I consider the interaction of a superposition of mesoscopic coherent states and its approach to a mixed state as a result of a suitably controlled environment. I show how the presence of a gain medium in a cavity can lead to diagonalization in coherent state basis in contrast to the standard model of decoherence. I further show how the new model of decoherence can lead to the generation of s-ordered quasidistributions.

Using the ideas of Lieb and Oxford [Int. J. Quantum Chem. $19,$ 427 (1981)], we show that the exchange-correlation energy, and indirect part of the Coulomb energy, are bounded from below by $-1.6358\int {\rho }^{4/3}\mathrm{}\left(x\right)dx,$ where $\rho \mathrm{}\left(x\right)\mathrm{}$ is the single-particle density.

By removing the structural x-ray-absorption fine-structure signal from K-edge absorption spectra of $4p$ elements from Ga to Br in chalcogenide glasses and other samples, the pure atomic absorption is recovered. Its main components are shake-up and shake-off channels of the $\left[1s3p\right]$ and $\left[1s3d\right]$ group. The $\left[1s3p\right]4p$ shake-up channel is observed to close off along the series as the $4p$ shell is progressively filled.

If electrons had an electric dipole moment (EDM) they would induce EDM’s of atoms. The ratio of the atomic EDM to the electron EDM for a particular atom is called the enhancement factor R. We calculate the enhancement factor for the francium and gold atoms, with the results $910\pm \sim 5\%$ for Fr and $260\pm \sim 15\%$ for Au. The large values of these enhancement factors make these atoms attractive for electron EDM measurements, and hence the search for time-reversal invariance violation.

The paper reports on calculations of the flow properties in low-temperature He free-jet expansions by means of the Lennard-Jones potential and the recently appearing He-He van der Waals potential based on perturbation theory. The calculated speed ratios are compared with both the values we measured in the 20–80 K source temperature range and data which were already reported. The comparison points out a better agreement of our experimental results with the speed ratio values obtained by the recent potential whenever the parallel temperature of the beam decreases below ${10}^{-2}\hspace{0.5em}\mathrm{K}.$ In the same temperature range the light He-cluster size is analyzed.

We present results of calculations of cross sections for Rayleigh (elastic) and Raman (inelastic) scattering of light from helium for wavelengths down to almost the threshold for double ionization. Our calculations were performed using a complex basis set expressed in perimetric coordinates.

In a recent theoretical paper Gorczyca et al. [Phys. Rev. A 56, 4742 (1997)] predicted interference between radiative and dielectronic recombination to show up in the total recombination rate coefficient of Ar-like ${\mathrm{Sc}}^{3+}{(3s}^2{3p}^6)$ with the signature of interference being an asymmetric line profile of the ${3s}^2{3p}^5{3d}^2{}^{2}F$ dielectronic recombination resonance. Due to limited counting statistics our experimental result obtained at the Test Storage Ring at the Heidelberg Max-Planck-Institut for Nuclear Physics does not permit a detailed line-shape analysis to be carried out. However, differences between theory and experiment remain to be explained.

The secondary-electron emission (SEE) yield has been measured at low-energy proton impact for clean Cu metal and nonmetallic targets. The observed SEE for Cu below 3 keV shows the apparent thresholdlike behavior and decreases roughly as $v_p^4$ when the proton velocity ${(v}_p)$ decreases; meanwhile, there seems to be no such behavior in nonmetallic targets whose SEE decreases as $v_p^1.$ The SEE for these two different types of targets is also compared with the electronic stopping powers.

The stability of polygonal patterns composed of a set of N point charges in a potential well is studied. It is shown that a right polygon is stable for $N<~5.$ A right polygon with additional charge in its center is stable for $N=5,\dots ,9.\mathrm{}$

We have observed periodic structures of atoms when a bichromatic laser combination, with frequencies near the $D1\mathrm{}$ and $D2\mathrm{}$ lines, is used to produce a magneto-optical trap of sodium atoms. These periodic spatial distributions could not be explained on the basis of rectified force. Our interpretation relies on a possible spatial modulation of the damping and spring constants, resulting from the interference between the two atomic transitions.

The possibility of subjecting molecules to intense electric fields of ultrashort duration using fast beams of highly charged ions is explored. The morphology of dissociative ionization of ${\mathrm{H}}_2\mathrm{O}$ in intense fields of picosecond and attosecond duration is found to be significantly different. Such fields, produced in laser-${\mathrm{H}}_2\mathrm{O}$ interactions and in collisions with 50–100 MeV ${\mathrm{Si}}^{3+,8+}$ ions, are of approximately the same magnitude but have different directional properties.

