multichannel quantum-defect theory, including the effects of long-range multipole interactions. The isolated-core approximation is extended to treat two-photon absorption. The calculations permit a detailed interpretation of recent experimental measurements in atomic strontium and barium.

Relativistic many-body perturbation theory is applied to study the elastic scattering of electrons from xenon and from the xenonlike ions ${\mathrm{Cs}}^{+}$ and ${\mathrm{Ba}}^{2+}$, below their respective inelastic thresholds. The quasiparticle equation for the electron scattering wave function is solved in second-order perturbation theory starting from the Hartree-Fock approximation. Phase shifts for partial waves with l=0–5 are determined as functions of electron momentum for each ion. The theoretical elastic scattering cross section for xenon is found to be in agreement with experiment, showing a Ramsauer-Townsend minimum below 1 eV. Agreement with experiment is also found for differential cross sections and scattering asymmetry functions for xenon. Differential cross sections and scattering asymmetry functions for ${\mathrm{Cs}}^{+}$ and ${\mathrm{Ba}}^{2+}$ are predicted at 10 eV.

Briggs’s representation [Phys. Rev. A 41, 539 (1990)] of the Mo/ller wave operator for multiparticle wave functions is applied to charged fragments using a limiting procedure to correctly account for the slow decrease of Coulomb interactions with distance. Approximate wave functions used to model (e,2e) angular correlation measurments are obtained. Computed and measured angular correlations are compared to clarify the region of applicability of two approximations.

Photoionization cross sections from the metastable state (1s2s) ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ of the He atom for photon energies between the ${\mathrm{He}}^{+}$ (N=2) and (N=3) thresholds are calculated using the hyperspherical close-coupling method. The calculated spectra are convoluted with an energy resolution of 5.4 meV and are compared with the spectra for photoionization from the ground state. It is found that among the four possible outgoing channels, the 1sεp channel, which is the dominant channel for photoionization from the ground state, makes negligible contributions to the total cross sections for photoionization from the metastable state. As a result, the propensity rule derived from the ground-state photoionization no longer applies and more series of the doubly excited states are populated with significant spectral intensity in photoionization from the metastable state. Photoionization cross sections from the metastable (1s2s) ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ state are also calculated and analyzed.

We extend the convergent-close-coupling method for the calculation of electron-hydrogen scattering to hydrogenlike targets, atoms, or ions. These include H, Li, Na, and K, atoms, as well as the multitude of ions which have the same isoelectronic sequence as any of these atoms. The reliability of the method is independent of the projectile energy, and we demonstrate its applicability by achieving excellent agreement with a large set of measurements for electron scattering on sodium at projectile energies ranging from 1 to 54.4 eV. These measurements include spin asymmetries, singlet and triplet ${\mathit{L}}_{\mathrm{\perp }}$, reduced Stokes parameters, differential, integrated, and total cross sections, as well as the total ionization spin asymmetry. The method is found to give better agreement with experiment than any other over this entire energy range.

Recent experimental determinations [J. E. Valdés, G. Martínez-Tamayo, G. H. Lantschner, J. C. Eckardt, and N. R. Arista, Nucl. Instrum. Methods B 73, 313 (1993)] of the energy loss of low-energy protons in Ag and Au show a departure from the velocity proportionality predicted by theory, while data for other metallic elements, such as Al, Sb, and Bi, are in agreement with this dependence. In this work we present measurements in Cu foils which show a similar behavior to that in Ag and Au. We give an interpretation of the differences between these cases due to the existence of small binding energies for the d electrons in Cu, Ag, and Au. We present a model based on transport cross section and density-functional theory, which introduces threshold effects in the calculation of the transport cross section of nonfree electrons, and explains the deviations from the velocity-proportional dependence.

The stopping-power effective charge (SPEC) has been formulated in the Brandt-Kitagawa (BK) model. In the effective-charge theory, the stopping power of heavy ions can be obtained by scaling using the SPEC with a mean charge state. Based on the BK treatment for the statistical ion screening and the dielectric function, a formula for the straggling effective charge (SGEC) has been obtained. The SGEC, which is different from the SPEC, provides the scaling of collisional (without charge exchange) straggling in the effective-charge theory. To properly describe the stopping power and straggling of heavy projectiles in condensed matter, partial effective charges (PEC’s) are introduced to describe the charge-state-dependent stopping power and straggling in a charge-state model. The partial stopping power and straggling of projectiles with a fixed charge state can be obtained by scaling to those of the bare nucleus with related PEC’s. This allows the calculation of the total stopping power and straggling of heavy projectiles in a charge-state description including the charge-exchange effect.