The effect of orbital relaxation, hole localization, and electron correlation on the transition energy, optical oscillator strength (OOS), and generalized oscillator strength (GOS) for the (1${\mathrm{\sigma }}_{\mathit{u}}$→1${\mathrm{\pi }}_{\mathit{g}}$,1${\mathrm{\sigma }}_{\mathit{g}}$→1${\mathrm{\pi }}_{\mathit{g}}$) process in nitrogen has been examined using the generalized multistructural (GMS) wave function. It is shown that the transition energy is largely affected by hole localization while for the GOS and OOS orbital relaxation effects dominate. The behavior of the GOS as a function of the transferred momentum is very well described by the GMS wave function, in the first Born approximation, for small and medium values of the transferred momentum ${\mathit{k}}^2$.

The 2p(3${\mathit{a}}_1$) to 6${\mathit{a}}_1$ and 3${\mathit{b}}_2$ photoexcitation resonance of ${\mathrm{H}}_2$S has been studied using synchrotron-radiation-excited photoelectron spectroscopy. The overall shape of the measured photoelectron spectrum of ${\mathrm{H}}_2$S is found to be in close resemblance with the normal Auger-electron spectrum of HCl. The spectrum is assumed to result mainly from the Auger decay in the excited HS molecule taking place after the molecular dissociation of the excited ${\mathrm{H}}_2$S molecule. The finding is compared with the results of multiconfiguration self-consistent-field calculations [A.N. de Brito and H. Agren, following paper, Phys. Rev. A 45, 7953 (1992)].

Ab initio studies of the 2p(3${\mathit{a}}_1$)-6${\mathit{a}}_1$ and 2p(3${\mathit{a}}_1$)-3${\mathit{b}}_2$ photoexcitation resonances of ${\mathrm{H}}_2$S have been carried out in order to support interpretations of recent photoelectron and autoionization spectra using synchrotron-radiation excitation. The full ${\mathrm{H}}_2$${\mathrm{S}}^{\mathrm{*}}$→S+${\mathrm{H}}_2$ and ${\mathrm{H}}_2$${\mathrm{S}}^{\mathrm{*}}$→S+H+H potential-energy surfaces as well as the resonant energies for autoionization (Auger) decay of core-excited states in the fragment HS species have been obtained by multiconfiguration self-consistent-field calculations. The results support an interpretation of the spectral fundings in terms of fast dissociation of the ${\mathrm{H}}_2$${\mathrm{S}}^{\mathrm{*}}$ 2${\mathit{p}}^{\mathrm{-}1}$6${\mathit{a}}_1^1$ and 2${\mathit{p}}^{\mathrm{-}1}$3${\mathit{b}}_2^1$ states to core-excited states in ${\mathrm{HS}}^{\mathrm{*}}$ followed by resonance Auger decay to ${\mathrm{HS}}^{+}$ valence single-hole states.

Observations of overlapping sequences of doubly excited resonances in the absorption spectrum of argon are reported at improved resolution, exhibiting more detail than earlier studies. Their appearance is compared with the doubly excited manifolds recently reported in the spectrum of He by Domke et al. [Phys. Rev. Lett. 66, 1306 (1991)] and by Zubek et al. [J. Phys. B 24, L342 (1991)]. The salient differences are pointed out and their origin is explained. The simplest case of overlapping channels has been accounted for by Lane’s theory [J. Phys. B 17, 2213 (1984)] for one broad intruder in a Rydberg series of autoionizing resonances. Numerous interesting examples of the q-reversal effect are uncovered.

Surface melting on clusters is investigated by a combination of analytic modeling and computer simulation. Homogeneous argonlike clusters bound by Lennard-Jones forces and Cu-like clusters bound by ‘‘embedded-atom’’ potentials are the systems considered. Molecular-dynamics calculations have been carried out for clusters with 40–147 atoms. Well below the bulk melting temperature, the surfaces become very soft, exhibiting well-defined diffusion constants even while the cores remain nearly rigid and solidlike. The simulations, particularly animations, of atomic motion reveal that the surface melting is associated not with amorphous, random surface structures in constant, irregular motion, but rather with large-amplitude, organized, collective motion of most of the surface atoms accompanied by a few detached atoms (‘‘floaters’’) and holes. At any time, a few of the surface atoms are out of the surface layer, leaving vacancies; these promoted particles wander diffusively, the holes also but less so; the floaters occasionally exchange with atoms in the surface layer. This result is the basis for an analytic, statistical model. The caloric curves, particularly the latent heats, together with the results from an analytical model, show that surface melting of clusters is a ‘‘phase change’’ different from the homogeneous melting of clusters.

