Laser cavities are open systems, in that energy can leak to the outside via output coupling. The ‘‘normal modes’’ are therefore quasinormal modes, with eigenvalues that are complex and eigenfunctions that extend outside the cavity, such that any normalization integral is dominated by the region outside; in short, such systems are non-Hermitian. This paper addresses the question: How is the complex eigenvalue (i.e., the mode frequency) changed when the cavity is perturbed by a small change of dielectric constant? The usual time-independent perturbation theory fails because of non-Hermiticity. By generalizing the work of Zeldovich [Sov. Phys.—JETP 12, 542 (1961)] for scalar fields in one dimension, we express the change of frequency in terms of matrix elements involving the unperturbed eigenfunctions, so that the problem is reduced to quadrature. We then apply the formalism to shape perturbations of a dielectric microdroplet, and give analytic formulas for the frequency shifts of the morphology-dependent resonances. These results are, surprisingly, independent of the radial wave function, so that all integrals can be performed and explicit algebraic expressions are given for axially symmetric perturbations.

Thermal fluctuations lead to nonspherical distortions of liquid microdroplets with typical amplitude Δ∼0.1 nm. The 2l+1 modes of a multiplet are then split in frequency by a fractional amount of order Δ/a∼${10}^{\mathrm{-}5}$, where a is the droplet radius. The width of the multiplet in the frequency domain implies a proportionate reduction in the photon storage lifetime and can limit the effective Q value defined by the photon lifetime to ${\mathit{Q}}_0$∼(a/Δɛ)∼(${10}^5$/ɛ), where ɛ (0<ɛ<1) is process dependent and expresses the fraction of the modes of the multiplet effectively coupled in the process. The theory behind this scenario is presented.

The bound-state energy spectrum of the generalized exponential cosine-screened Coulomb potential is obtained by employing the hypervirial equations with the Hellman-Feynman theorem applied to the screening parameters λ and μ independently. The energy eigenvalues are obtained up to the fourth order of the screening parameters.

Three-body L=1 symmetrized hyperspherical harmonic functions, realizing irreducible representations of the permutation group of three particles, are expressed in terms of Wigner D functions. Matrix elements of arbitrary two-body potentials between these hyperspherical states, including their velocity-dependent parts, are calculated analytically and expressed through the sum of products of the Wigner 3-j symbols and explicitly written functions of the radial variable.

The partial photoionization cross sections (1s-2p${)}^1$P→(1s-ks${)}^1$S,(2s-ks${)}^1$S, (2p-kp${)}^1$S are computed in the vicinity of the (3,3b) resonance. The multichannel version of configuration-interaction in the continuum is used with a basis set of hyperspherical coordinates developed earlier [Cordes and Altick, Phys. Rev. A 40, 20 (1989)]. Qualitative agreement is found with the one existing calculation—the 2p cross section is favored and the 1s cross section is very small. The resonance parameters are found by fitting to the calculated results.

Measurements are reported of the cross sections for the capture and loss of an electron by 15–130-keV bromine atoms during single collisions with rare-gas atoms. The bromine data fit well with similar data for the other halogens. The electron-capture cross sections peak at collision velocities that agree with the simple two-state model. Competition between target ionization and beam ionization, which is determined by relative ionization potentials, seems to account for the observed differences in the electron-loss cross sections.

Several distorted-wave approximations that have recently been applied to relativistic atomic collisions are analyzed. It is shown analytically and numerically that the symmetric eikonal approximation contains unphysical spin-flip contributions. Similar arguments hold for the continuum-distorted-wave approximation. It is concluded that an improvement in describing the distortion in both entrance and exit channels does not necessarily lead to better predictions for experimental data.

White-light-induced drift is a process whereby white light can separate species in a gas mixture. The effectiveness of the process is determined by the competition between the increasing velocity selectivity of excitation and the loss of excitation as the propagating light is absorbed and reemitted. We show that the divergence of real light sources affects this competition and find conditions relating source dimensions and densities for which the white-light-induced drift effect should be strong. Geometries with cylindrical and spherical symmetries are considered.

The optical forces exerted on a slowly moving two-level atom in a standing wave are profoundly modified when the normal decay rates of the atom are altered. The theory presented in this paper shows that the stimulated emission force becomes dominant at a very low laser intensity. Furthermore, it can change sign to provide an additional cooling force instead of a heating force at laser detunings below the atomic resonance. These phenomena can lead to equilibrium temperatures below the Doppler limit.

