We present the theoretical near-threshold photoionization cross sections of He atoms from bound excited states, using a B-spline-based configuration-interaction method. Our study has shown that in the absence of a strongly energy-dependent feature such as a doubly excited resonance, the photoionization cross sections in the immediate vicinity of the ionization threshold depend linearly on the wavelengths of the incident light. Qualitatively, following the quantum-defect theory, this linear dependence can be linked directly to the expected nearly energy-independent scattering phase shift corresponding to the wave function of the ionized electron at near-zero photoelectron energy.

The dielectronic satellite radiation emitted from ions excited by resonant capture of an electron from a directed electron beam can be strongly linearly polarized. We have carried out relativistic calculations of angular asymmetry parameters and polarization of dielectronic satellite lines for H-like ions with Z=9, 22, 28, 42, and 92 using the multiconfiguration Dirac-Fock model. We found that many transitions exhibit strong angular asymmetry and a large degree of polarization. In addition, we found that the polarization of the resulting radiation is independent of atomic number in the nonrelativistic limit. But when the effects of relativity are taken into account, the polarization becomes markedly Z dependent.

A theoretical investigation of electric-field effects on the doubly excited resonances of ${\mathrm{H}}^{\mathrm{-}}$ between the H(N=3) and H(N=4) thresholds is made using the method of complex-coordinate rotation. Products of Slater orbitals are used to represent the two-electron wave functions, while ${\mathit{l}}_{\max }$=8 is employed for an individual electron. Block matrices up to ${\mathit{L}}_{\max }$=7 (K states) are used. Convergence behaviors for the resonance parameters (resonance energy and width) are examined by using different values of ${\mathit{L}}_{\max }$. Results for the electric-field effects on the Feshbach ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$(1), ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$(1), ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$(1), ${}_{}{}^1{}_{}{}^{}$${\mathit{F}}^{\mathit{o}}$(1), and ${}_{}{}^1{}_{}{}^{}$${\mathit{G}}^{\mathit{e}}$(1) resonances are given, as well as for the effects on the ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ shape resonance lying above the H(N=3) threshold.

By means of differential energy-transfer measurements, excitation mechanisms in ${\mathrm{Na}}^{+}$-Ar collisions have been studied at laboratory collision energies of 50≤${\mathit{E}}_{\mathrm{lab}}$≤1500 eV. In these experiments, doubly differential cross sections have been measured over nearly the whole angular range in the center-of-mass system by simultaneously detecting both scattered and recoiled particles (${\mathrm{Na}}^{+}$, Na, ${\mathrm{Ar}}^{+}$, and Ar) at laboratory angles of 2°≤θ≤92°. The ${\mathrm{Na}}^{+}$ ions and Na atoms scattered inelastically were observed at reduced angles of τ>3.5 keV deg and collision energies of ${\mathit{E}}_{\mathrm{lab}}$>500 eV. For energies of 500<${\mathit{E}}_{\mathrm{lab}}$<1000 eV, the dominant inelastic signal is due to one-electron charge transfer, while for ${\mathit{E}}_{\mathrm{lab}}$>1000 eV, two-electron excitation as well as one-electron excitation were observed. The electronic transitions in the ${\mathrm{Na}}^{+}$-Ar collisions are classified into two types of excitation mechanisms. One type is the one-electron transition that takes place at internuclear distances of R<${\mathit{R}}_{\mathit{C}1}$=1.07 Å. The other type is the one- and two-electron transitions that occur at distances of R<${\mathit{R}}_{\mathit{C}2}$=0.45 Å. These critical distances ${\mathit{R}}_{\mathit{C}}$=1.07 and 0.45 Å are close to ‖${\mathit{r}}_{\mathit{i}}$±${\mathit{r}}_{\mathit{j}}$‖=0.96 and 0.38 Å, respectively, evaluated from the ionic and atomic radii.

