Resonant dipole-dipole interaction (RDDI) in a Fabry-Pérot cavity is studied by the mode-expansion method. It is shown that the RDDI depends upon three factors: the length of the cavity, the positions of molecules, and the polarization of dipoles. It is also revealed by the mode-expansion calculation that, instead of contributions only from the propagating modes, the RDDI can also be assisted by the evanescent photons in the cavity. The expressions of energy transfer rate are obtained both for the two-level model and multilevel one in terms of observed spectral shape functions.

Measurements of the photodetachment cross section of ${\mathrm{B}}^{\mathrm{-}}$(2${\mathit{s}}^2$2${\mathit{p}}^2$${\mathrm{}}^3$P) have been performed in the vicinity of the B(2s2${\mathit{p}}^2$${\mathrm{}}^4$P) threshold with photon energies ranging from 3.37 to 4.83 eV. The experimental technique is based on a crossed-ion-laser-beam setup using a dye laser combined with standard nonlinear techniques. The behavior of the cross section above the threshold indicates the presence of the theoretically predicted quasibound ${\mathrm{B}}^{\mathrm{-}}$(2s2${\mathit{p}}^3$${\mathrm{}}^3$P${,}^3$D) states.

Ionization and charge transfer in slow collisions between highly charged projectiles and helium atoms are studied by the two-center atomic orbital close-coupling method within the independent electron approximation. The relative importance of single ionization to single capture is investigated. Our calculations show that ionization cross sections for ${\mathrm{C}}^{6+}$, ${\mathrm{N}}^{7+}$, and ${\mathrm{O}}^{8+}$ projectiles increase rapidly between v=1 and 2 a.u. The onset is dependent on projectile charge state. At a fixed velocity, ionization cross sections decrease with increasing projectile charge. It is also found that ionization and charge transfer occur at about the same impact parameter range.

The cross sections for antihydrogen formation in the n=1, 2, 3, 4, 5, 6, and 7 levels from antiproton-positronium collisions are computed in the unitarized Born approximation (UBA). Twenty-seven physical states of antihydrogen (1s→7h) and ten physical states (1s→4f) of positronium are included in the UBA basis. The peak cross section for antihydrogen formation from excited positronium targets is much larger than that from a ground-state positronium target at low incident energies. The high-n antihydrogen levels make a significant contribution to the total antihydrogen formation cross section, especially for incident positronium atoms in the Ps(n=3) and Ps(n=4) levels.

The theory of Demkov et al. (Zh. Eksp. Teor. Fiz. 66, 125 (1974) [Sov. Phys. JETP 39, 57 (1974)]) for collisional mixing of degenerate states of a hydrogenlike target is generalized to nonhydrogenic targets parametrized by a set of quantum defects, {${\mathrm{\mu }}_{\mathit{l}}$}, with l the orbital angular-momentum quantum number of the Rydberg electron. Schrödinger’s equation is written in periodic form in the impact-parameter approximation by using the angle of rotation of the internuclear line, as opposed to the time, as the independent variable. Quasiperiodic solutions are constructed by Floquet analysis, as in studies of Rydberg-state mixing by oscillatory fields, leading to an infinite-dimensional eigenvalue equation whose eigenvectors and eigenvalues determine the evolution operator of the collision system. An initial application to ‘‘l-mixing collisions’’ of ions with alkali-metal atoms indicates that dipole-forbidden transitions dominate depopulation cross sections at intermediate collision speeds.

With a relativistic partial-wave method we calculated numerically the positron energy-angle distributions of pair production in the field of atoms with atomic numbers $Z=1\mathrm{} \mathrm{and} 92$ for photons of energies $k=5\mathrm{}{m}_e{c}^2 \mathrm{and} 10m_e{c}^2$. We have found that in this intermediate-photon-energy region, the Born approximation works fine for low-$Z$ elements, as expected. Our partial-wave results indicate that the Born approximation prediction for the shape of positron energy-angle distributions (especially at small positron angles) is better than its prediction for positron energy spectra of pair production in this intermediate-photon-energy region. Our results also show that the shape of positron energy-angle distributions is almost independent of atomic electron screening.

We have calculated the resonant-excitation-double-autoionization (REDA) cross sections for sodiumlike selenium. The REDA process is treated as a three-step process (i.e., resonant electron capture followed by sequential double Auger emission). The necessary atomic data are computed using the multiconfiguration Dirac-Fock model. The effects of radiative damping can reduce the calculated REDA cross sections by a factor of 6, and the loss channels can further reduce these cross sections by a factor of 2. The REDA processes produce many strong narrow resonances and contribute 10-20% to the average total ionization cross sections.

In this paper, we present a detailed theoretical description for the coherent electron-impact excitation, the subsequent time evolution, and the cascading decay process of the singlet-triplet mixed $1s4f$ state of helium. The excitation amplitude and phase of each sublevel of this state are related to measurable coincidence intensities and polarizations of the emitted photons. It is found that the intensity and polarization of the emitted photons are time modulated due to the singlet and triplet mixing in the $1s4f$ state.