A classical mechanism is proposed for the many-photon ionization of Rydberg atoms in a quasimonochromatic oscillating electric field with an amplitude smaller than the stochasticity threshold. The mechanism is based on the dynamic autoresonance between the slowly varying frequency of the oscillating field and the Keplerian frequency (or one of its harmonics) of atomic electrons. It is shown that a large fraction of an initial ensemble of atoms can be efficiently excited by this coherent mechanism and then ionized via the stochastic instability.

We have reexamined the two-level atom with decay from the excited state out of the system and have noticed a rather surprising result. The intuitive exponential decay of the ground state at the damping rate of the excited state is actually reduced substantially when this damping rate is much greater than the Rabi frequency between the two states.

A method to evaluate matrix elements of anharmonic unitary operators when the usual expansion techniques lead to nonconvergent series is suggested. This problem is particularly relevant in nonlinear quantum optics, when multiphoton processes are analyzed, and two examples of applications in this case are given.

We have computed correlated wave functions for a special class of ${}_{}{}^4{}_{}{}^{}$${\mathit{S}}^0$ triply excited states of Li that exhibit strong localization at the corners of an equilateral triangle as the three-electron ionization threshold is approached. Starting from this finding, we have derived the corresponding spectral formula -${\mathit{E}}_{\mathit{n}}$=Cn(n-1)/${\mathit{r}}_{\mathit{n}}^2$, with ${\mathit{r}}_{\mathit{n}}$∼${\mathit{n}}^2$ for n≥3. C is dependent on the solid angle of the pyramid only for low n, acquiring a ${\mathrm{\theta }}_{\mathit{n}}$-independent value of 3.36 for n≥12, where ${\mathrm{\theta }}_{\mathit{n}}$ is the angle formed by the position vectors of any pair of electrons. The above simple formula characterizes other many-electron ionization ladders as well and, for large n, scales like the Bohr formula for the hydrogenic spectra.

The QED contributions to the energy intervals 3${\mathit{s}}_{1/2}$-3${\mathit{p}}_{1/2}$, 3${\mathit{p}}_{1/2}$-3${\mathit{p}}_{3/2}$, and 3${\mathit{s}}_{1/2}$-3${\mathit{p}}_{3/2}$ in highly charged Na-like ions were determined by comparing the transition energies derived from the observations with the relativistic many-body perturbation-theory calculations of Johnson, Blundell, and Sapirstein [Phys. Rev. A 38, 2699 (1988)] that do not include QED effects. The determined QED contributions differ considerably from the one-electron QED contributions and are in better agreement with screened-nucleus QED contributions. Predicted transition energies and wavelengths are presented for the Na-like ions with atomic numbers up to Z=92.

Energies of the n=3 to n=2 transitions in Li-like and Be-like xenon were measured on the Grand Accelerateur National d’Ions Lourds at Caen, France, using the Lawrence Livermore National Laboratory high-resolution dual-crystal spectrometer. These measurements, on the highest available Z atom, cover the 5.0–5.5-keV energy region with precisions on the order of 150 ppm. Original theoretical results based on the multiconfiguration Dirac-Fock and relativistic many-body perturbation theory methods are shown for comparison.

Analytic solutions describing soliton interaction in bimodal optical fibers are obtained in the case of equal cross- and self-phase modulation effects. Conditions are established for integral pulse-shape switching and a periodic linearlike behavior of the energy coupling is emphasized.

Energy-transfer collisions between two 6 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ laser-excited indium atoms leading either to ionization of one of the colliding atoms or to population of its ${\mathit{n}}^2$P Rydberg states, have been studied. Atomic lines corresponding to the 5s5${\mathit{p}}^2$ ${}_{}{}^2{}_{}{}^{}$D→5${\mathit{s}}^2$6p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2,3/2}$ interconfiguration transitions have been recorded as evidence of the energy-pooling ionization via an autoionizing state. Atomic lines corresponding to the ${\mathit{n}}^2$P→6 ${}_{}{}^2{}_{}{}^{}$S transitions for n=9, 10, and 11 as well as to the 5 ${}_{}{}^2{}_{}{}^{}$D→5 ${}_{}{}^2{}_{}{}^{}$P transitions have proven that the population of highly excited indium atomic states is through the energy-poolimg collisions.

We describe a method for generating superpositions of classically distinguishable quantum states in an optical-frequency mode. The method uses the optical back-action-evading apparatus demonstrated recently by La Porta, Slusher, and Yurke [Phys. Rev. Lett. 62, 28 (1989)].