We propose a simple stability condition of collisionally fused aggregates. The model provides a description of the fusion cross section for colliding nuclei, clusters, and liquid droplets at large energies.

The very high symmetry of icosahedral ${\mathrm{C}}_{60}$ suggests that as a first approximation an atom trapped inside ${\mathrm{C}}_{60}$ would be subject to a potential that is radially symmetric about the center. All-electron local-density-functional calculations of the total energy of a sodium ion as a function of radial displacement from the center along the fivefold axis of ${\mathrm{C}}_{60}$ serve to refine such a radial potential. In particular the calculations suggest studying potentials that have minima displaced from the center. An analytic functional form for a radial potential having a positive cusp at the origin is proposed, and the s-wave radial solutions of the corresponding Schrödinger equation are examined.

We report an observation of Δm=2 ground-state coherences in selective-reflection spectra at a glass–metal-vapor interface. The coherence was created by a linearly polarized laser beam tuned to a sub-Doppler resonance in the selective-reflection spectrum of the cesium ${\mathit{D}}_2$ line and detected through the magnetorotation of the plane of polarization of the reflected beam. The coherence effect appears as a dispersive shaped resonance of subnatural width. The dependence of the width on the intensity of the optical field shows a saturation behavior. For vanishing light intensity and cesium vapor pressure the linewidth is limited by transverse time-of-flight broadening, whereas its pressure dependence indicates an additional contribution from the finite longitudinal interaction time.

The theory of pulsed polarization spectroscopy in the case of a saturating pump pulse and an optically thick sample is presented, both with and without inhomogeneous broadening. It is found that the molecular anisotropy produced by pumping an R- or P-branch transition is maximized by using a pulse whose flip angle is near 2π for the M component with the largest Rabi frequency. Calculations with no or extreme inhomogeneous broadening differ insignificantly. Such a pump pulse produces an anisotropy (and thus polarization rotation of the probe beam) of the opposite sign of that produced by weak-field excitation. Pulse-propagation calculations obtained by numerically solving the coupled Maxwell-Bloch equations demonstrate that there exist ‘‘stable-area’’ pulses, much like for a two-level system. The lowest such stable pulse produces a flip angle of 2.21π for the M=0 level and produces close to the maximum polarization anisotropy. This pulse still weakly excites the sample, and thus lengthens as it propagates to conserve area. The effective absorption coefficient, however, is much less than that for weak pulses. It is expected that such pulses should provide an order of magnitude or more sensitivity for polarization spectroscopy than that obtained with nonsaturating pulses.

A phenomenon occurring in a near-resonant collision between two dissimilar atoms in the presence of a laser field is described. Owing to a collision-induced coherence, the adiabatic collisional dressed states are found to behave as superradiant and subradiant Dicke states, giving rise to a marked asymmetry in the far wings of the absorption line shape.

Ligh-induced drift of ethylene (${\mathrm{C}}_2$${\mathrm{H}}_4$) molecules upon rovibrational excitation is found to exhibit anomalous behavior as a function of the frequency detuning. This behavior, which shows a strong dependence on the rotational sublevels involved, has been investigated experimentally for ${\mathrm{C}}_2$${\mathrm{H}}_4$ immersed in Kr. For a qualitative explanation of these results a model is proposed that contains transport collision rates dependent not only on molecular vibration but also on molecular rotation.

A general equation for the interaction of atoms with many separated traveling waves is introduced, and the oscillating terms corresponding to the Ramsey fringes are found in the equation. The features of the multizone Ramsey resonance are quite comprehensible with the aid of the diagrammatic representation of the atomic beam and laser fields. The resonance condition and the narrowing effect of the fringe width are discussed. Using the intercombination line of a Ca atom, the multizone Ramsey resonances of up to ten separated fields were demonstrated. With the ten separated fields, the Ramsey fringes with a contrast of 130% to the saturation dip could be obtained at a relatively small excitation power.

We present calculations of ionization rates, angular distributions, and above-threshold-ionization spectra for a hydrogen atom in a strong, two-color laser field. The two lasers are first- and second-harmonic fields with the same intensity and a constant relative phase difference between them. At longer wavelengths (1064 nm) and higher intensities (>${10}^{13}$ W/${\mathrm{cm}}^2$), there is clear evidence in the phase dependence of the ionization rates that ionization takes place primarily through tunneling. Even though the total ionization rate in this regime depends only on the peak value of the time-dependent electric field, the angular distributions show additional phase-sensitive effects. In particular, there is a large forward-backward asymmetry in the emitted electron distributions that is not simply correlated with the maximum electric field. At shorter wavelengths and/or lower intensities, there is a transition to multiphoton ionization with interference between ionization paths from the two lasers evident in the total ionization rates. In all cases, we find that the total ponderomotive shift of the ionization limit is the sum of the shifts for the two individual fields.