We have used the CRYRING heavy-ion storage ring at the Manne Siegbahn Laboratory in Stockholm to store and cool beams of ${\mathrm{H}}_2$${\mathrm{D}}^{+}$. These beams circulate with energies of MeV per amu and are merged with an electron beam traveling at nearly the same velocity. In this way, relative energies from a fraction of a meV to tens of an eV are achieved. When dissociative recombination (DR) takes place, the neutral fragments exit the ring and strike a detector, enabling the measurement of total DR cross sections. Using this technique, coupled with a ‘‘translucent barrier’’ in which some particles pass directly through holes and others pass through a thin solid and lose part of their energy before striking the detector, we are able to measure the total cross section for DR in ${\mathrm{H}}_3^{+}$ and the branching into the three-body (H+H+H) channel and the two-body (${\mathrm{H}}_2$+H) channel. Coupling this method with the use of energy absorbers, we are able to measure these quantities for ${\mathrm{H}}_2$${\mathrm{D}}^{+}$ and the separate two-body channels HD+H and ${\mathrm{H}}_2$+D. An isotope effect is observed favoring the formation of HD.

Recombination of ${\mathrm{Kr}}^{34+}$ ions in collisions with ${\mathrm{H}}_2$ has been investigated. The principal motivation for this work was to search for resonant electron-transfer double excitation (RT2E), a three-electron correlation mechanism that is expected to occur in an ion-atom collision when electron transfer is accompanied by the simultaneous (resonant) excitation of two electrons to form an intermediate excited state. As a benchmark for RT2E, measurements of resonant transfer excitation (RTE) were made first. In this work, Kr (mostly K-shell) x-ray emission associated with single-electron-capture events was detected. Events associated with RTE were readily observed, but no events associated with RT2E were observed to a sensitivity of about ${10}^{\mathrm{-}25}$ ${\mathrm{cm}}^2$. However, x rays associated with capture did occur at about twice the Kr K x-ray energy. The change in the x-ray energy of these latter events with beam energy indicates that radiative electron capture (REC) (inverse photoelectric effect) is the mechanism responsible. The observed cross sections for both the RTE and REC mechanisms are in reasonable agreement with theoretical calculations.

Retardation effects on the cross sections for double photoionization of He${(}^1$${\mathit{S}}_0$) targets are studied at photon energies above 1.0 keV. The dipole asymmetry parameter β is examined first and the importance of correlation in the final and initial states for this quantity is examined. The effects of retardation on the electron angular distributions are studied and the range of validity of a first-order retardation correction is evaluated. Finally, by using a simple model to calculate the total cross section, the contribution of retardation on the double ionization ${\mathrm{\sigma }}^{2+}$ is calculated and it is found that the ratio of double to single photoionization, including retardation, remains the same as when the dipole velocity approximation is employed.

The difference R(z) between the resolvents of the interacting and free Hamiltonians is inherently associated with (pair) particle correlations and can also be used for the evaluation of the second virial coefficient. This paper explores the operator properties of R(z) and the analytical continuation of its momentum matrix elements. Correlated-state wave functions are identified when making a pole expansion of the analytically continued matrix elements. These square-integrable wave functions have a one-to-one correspondence with resolvent poles, and as such are associated with resonance-bound-, and virtual-state momenta. Their properties and use in evaluating the second virial coefficient are discussed. Except for the bound states, these wave functions are not eigenvectors of the interacting Hamiltonian. The separable Yamaguchi potential is used to illustrate these properties.

In this paper we investigate the spectra of complex scaled two-dimensional Hamiltonians. More specifically, we compare the spectra of the two-dimensional Hamiltonians to those of underlying one-dimensional ones and find that phenomenologically the spectra also of nonseparable two-dimensional Hamiltonians can be understood as a direct sum of one-dimensional spectra. Moreover, certain parameters describing a string or string curve can be easily carried over to two dimensions. Furthermore, we address the connection to the emergence of coherent-dissipative structures and argue that no specific form of interaction is necessary for the latter, since in almost every case enough eigenvalues are available to fulfill the necessary quantization conditions.

The Xe 4d photoelectron satellites have been studied in the photon energy region 90–200 eV. The substantially improved energy resolution has enabled us to determine a wealth of satellites. A comparison of the satellite spectra at the intermediate excitation energies to the threshold-electron and the high-excitation-energy spectra shows the existence of several kinds of characteristic photon energy dependences of the Xe 4d satellite intensities. At the intermediate excitation energies, the satellites due to final states of both the 4${\mathit{d}}^{\mathrm{-}1}$5${\mathit{p}}^{\mathrm{-}1}$np and 4${\mathit{d}}^{\mathrm{-}1}$5${\mathit{p}}^{\mathrm{-}1}$md,ns (n≥6, m≥5) configurations occur.