Some kinematical arrangements in He(e,3e) process have been explored to ascertain the sensitivity of continuum electron-electron angular correlation to the electron-electron (e-e) correlation in the target wave function. The calculation has been done in the Coulomb-projected Born approximation with suitably chosen effective charges. It is found that the variation in the cross section is quite sensitive to the target e-e correlation when (i) the two low-energy ejected electrons are detected at fixed angles ${\mathrm{\theta }}_{\mathit{b}}$ and ${\mathrm{\theta }}_{\mathit{c}}$ but their energies ${\mathit{E}}_{\mathit{b}}$ and ${\mathit{E}}_{\mathit{c}}$ are varied such that ${\mathit{E}}_{\mathit{b}}$+${\mathit{E}}_{\mathit{c}}$ is held fixed, or (ii) one of the electrons is detected at some large fixed angle (say, 90°) and the other angle ${\mathrm{\theta }}_{\mathit{c}}$ is varied at fixed energies.

The ionization and dissociation, induced by positrons having kinetic energies in the range 0.3–3 eV, of a large number of organic molecules have been studied. The required energies for ionization and dissociation are supplied through the annihilation of electrons by the positrons. Cross sections for these interactions are strong functions of molecular size, molecular structure, and bond type. Fragmentation usually occurs between atoms linked by σ bonds. Multiple bonds tend to stabilize the molecules against fragmentation. Fragmentation induced under the low-energy process occurs preferably in the linear groups of molecules rather than in ring structures. Ionization and dissociation cross sections of large alkane molecules are higher than those of smaller molecules. A possible explanation of the fragmentation phenomena is that positrons annihilate electrons in energy levels below the highest occupied molecular orbital (HOMO), leaving the molecules in excited states. Multiple bonds in molecules tend to promote higher populations near the HOMO state; annihilation of electrons near the HOMO state does not result in fragmentation.

The partial photoionization cross sections and angular distributions of the C 1${\mathit{s}}^{\mathrm{-}1}$ and O 1${\mathit{s}}^{\mathrm{-}1}$ single-hole states of ${\mathrm{CO}}_2$ have been measured with high precision near threshold. The 4${\mathrm{\sigma }}_{\mathit{u}}^{\mathrm{*}}$ shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections ${\mathrm{\sigma }}_{\mathrm{SH}}$ due to multielectron excitations: virtually all spectral details in the total 1s cross sections ${\mathrm{\sigma }}_{1\mathit{s}}$ are also present in ${\mathrm{\sigma }}_{\mathrm{SH}}$ implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of ${\mathrm{CO}}_2$ have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.

We report on a study of total and differential excitation cross sections for the electron-impact excitation of hydrogenlike carbon for the transitions 1s ${}_{}{}^2{}_{}{}^{}$S→2s ${}_{}{}^2{}_{}{}^{}$S and 2p ${}_{}{}^2{}_{}{}^{}$P. These calculations have been performed for electron-impact energies ranging from 28.2 to 29.5 Ry using the close-coupling R-matrix method. This energy region has been chosen because in addition to the direct excitation of 2s and 2p, there is a sizable contribution to the excitation cross section from the doubly excited two-electron states (3l3l′), which decay predominantly to n=2 via autoionization. Differential cross sections for both the direct nonresonant channel and in the vicinity of several autoionizing resonant states are presented. The differential cross sections in the vicinity of the autoionizing resonant states are shown to be very rich in structure. We discuss the relevance of these calculations to a recent experiment that measured the electron emission spectra for the collision of ${\mathrm{C}}^{5+}$+${\mathrm{H}}_2$.

We present a time resolved study of excitation transfer processes between the low lying excited states of barium (6s6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_{\mathit{J}}$, 6s5d ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$, 6s5d ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_{\mathit{J}}$) induced by collisions with noble gases. The transient excited state populations have been measured by means of fluorescence and absorption spectroscopy. Cross sections for spin-changing collisions from ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$ to ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ and from ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ to ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_3$ as well as for fine-structure mixing within the ${}_{}{}^3{}_{}{}^{}$D multiplet were obtained by fitting the numerical solution of a seven-state system of rate equations including diffusion to the experimental data. We compare our results to experimental findings for other alkaline earth metals and report an improved value of the lifetime of the 6s6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$ state of barium.

