The far-infrared pure rotational absorption spectra of gaseous HD and HD-X (X=${\mathrm{H}}_2$, He, Ne, Ar, Kr, and ${\mathrm{N}}_2$) were measured at 195 and 296 K. Values of the allowed dipole-moment matrix elements, the absolute frequencies, the spectral-line-shape parameters (broadening and frequency-shift coefficients), and the interference parameters, for the first four rotational lines, were deduced from the spectra. Theoretical calculations of the interference parameter a and the linewidths were performed, based, respectively, on the theory developed by Herman, Tipping, and Poll [Phys. Rev. A 20, 2006 (1979)] and a semiclassical theory developed by Robert and Bonamy [D. Robert and J. Bonamy, J. Phys. 40, 923 (1979)]. The induced dipole moments for HD-Ne and HD-${\mathrm{N}}_2$ were estimated. The analysis includes the present experimental results, together with previous determinations at 77 and 296 K [L. Ulivi, Z. Lu, and G. C. Tabisz, Phys. Rev. A 40, 642 (1989); P. Drakopoulos and G. C. Tabisz, ibid. $bold 36 —, 5556 (1987)]. The behavior of the temperature dependence of the collisional interference in the pure rotational band for HD and its mixtures is confirmed to be more complicated than predictions of the intracollisional theory.

A very weak perturbation around the 6${\mathit{s}}^2$52d ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ high Rydberg series of Lu has been studied. A line-intensity distribution of the 6${\mathit{s}}^2$nd ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{3/2}$ optical transitions exhibits a well-developed Beutler-Fano interference profile with a significant decrease of the radiative lifetime by the configuration interaction. An accurate energy-level structure of the perturbed series is determined by measuring microwave two-photon transitions among the 6${\mathit{s}}^2$${\mathit{nd}}^2$${\mathit{D}}_{3/2}$ series. The optical line-intensity profiles are analyzed directly by using the eigenfunctions of the perturbed Rydberg series derived from the microwave data. The results are compared with those obtained by the configuration-interaction theory developed by Fano [Phys. Rev. 124, 1866 (1961)].

The photoabsorption spectrum of the ${\mathrm{C}}_{60}$ molecule and those of Xe and Ba atoms inside the ${\mathrm{C}}_{60}$ molecule are calculated using a jelliumlike model for the confining cage. The dynamic electronic response to an external electric field is obtained through time-dependent density-functional theory. The photoabsorption cross section for ${\mathrm{C}}_{60}$ shows strong collective resonances corresponding to plasmonlike excitations. The resonant 4d photoemission of the free atom is suppressed by the carbon cage, resulting in a weakly oscillating spectrum.

We have measured the Stark effect on absorption spectra of highly excited Rydberg levels near the ionization threshold of Kr. The spectra were recorded against the continuum from a synchrotron-radiation (SR) light source using a high-resolution vacuum-ultraviolet spectrometer and applying electric fields that were parallel or perpendicular to the polarization of the SR. The main features are hydrogenlike Stark manifolds that arise from mixing of high-orbital-angular-momentum states with negligible quantum defects, and a number of lines belonging to the np and nd (J=2) series, the appearance of the latter being strongly dependent on the direction of the external field with respect to the SR polarization. Based on the diagonalization of the Stark energy matrix including more than 450 configurations, the np and nd (J=2) series are correlated with the zero-field states. The term values of the zero-field states are obtained by extrapolation of observed energy values to the zero electric field. A multichannel-quantum-defect-theory analysis has been carried out for the J=1 even-parity energy levels to derive the eigenchannel quantum defects and transformation matrix elements.

Using the first Born approximation and the A⋅p gauge, the Ps formation in a laser-assisted positron-helium collision is studied. In reducing the S matrix, we have developed an exponential-parameter integration technique which causes the laser-modified scattering amplitude to be reduced from nine-dimensional infinite integrals to one-dimensional finite ones without a singularity. Our results indicate that when the laser electric vector is parallel to the incident momentum, the integral cross section for Ps formation decreases, but when the electric vector is perpendicular to the incident momentum, the integral cross section increases. These results are quite different from that for an atomic-hydrogen target.