This paper presents a distorted-wave generalization of the S-matrix version of the Kohn variational principle developed by Zhang, Chu, and Miller [J. Chem. Phys. 88, 10 (1988)]. For scattering in the presence of a long-range interaction, the large-r asymptotic solution to the Schrödinger equation is built into the Kohn variational principle order by order in an effort to accelerate the convergence of the short-range square integrable part of the basis-set expansion. The improvement in the rate of convergence is demonstrated by applying the method to a long-range model potential. Multichannel scattering is discussed.

Results of two different theoretical models for the electronic structure of ${\mathrm{C}}_{60}$ targets were used in order to predict the population dynamics of target and projectile levels, charge-state evolutions, projectile electron emission, and the projectile deflection function in soft collisions with 80 keV ${\mathrm{Ar}}^{8+}$ ions. The calculations were performed within a previously developed semiclassical overbarrier model for nondestructive, long-range charge exchange. The two target models, a local-density-approximation-type, spherical-shell approach and our self-consistent Dirac-Fock-Slater calculation, lead to charge states and deflection functions that agree at the 10% per cent level.

A superradiance theory is developed for two identical hydrogenic ions (four states each) in a microtrap, as in recent experiments. The ions oscillate (micromotion) due to the trap’s radio frequency (rf) electric quadrupole field. One signature of superradiance is a deviation of the two-ion average upper state decay rate γ¯ from the one-ion value γ. A master equation is derived, giving a fractional change in the upper state lifetime γ/γ¯-1=sinkR/2kR [${\mathit{J}}_0^2$(z)-2${\mathit{J}}_1^2$(z)+...]cosΦ, where k=2π/λ, λ is the emission wavelength, R is the ion-ion distance, ${\mathit{J}}_{\mathit{n}}$(z) is a Bessel function of integer order n, and z=ka, a is the ion amplitude of motion, and Φ is the two-ion relative phase due to the preparation. In the Lamb-Dicke regime, a<λ, ${\mathit{J}}_0^2$(z)≊1 and thus superradiance is not influenced significantly by ion motion. This damped sinusoid is diluted by the factor of 1/2 due to destructive interference. Superradiance vanishes in the absence of coherent preparation, e.g., with inversion, as indicated by the time evolution of the two-ion dipole correlation function. Fringes and a beat at the rf are predicted in forward scattering, the most elementary form of optical free induction decay.

We have studied the effect of laser intensity on the photoelectron angular distribution (PEAD) in (1 + $1^{\prime }$)-photon resonance-enhanced multiphoton ionization (REMPI) of the ${\mathrm{H}}_2$ molecule via $B{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}(v=4; j=1, 2)$ levels. It has been found that both the total and the vibrationally resolved angular distributions [for the formation of ${\mathrm{H}}_2^{+}$ ion in ground $X{}_{}{}^2{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}(v^{+}=0, 1)$ levels] vary with laser intensity. Intensity dependence of corresponding asymmetry parameters has also been studied. We have shown that this variation with laser intensity is caused by two effects: (i) the effect of interference of ionization channels via different vibrational levels of $B{}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}$ and $C{}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ states, and (ii) the effect of coupling between these levels (Raman-like transitions) via continuum. Furthermore, the nature of variation of the PEAD and of the corresponding asymmetry parameters with laser intensity have also been found to depend on the choice of different rotational levels as resonances. This difference in variation is due to the variation in strength of interference and the Raman-like couplings effective in these REMPI processes.

The time-dependent Schrödinger equation for ${\mathrm{H}}_2^{+}$ in a strong laser field is solved numerically for a model that uses the exact three-body Hamiltonian with one-dimensional nuclear motion restricted to the direction of the laser electric field. The influence of ionization on possible stabilization against dissociation is investigated. Unexpectedly high ionization rates from high vibrational states, exceeding those of neutral atomic hydrogen, are found. The ionization rates as functions of the internuclear distance R were also calculated for fixed nuclei, and these exhibit two strong maxima at large R, which explain the full dynamical results. A series of peaks seen in the calculated proton energy spectra can therefore be interpreted as occurring preferentially at (i) turning points of laser-induced vibrationally trapped states, and (ii) at the ionization maxima that occur at large internuclear distances of ${\mathrm{H}}_2^{+}$.

A theory of molecular photofragmentation is developed for coherent and incoherent light and for the case where dissociation lifetimes of field-induced resonances are shorter than pulse lengths. Using a mixed representation for the transition operator (a description of the initial state of the field with the coherent representation and of the final state with the number representation), we show how observables are modified at different field regimes by Poisson distributions. Applications to ${\mathrm{H}}_2^{+}$ and ${\mathrm{D}}_2^{+}$ are presented and discussed. They include line shapes and final distributions of kinetic energies for weak, intermediate, and strong fields. Results show that the interference of resonance states plays an important role even after averages over the field phases have been carried out to account for incoherent effects.