It is pointed out that the kernel of the inverse response function $\delta v(\mathbf{r},t)/\delta \rho \left({\mathbf{r}}^{\prime }{,t}^{\prime }\right),$ whose existence is guaranteed by the Runge-Gross theorem, cannot vanish for $t<\mathrm{}{t}^{\prime }.$ As such the kernel $f_{\mathrm{xc}},$ which is the functional derivative of the time-dependent exchange-correlation potential with respect to the density, can be symmetric with respect to an interchange between t and $t^{\prime }$ without violating causality. Thus there is no conflict between the symmetry requirement of this kernel and causality in time-dependent density-functional theory.

Both Hartree-Fock and density-functional theories have been used to investigate the possible existence of the plutonium dimer. Energy considerations indicate positive binding, while molecular orbital analyses indicate that the f electrons are clearly localized and the bonding between the two Pu atoms is rather weak.

The differential cross section (DCS), Stokes parameters, and the STU parameters for electron scattering from a cesium atom are calculated by two kinds of distorted-wave methods, each including different relativistic effects. Comparing our results with those from the relativistic distorted-wave method of the Toronto group, we find that, at an incident electron energy of 50 eV, the direct relativistic effect is more important than the indirect relativistic effect for cesium. Further the direct relativistic effect leads to the invalidity of the “fine-structure approximation” for the case of cesium. We also calculated the DCS and the Stokes parameters for an electron-gold collision at incident electron energy of 50 eV. Unlike the cesium case, the indirect relativistic effect is more important for gold.

The results of accurate hyperspherical calculations of the reaction $d\mu {(n}_i)+\overrightarrow{t}t\mu {(n}_f)+d$ between states of the $n_i{=n}_f=1\mathrm{}$ and $n_i{=n}_f=2\mathrm{}$ manifolds for zero total angular momentum of the collision system are reported. A parametrization of the threshold behavior of the ground-to-ground–state muon transfer cross section in the spirit of the effective range theory is discussed.

We have investigated the generalized oscillator strength (GOS) for a transition of the type $n\overrightarrow{p}\mathrm{}(n+1\mathrm{})s,$ where n is the principal quantum number of the outermost filled shell of the atomic ground state, using the random-phase approximation with exchange. We find that the influence of correlation and exchange effects on the position of the characteristic minimum in the GOS of $\mathrm{Ar}\mathrm{}(n=3\mathrm{})\mathrm{}$ is insignificant. Also, our first Born approximation predicts the position of the minimum accurately provided that accurate target wave functions are employed. Our results agree excellently with measurements and are expected to be applicable equally to the corresponding subshells of $\mathrm{Ne}\mathrm{}(n=2\mathrm{}),$ $\mathrm{Kr}\mathrm{}(n=4\mathrm{}),$ and $\mathrm{Xe}\mathrm{}(n=5\mathrm{}).$

Using the electron-hydrogen scattering Temkin-Poet model we investigate the behavior of the cross sections for excitation of all of the states used in the convergent close-coupling (CCC) formalism. In the triplet channel, it is found that the cross section for exciting the positive-energy states is approximately zero near-threshold and remains so until a further energy, equal to the energy of the state, is added to the system. This is consistent with the step-function hypothesis [Bray, Phys. Rev. Lett. 78, 4721 (1997)] and inconsistent with the expectations of Bencze and Chandler [Phys. Rev. A 59, 3129 (1999)]. Furthermore, we compare the results of the CCC-calculated triplet and singlet single differential cross sections with the recent benchmark results of Baertschy et al. [Phys. Rev. A 60 13, (1999)], and find consistent agreement.

We report on the observation of stabilization of laser-excited rubidium Stark atoms above the classical ionization limit with the help of short (9 ns) unipolar electric-field pulses. Dipping the field down from 40 to 0 V/cm and back to 40 V/cm, 30 ns after photoexcitation into the continuum, yields a large bound-state population. The results suggest an additional mechanism of the recombination as seen in the THz experiment by Bensky, Campbell, and Jones, [Phys. Rev. Lett. 81, 3112 (1998)].

It is shown that the exchange of an odd number of photons in laser-assisted charge-particle scattering is forbidden when the laser polarization is perpendicular to the scattering plane. It is proposed that this selection rule may be used to test the satisfaction of single-collision conditions under critical geometrical conditions where experimental count rates tend to become prohibitively small.

We use a diffusion Monte Carlo method to calculate the lowest-energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low density the energy per particle follows a universal behavior fixed by the gas parameter ${\mathrm{na}}^3.$ In the regime of densities typical for experiments in trapped Bose-condensed gases, the corrections to the mean-field energy greatly exceed the differences due to the details of the potential.