The stability of Rydberg states of triatomic hydrogen in an external electric field is investigated for principal quantum numbers ranging from n=50 to 110. When excited in the presence of an electric field, the states with an electric-field projection of the total angular momentum ${\mathit{M}}_{\mathit{N}}$=0 are found to be rapidly field ionized above the classical saddle-point energy. However, a significant proportion of the states for ${\mathit{M}}_{\mathit{N}}$=1 is observed to be stable above this threshold, with lifetimes exceeding ${10}^{\mathrm{-}6}$ s. A perturbative treatment of the molecular Stark effect indicates that different behavior of ${\mathit{M}}_{\mathit{N}}$=0 and 1 Stark states is a consequence of the stronger coupling among the states of the ${\mathit{M}}_{\mathit{N}}$=0 manifold. Following excitation under near-zero field conditions, a predominantly diabatic evolution of the Stark manifold of ${\mathrm{H}}_3$ is observed, for slew rates of a few times ${10}^7$ (V/cm)/s. This behavior results from the nondefinition of the projection of the total angular momentum of the molecule excited at near-zero field, where a broad manifold of l states is prepared within the bandwidth of the laser. For a comparison, similar experiments are carried out in atomic hydrogen and helium.

We extend our previous discussion of the resonant nonlinear Faraday effect in terms of the precession of an induced quadrupole moment to the realistic experimental situations where depopulation, repopulation, and hyperfine optical pumping contribute. We derive, to lowest order in the light intensity, an analytical expression for the magnetorotation angle near an electric-dipole resonance in an atomic medium of arbitrary spins. We find excellent quantitative agreement with absolute measurements performed on the Cs ${\mathit{D}}_2$ line.

We investigate the effect of the Stark shifts on the atomic-inversion dynamics when a multilevel atom interacts with a two-mode quantized radiation field. We utilize Hamiltonians obtained by adiabatically eliminating all but two of the atomic levels and including terms that give rise to the Stark shifts for a Raman-type process as well as for equal- and unequal-frequency two-photon absorption. We find that the Stark effect is relatively unimportant for Raman scattering but should not be ignored in equal- or unequal-frequency absorption. In equal-frequency absorption, the atomic dynamics can be made exactly periodic through an appropriate choice of detuning.

We study the quantum dynamics of the k-photon down-conversion process. We concentrate our attention on the entanglement between the pump and the signal modes. We show that the degree of the entanglement between the pump and the signal depends on the initial statistics of the light field in both modes. Moreover, the higher the nonlinear process the stronger the entanglement is. The entanglement between the modes is related to the marginal entropy in the pump (signal) mode; i.e., the larger the entanglement, the larger the entropy is. In the quantum k-photon down-conversion the signal mode at t>0 is not generally in a pure state which restricts applicability of the parametric approximation.

A self-consistent generalization of the Maxwell-Bloch formulation is presented here for a nonlinear system of saturable two-level atoms interacting with the electromagnetic field, under the condition that the atomic density is such that there are, on the average, many atoms within a cubic resonance wavelength such that near–dipole-dipole interactions, which lead to local-field corrections, cannot be neglected.

We analyze qualitative aspects of the propagation of multiple discrete frequency waves in the normal dispersion regime in an optical fiber, analytically and numerically, using the nonlinear Schrödinger equation and associated scattering theory. Results illustrating phase-locked, periodic, and quasiperiodic propagation are presented in the context of multiphase dark solitary wave trains.

The global dynamical behavior of a model describing the formation of one-dimensional transverse patterns in a laser has been investigated. Solutions of the equation for the complex electric field are compared to those of a reduced phase equation, the laser Kuramoto-Sivashinsky equation, which should be a good approximation for small detuning. The numerical results show strong evidence of the leading role of the dynamics of the phase of the field in the formation of transverse patterns in a laser with small detuning.

Conditions for noise minimization in laser systems are studied and formulated as a principle. It involves the correlation of the internal degrees of freedom of the system and the relative rates of transitions between them. The minimization principle proposed is appropriate for a variety of lasing schemes including nonunidirectional pump processes. It allows one to interpret a wide scope of experimental results. It is expected to be serviceable in further exploring of the squeezing phenomenon.

A theory of radiation pressure and its effects on the gross motion of an aggregrate of charges is established within a canonical framework. The theory indicates the existence of radiation-pressure effects arising from an additional interaction–the Röntgen interaction–whose origin is the classical Röntgen current. A careful development of the formalism reveals the importance of distinguishing between the canonical and mechanical momenta of the gross motion of the aggregate and of incorporating the Röntgen-type interaction terms when calculating the gross-motion dynamics. A form of the Röntgen interaction is present even in the dipole approximation: an effect which has previously been ignored but which is necessary to ensure gauge invariance of the radiation-induced mechanical force. Explicit calculations of the rates of change of canonical and mechanical momenta are presented for a general atomic dipole, with specializations to a two-level atom, revealing the presence of velocity-dependent terms in a natural way. The formalism is consistent with the Minkowski form of the classical momentum density of an electromagnetic field.