The nonlinear two-color photodetachment of a negative ion is considered for two limits: rapid and adiabatic time dependence of the field. It is shown that the resulting ponderomotive shift depends strongly on an adiabatic parameter and can be observed only for the ‘‘rapid’’ limit. In the opposite ‘‘adiabatic’’ limit, the spectrum of detached electron energies can be represented by a ‘‘smooth’’ function without any ponderomotive shift. The results of the model are in qualitative agreement with recent experimental studies of the two-color photodetachment of ${\mathrm{Cl}}^{\mathrm{-}}$.

We describe the quantum dynamics of a two-atom Raman coupled model interacting with two quantized cavity electromagnetic fields as an extension of the previously discussed single-atom Raman coupled model [C. C. Gerry and J. H. Eberly, Phys. Rev. A 42, 6805 (1990)]. In particular we study the atomic-population dynamics and the dynamics of the photon statistics in the two cavity modes. We present evidence of cooperative effects.

We present a theoretical analysis of the intensity correlations between the spectral components of the resonance fluorescence triplet while allowing for detection time differences that are smaller than the inverse frequency width of the frequency filter. Explicit expressions are derived for the intensity correlation functions that are valid for all times. Furthermore, we present the results of measurements on these correlation functions for the ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$${\mathrm{\leftrightarrows }}^1$${\mathit{P}}_1$ resonance transition of natural barium. In general, the results confirm the theoretical predictions.

Time-dependent transition amplitudes between bound and continuum states coupled by an intense laser field are expressed in terms of their energy-dependent analogs maintaining the coherence effects through the intervention of complex molecular dressed states (field-induced resonances). A coupled-equation method combined with an artificial-channel technique, properly describing the preparation step, is used to calculate absorption line shapes and branching ratios of the Floquet states involved in the multiphoton absorption-emission processes. The formalism is applied to the photodissociation of ${\mathrm{H}}_2^{+}$(1s${\mathrm{\sigma }}_{\mathit{g}}$,v=0,j=1→2p${\mathrm{\sigma }}_{\mathit{u}}$). The intense-field behavior of laser-induced resonance Fano profiles are analyzed in relation with the dissociation line shapes. Strong mixings appear for intensities larger than ${10}^{13}$ W/${\mathrm{cm}}^2$ and the distribution of higher-energy peaks decreases with increasing intensity, due to stimulated emissions of the dissociating fragments.

We formulate a continuous-mode quantum theory of nonlinear loop mirrors. We exploit the ‘‘conservation of squeezing’’ property at a beam splitter to provide a particularly simple description of the output noise spectra from such devices. We also derive a general condition for which one may obtain a squeezed vacuum from one of the output ports of the loop mirror.

Photon entanglement can have a major effect in macroscopic systems containing large numbers of photons and atoms, where it can eliminate the interference between two high-intensity beams of light that would otherwise be expected. The nonclassical nature of these effects is related to the lack of classical chaos below some threshold of nonlinearity.

The Pegg-Barnett phase-operator formalism utilizes a finite basis set to represent operators of the harmonic oscillator; this enables the phase to be represented by a Hermitian operator, but rests on taking the dimensionality of the basis set to infinity for observable quantities. Simultaneously, in their approach Pegg and Barnett consider quantum states of a harmonic oscillator which are normalized in the Fock space, i.e., the dimensionality of the basis set in which the states of the harmonic oscillator are defined is supposed to be infinite, while the phase operator is defined in the finite-dimensional basis set. In this paper we address the problem of a consistent definition of a coherent state within a finite state basis. We employ displacement operators to define such coherent states and numerically evaluate observables as a function of the size of the basis set. We investigate phase properties of these coherent states. We find that if the dimensionality of the state space is much larger than the mean occupation number of the coherent states, then the results obtained in the finite-dimensional basis are applicable in the case of a ordinary quantum-mechanical harmonic oscillator. These coherent states are minimum uncertainty states with respect to quadrature operators (i.e., the position and momentum operators) and do not exhibit quadrature squeezing. A weakly excited (compared with the dimensionality of the state space) coherent state in finite-dimensional basis is not strictly speaking a minimum uncertainty state with respect to the number and phase operators. We give definitions of amplitude and phase squeezing and show that weakly excited coherent states can be amplitude squeezed. In the high-intensity limit (again compared with the dimensionality of the state space) these states exhibit phase squeezing.