We show that the condensate occupation of a superfluid Bose liquid quite generally exhibits anomalously large fluctuations at finite temperatures $T\ne 0.$ In three dimensions, the variance $〈\delta N_0^2〉$ of the number $N_0$ of particles in the condensate scales nonlinearly with volume V like $T^2{V}^{4/3}$ at low T, generalizing the result obtained by Giorgini, Pitaevskii, and Stringari for a weakly interacting Bose gas. In two dimensions there is only a quasicondensate whose fluctuations are of the same order as the mean value.

We suggest a method to measure the canonical (London) phase distribution of a single-mode radiation field by a heterodyne or a multiport homodyne detector. The technique is based on sampling the overlap between the signal mode and a phase coherent state.

In optical superradiance studies, data were reported supporting a relation between the delay time $\left({\tau }_D\right)$ and the tipping angle $\left({\delta }_0\right),$ ${\tau }_D=({\tau }_R/4)\left[\ln \right({\delta }_0/2\mathrm{}\pi ){]}^2,$ where ${\tau }_R$ is the radiation damping time constant. We found that the same data also satisfy another relation, ${\tau }_D={\tau }_R\ln \left[\cot \right({\delta }_0/2\left)\right].$ The derivation of the two relations is reexamined. It is found that the former relation does not have sound theoretical background, and could lead to considerable errors when it is used to interpret experimental results.

Makowski and Konkel [Phys. Rev. A 58, 4975 (1998)] have obtained certain classes of potentials which lead to identical classical and quantum Hamilton-Jacobi equations. We obtain the most general form of these potentials.

We show briefly that claims in the preceding Comment by Mostafazadeh [Phys. Rev. A 60, 5144 (1999)] are based mostly on an empty premise.

Using analytical solutions of the mean fields, it is shown that the vibron model solution for water given by Shao et al. [Phys. Rev. A $47,$ 2064 (1993)] corresponds to a linear molecule. The numerical fit obtained in this paper appears to be a runaway solution, nothing to do with the actual properties of water molecule.

The polarization state of biphoton light generated via colinear frequency-degenerate spontaneous parametric down-conversion is considered. A biphoton is described by a three-component polarization vector, its arbitrary transformations relating to the SU(3) group. A subset of such transformations, available with retardation plates, is realized experimentally. In particular, two biphoton beams, formed by colinearly polarized photons (type I) are transformed into a single biphoton beam formed by orthogonally polarized photons (type II). Polarized biphotons are suggested as ternary analogs of two-state quantum systems (qubits).

Relative and absolute cross sections are presented for fluorine KLL Auger-electron production in collisions of 2.0-MeV ${\mathrm{H}}^{+}$ and 0.5-MeV/amu ${\mathrm{Li}}^{2+}$ with various fluorocarbon targets. Auger yields were measured for molecular targets of ${\mathrm{CH}}_3\mathrm{F},$ ${\mathrm{CH}}_2{\mathrm{F}}_2,$ ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{F}}_2,$ ${\mathrm{CHF}}_3,$ ${\mathrm{CF}}_4,$ ${\mathrm{C}}_2{\mathrm{F}}_6,$ and ${\mathrm{C}}_4{\mathrm{F}}_8.$ The fluorine Auger cross sections for these collision systems were found to be independent of the chemical environment, i.e., the atomic cross sections were found to obey additivity for these molecules. This is in contrast to recently reported fluorine K-shell ionization cross sections found for ${\mathrm{He}}^{+}$ impact on fluorocarbon targets, where the atomic cross sections were found to differ by up to a factor of 3.

An analytic method that exploits the properties of the eigenphase sum and its derivative is developed and implemented to study resonance structure found in the phase shift for electron scattering by diatomic molecular cations. The approach is used to locate narrow resonances and identify interloper resonances found within dominant Rydberg series. The power of the technique is illustrated for two molecular cations (oxygen and titanium oxide).

Relativistic and resonance effects in the photoionization of Mg-like Fe XV are investigated using the Breit-Pauli R-matrix method at near-threshold and intermediate energies, complemented by the relativistic random-phase approximation and multichannel quantum-defect theory in the energy region up to the L-shell ionization thresholds. The cross sections exhibit extensive resonance structures that considerably enhance the effective photoionization of Fe XV. These results should be of general interest in photoionization modeling of x-ray sources observed by space observatories.

We demonstrate control of quantum dynamics in a finite model system described by a tight-binding Hamiltonian, through interaction with a multifrequency external field. Effective defects can be introduced into the lattice by a two-frequency field, and the character of the defects can be controlled by the relative phase between the two field components. These field-induced defects imply robust localization of dressed (Floquet) states on lattice sites. Implications for a spin system in crossed magnetic fields are discussed.