An ideal gas trapped in a generic power-law potential in any dimensional space is studied. The density of states of this system is derived. Both Fermi and Bose gases are discussed, and their properties are derived straightforwardly from the density of states.

For confined systems of identical particles, either bosons or fermions, we argue that the parabolic nature of the confinement potential is a prerequisite for the nondissipative character of the center-of-mass response to a uniform probe. For an excitation in a parabolic confining potential, the half-width of the density response function depends nevertheless quantitatively on properties of the internal degrees of freedom, as is illustrated here for an ideal confined gas of identical particles with harmonic interparticle interactions.

Sum-frequency generation with electromagnetically induced transparency (EIT) has been examined in a hydrogen discharge. EIT is demonstrated in the $3p-1s$ and $4p-1s$ transitions at 102.6 and 97.3 nm, respectively, with increased efficiency of generation arising from higher H-atom density, although competing effects due to the presence of ${\mathrm{H}}_2$ distort the line shapes of these spectra. Only for shorter-wavelength transitions (such as $7p-1s$ and $8p-1s)$ that show very large shifts and distortions is there a decrease in intensity in comparison with generated radiation from H atoms external to the discharge.

We consider second-harmonic generation in a plane nonlinear antiwaveguide with an isotropic core and cladding and a reverse (relative to waveguide) distribution of the linear refractive index. The structure may support a light beam near the core in a certain interval of optical power. The cross section of the beam in such stability regions is very small, of the order of the light wavelength, whereas the optical density is very high, of the order of a self-focusing one. The second-harmonic generation in the structure is stimulated by the high density and applying a constant electric field. The waveguide for the second harmonic is produced by the powerful fundamental and depends on the form of its eigenmodes. We show that in parametric regions corresponding to the stability regions a continuum of second-harmonic eigenmodes at an invariable propagation constant exists and find necessary conditions for the phase matching.

The correct wave function for the problem of a harmonic oscillator of time-dependent mass and frequency is obtained following the same approach used in the paper of Dantas et al. [Phys. Rev. A 45, 1320 (1992)].

In a recent paper Croxson [Phys. Rev. A $49,$ 588 (1994)] reported dissipation-induced transition probabilities from the ground state to higher states for a quantum harmonic oscillator. A partial approximate result was given for the oscillator to remain in its ground state, while for higher transitions the situation became complicated. Our approach provides an exact simple form closed expression for any order of transition. In addition, we supply the evolution of the work done by the system on its environment.

The very successful convergent close-coupling (CCC) method [I. Bray and D. V. Fursa, Phys. Rev. A $54,$ 2991 (1996)] for calculating cross sections for the ionization of atoms by electron impact is examined for the case of hydrogen ionization. The theory is recast in an operator theoretic form, allowing explicit formulas to be given for the solutions of the truncated CCC equations. The limit to which these solutions tend as ever more terms are included in the truncation is shown to be the exact transition operator for the system. This analytical result is not consistent with the numerical CCC results, which yield amplitudes that violate the symmetrization principle of quantum mechanics and which depend for success on treating the electrons as classically, rather than quantally, indistinguishable particles. This new result is also inconsistent with the “step function hypothesis” [I. Bray, Phys. Rev. Lett. $78,$ 4721 (1997)] that was introduced to restore the consistency of the CCC calculations with the symmetrization postulate. In addition, the CCC method does not include long-range effects of the Coulomb interaction that are expected to be important in certain kinematic configurations at low energies. Consequently, it is not justified yet to interpret the numerical successes of the CCC method as confirmation of a fundamental dynamical understanding of electron-induced atomic ionization.

Bencze and Chandler, in the preceding paper, are highly critical of the convergent close-coupling method as applied to the calculation of differential ionization processes. We argue that the criticism is unwarranted and arises from a misunderstanding of the close-coupling theory.

Recently, Sharma et al. [Phys. Rev. A 56, 694 (1997)] proposed a q-deformed oscillator trap model to investigate the time evolution of a single two-level ion in an anharmonic oscillator trap, with the anharmonicity being parametrized by the deformation parameter q. The model is obtained by simply replacing the usual bosonic creation and annihilation operators in the harmonic trap model by their q-deformed counterparts. In the present paper we have reexamined the validity of the q-deformed oscillator trap model, and our analysis indicates that such a naive generalization actually does not exist. This is accomplished by demonstrating that the eigenstates of this q-deformed oscillator trap model are not normalizable.