A pair of correlated photons generated from parametric down-conversion was sent to two independent Michelson interferometers. Second-order interference was studied by means of a coincidence measurement between the outputs of two interferometers. The reported experiment and analysis studied this second-order interference phenomenon from the point of view of the Einstein-Podolsky-Rosen (EPR) paradox. The experiment was done in two steps. The first step of the experiment used 50-psec and 3-nsec coincidence time windows simultaneously. The 50-psec window was able to distinguish a 1.5-cm optical path difference in the interferometers. The interference visibility was measured to be 38% and 21% for the 50-psec time window and 22% and 7% for the 3-nsec time window, when the optical path differences of the interferometers were 2 cm and 4 cm, respectively. By comparing the visibilities between these two windows, the experiment showed the nonclassical effect which resulted from an EPR state. The second step of the experiment used a 20-psec coincidence time window, which was able to distinguish a 6-mm optical path difference in the interferometers. The interference visibilities were measured to be 59% for an optical path difference of 7 mm. This is an observation of visibility greater than 50% for a two-interferometer EPR experiment which demonstrates nonclassical correlation of space-time variables.

A theory of four-wave mixing is developed using an amplitude approach in the Schrödinger picture. It is shown that the four-wave mixing signal produced when three fields are incident on an ensemble of two-level atoms can be viewed as arising from constructive interference of the signals emitted at the various atomic sites. Fourteen distinct amplitudes contribute to the four-wave mixing signal. Some of these amplitudes involve final states in which one or two atoms are excited, and some involve final states in which photons are emitted into modes other than a phase-matched mode of interest. The amplitude approach has the advantage of clearly showing the atom-field correlations. It helps one to understand the underlying physical mechanisms responsible for creation of the four-wave mixing signal, as well as features in the line shape resulting from collisions and effects relating to atomic recoil on the absorption or emission of radiation. These so-called collision- and recoil-induced resonances are calculated explicitly using the amplitude approach, and an interpretation of the results in terms of the interference between different amplitudes is given. Our results, calculated for a finite interaction time between the atoms and the fields, are compared with analogous calculations in which an effective ground-state decay time is assumed. It is shown that the results differ if the atom-field interaction time is not the longest relevant time parameter in the problem.

Within the framework of the cavity QED model we consider a cavity-field-assisted dissipation process via a Raman-type coupling of the atom-cavity system (under off-resonance conditions) to a reservoir. We show that at higher applied field strength the Mollow triplet gets significantly suppressed due to this cavity-field-dependent atomic relaxation.

A method to generate low-energy and high-multipolarity nuclear transitions through a laser-assisted, resonant, inverse electronic-bridge process is presented. The cross section of the suggested nuclear-excitation process is calculated in a simple model. The yield is compared with the yields of ordinary γ-ray absorption and Coulomb excitation. The excitation of the ${}_{}{}^{235\mathit{m}}{}_{}{}^{}\mathrm{U}$ isomeric state of energy 73.5 eV by an E3 transition is traced numerically. The excitation of an electron from the ${\mathit{O}}_4$ (5${\mathit{d}}_{3/2}$) electronic shell of binding energy 105 eV can be followed by a ${\mathit{P}}_3$ (6${\mathit{p}}_{3/2}$)→${\mathit{O}}_4$ transition, which can be tuned by the laser to resonance with the nuclear transition. In case of resonance, the electronic excitation energy is transferred to the nucleus with a high efficiency.

A spectrum based on the correspondence principle leads directly to the fundamental dipole moment correlation function for light scattering. We consider the distinction between mean-field scattering (coherent scattering) and scattering by fluctuations (incoherent scattering). We calculate the corresponding spectra from ab initio atomic wave functions associated with (i) a two-level system and (ii) an atom in two situations: while undergoing multiphoton ionization, and during electron-atom scattering.

A model radiating system consisting of two atoms in close proximity is analyzed. This system demonstrates the influence of spatial coherence on the spectrum of the radiation field. Explicit expressions for the degree of coherence, the source spectrum, and the spectrum of the radiation field are derived. The results are discussed in terms of Wolf’s work [Phys. Rev. Lett. 56, 1370 (1986)] on this effect, which can be considered in terms of a multiple-atom analog of the effects of radiation reaction on a single atom, i.e., spontaneous decay and the Lamb shift.