The first experiments measuring the impact-parameter-dependent L-K vacancy sharing in ion–solid-target collisions started almost two decades ago. A rather qualitative explanation of the vacancy sharing process was achieved in the framework of the 2pπ-2pσ coupling scheme developed by Briggs and Taulbjerg, but for most of the collision systems significant discrepancies remained. Even a multiple collision calculation using the 2pπ-2pσ coupling scheme, as undertaken by Schuch et al., did not improve the agreement with the experimental results and left a lack of an explanation, calling for first-principles calculations. Using our ab initio relativistic Dirac-Fock-Slater many-electron basis set method, we confirm that both a qualitative and quantitative explanation can be given for the L-K vacancy sharing process in heavy ion–solid-target collisions. As an example, we compare our results with measurements for 108-MeV Br on Ni-solid targets.

A crossed beam experiment has been used to measure absolute charge capture cross sections in collisions of slow highly charged xenon ions with laser excited Rydberg atoms. The cross sections were measured for scaled projectile velocities ${\mathit{nv}}_{\mathit{p}}$ from 1.0 to 6.0, for projectile charges of 8, 16, 32, and 40, where n is the principal quantum number of the target electron. Experimental cross sections are compared with predictions of classical models.

Universal, spherical, nonsingular van der Waals interactions including retardation effect are developed for atoms and small molecules through a semiclassical field approach. Consideration of the finite molecular size effect removes the short-distance singular behavior inherent in the widely used potentials obtained from the point-molecule approximation. Physical arguments lead to the molecular size parameter a (in atomic units) as 1/a=1.25(I/${\mathit{I}}_{\mathrm{H}}$${)}^{1/2}$, except for a system that involves at least an atom or a molecule with very different first and second ionization potentials, and for such a system the above numerical factor 1.25 is replaced by unity. Here I and ${\mathit{I}}_{\mathrm{H}}$ are the first ionization potentials for the atom or molecule considered and for a hydrogen atom, respectively. The nonsingular potentials have been tested for the following representative systems: ${\mathit{H}}_2$ ${(}^3$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$), ${\mathrm{He}}_2$, ${\mathrm{Ar}}_2$, NaK ${(}^3$${\mathrm{\Sigma }}^{+}$), LiHg ${(}^2$${\mathrm{\Sigma }}^{+}$), He-HF, Ne-HF, HF-HF, and Ar-HCl. Very good agreement has been found for each of the systems. Based on the above systems studied, an empirical relation has been obtained between the parameter b in the Born-Mayer repulsive potential ${\mathit{Ae}}^{\mathrm{-}\mathit{b}\mathit{R}}$ and the molecular size parameters (${\mathit{a}}_1$ and ${\mathit{a}}_2$). Applying this relation to dozens of systems with known b from either self-consistent-field calculations or experiments, surprisingly good agreements have been obtained. By the same token, another empirical formula is obtained that relates the van der Waals minimum well parameter ${\mathit{R}}_{\mathit{m}}$ to the molecular size parameters (${\mathit{a}}_1$ and ${\mathit{a}}_2$) and the first ionization potentials (${\mathit{I}}_{\mathit{x}}$ and ${\mathit{I}}_{\mathit{y}}$) of interacting species. Again, very good agreements have been achieved in comparison with dozens of systems with known experimental ${\mathit{R}}_{\mathit{m}}$’s.

A scheme is proposed to manipulate the quantum-statistical properties of a neutral atomic Bose-Einstein condensate in an ultracold alkali-atom-trap system with atoms in the ground hyperfine F=1, ${\mathit{M}}_{\mathit{F}}$=±1,0 states (labeled respectively as ‖${\mathit{g}}_{\pm ,0\mathrm{}}$〉). Initially, the atomic condensate is assumed to be prepared in a hyperfine sublevel ‖${\mathit{g}}_{\mathrm{-}}$〉. By effective two-photon excitations with two classical (${\mathrm{\sigma }}^{\mathrm{-}}$) and quantum (${\mathrm{\sigma }}^{+}$) nonresonant copropagating traveling-wave light fields, we show that the atomic system can be settled into a coherent superposition of two atomic Bose-Einstein condensates corresponding to different ground hyperfine sublevels and different quantum statistics. Furthermore, with an appropriate choice of interaction time, atom-field coupling strengths, and quantum features of the quantized laser field, a nonclassical condensate can be prepared in the hyperfine sublevel ‖${\mathit{g}}_{+}$〉.

We investigate schemes for the clean splitting of beams of three-level atoms using two standing-wave laser fields within an optical cavity. The proposed beam splitter is shown to work for atoms in the Λ ladder, and ssV configurations. For appropriate values of Rabi frequencies and detunings, we obtain a triangular type of potential for the atomic states of interest. As well as modeling the coherent evolution of the systems, we have used quantum Monte Carlo wave-function methods to model the effects of spontaneous emission on the resulting diffraction pattern, finding significant differences between the three configurations. We also investigate the limits of the Raman-Nath approximation for our systems, using the symmetric split-operator technique to include the effects of the kinetic term in the Hamiltonian. We also present the results of calculations in which the split output beams are recombined, demonstrating the expected interference for differently prepared input beams. In comparison with two-level beam splitters using a single standing wave, we obtain a superior splitting, while, in comparison with magneto-optical beam splitters, our system possesses the worthwhile practical advantages of experimental simplicity.

We perform a theoretical study of the response of neutral and ionized metal clusters to the elastic impact of slow electrons. We center our attention on ${\mathrm{Na}}_{40}$ and ${\mathrm{Na}}_{41}^{+}$ closed-shell clusters. A phase-shift analysis is performed for both systems. The total cross section for the neutral cluster presents resonances related with the existence of quasibound states when studied as a function of incident energy. For the ionized cluster we show the calculated ratio of the angular distribution to the Rutherford differential cross section, near and far from a resonance. The existence of resonances as well as the associated energies are very sensitive to the mean interaction potential. As a consequence, elastic scattering of slow electrons on clusters could be a useful tool in the study of the electronic structure of clusters.

Surface light-induced drift (SLID) in transparent nonadsorbing porous media is analyzed. In a simple model of long, narrow, converging or diverging, straight nonbranching pores, and neglecting the light scattering by the porous medium, steady-state molecular flows are calculated for the cases of Knudsen diffusion and of a dense gas. The SLID flux is found to be weakly dependent on the dispersion of the pore lenghts, but strongly dependent on the dispersion of pore radii and orientations. The effect can be used for studying the state specificity of heterogeneous chemical reactions and catalysis, and potentially for isotope separations.

Several approximations to the T matrix for collisions by a static short-range potential in the presence of a strong static magnetic field and a collinear polarized radiation field are considered. We discuss their different ranges of validity and compare the expressions of the T matrix in the different approximations.

A multilevel system driven strongly by several near-resonant and arbitrary modulated optical fields is considered. The orthonormal basis of the time-dependent quasienergy states is introduced to represent in a matrix form the density operator and the operators describing atom-field interaction and relaxation. The above formalism simplifies the density-matrix equations of motion and provides a uniform treatment of resonance phenomena such as probe wave resonances, parametric resonances, and quantum interference. Special consideration is given to the steady-state and transient-time evolution of two- and three-level systems.

We have made several different types of measurements of the three-photon ionization of Li produced by 3-ps laser pulses and describe the results using a Floquet picture. Over the photon frequency range 15 000 to 15 800 ${\mathrm{cm}}^{\mathrm{-}1}$, Li represents a strongly coupled three-state system with the 2s ground state coupled to the 2p and 3d states by one and two photons, respectively. Energy analysis of the photoelectrons allows the measurement of the intensity dependent shift of the 2s Floquet state during the laser pulse. The shift shows a strong frequency dependence that is not predicted by first-order perturbation theory. We have also measured the total ionization spectrum over several ranges of frequency, as well as the angular distribution of the ionization and the first above-threshold ionization peak for frequencies where the ground state is near resonance with the 4s and 4d excited states. Calculations based on the Floquet Hamiltonian indicate that all of these processes may be understood in terms of a Floquet description.

Starting from a three-level atom coupled to two modes of radiation field, we derive a Raman-coupled Hamiltonian by a unitary transformation, evaluated perturbatively in coupling constants. The Rabi oscillation frequency and the collapse and revival times of the atomic coherence are found to have strikingly different photon-intensity dependence than those found previously.