The Boltzmann equation and a detailed collisional-radiative model are solved simultaneously as a function of time to model the time-integrated x-ray spectra of the transient plasma produced by a high intensity ultrafast laser source. Level populations are calculated by solving the rate equations as a function of time using rate coefficients corresponding to a time varying electron energy distribution function (EEDF) determined by the solution to the Boltzmann equation. Electron-electron interactions are included through the solution of the Fokker-Planck equation. It is assumed that all the ions are initially in the Ne-like ground state due to the laser prepulse and that all free electrons have high energy (5 keV) from the fast laser deposition. The collisional-radiative model included over 3000 levels in the Ne-like through H-like ion stages of argon. The results are in agreement with highly resolved F-like to He-like K-shell emission spectra recorded recently during ultrashort laser experiments with argon cluster targets in Japan. The calculated time scale for emission is consistent with estimates of cluster decay times for these conditions. The calculations also show that the typical Li-like and Be-like satellite structure, sometimes attributed to a hot-electron component in the EEDF, can also be due to transient effects in a high-temperature ionizing plasma.

We present a combined theoretical and experimental study of dissociative charge transfer in collisions of slow ${\mathrm{Li}}_{31}^{2+}$ clusters with Cs atoms. We provide a direct quantitative comparison between theory and experiment and show that good agreement is only found when the experimental time-of-flight and initial cluster temperature are taken into account in the theoretical modeling. This model explains evaporation as resulting from a collisional energy deposit due to cluster electronic excitation during charge transfer. We discuss in detail the basic mechanisms that are responsible for the charge-transfer reaction and different approximations to evaluate the energy deposit.

Angular distributions of photoelectrons, produced by single ionization of ethylene or benzene molecules with an intense linearly polarized laser field, are calculated. The anisotropy and the yield of the photoelectrons depend strongly on the orientation between the molecular geometry and the field. In particular, electron emission in nodal planes is suppressed. The orientational dependence, also found in the total ionization rate, is a consequence of the symmetry of the highest occupied molecular orbital.

A photon echo experiment has been performed using accumulated highly attenuated laser pulses. We show experimentally that the photon echo process can be performed with, on the average, less than one photon in each pair of excitation pulses. The results support an interpretation where this nonlinear process can be performed with, on the average, less than one photon shared between two of the optical fields involved in the degenerate four-wave-mixing process. Further, we argue that the experiment can be interpreted as a form of delayed self-interference for photon wave packets that do not overlap in both space and time.

The interactions of hydrogen molecular ions $\left({\mathrm{H}}_2^{+}\right)$ and protons in the low energy range $(E<\mathrm{}7.5\mathrm{}\mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u})$ with very thin foils of monocrystalline gold are experimentally studied. Measurements of energy loss distributions of molecular fragments, recombined molecules, and protons channeled in the $〈100〉$ direction of a monocrystalline gold thin film have been performed. From the energy loss data we determine the stopping power ratio, which shows a significant negative “vicinage effect,” giving a reduced energy loss of molecular fragments and recombined molecules as compared to the energy loss of protons. This effect is more pronounced in the case of detected molecules. Computational simulations, where nonlinear models are included, qualitatively agree with our experimental results and suggest that some pairs of ions travel and emerge from the crystal with appropriate internuclear distances and relative velocities and are able to recombine at the exit.

We study the three-dimensional (3D) unidirectionally kicked Rydberg atom. For parabolic initial states elongated in the direction of the kicks we show that the ionization of the quantum system is suppressed as compared to the classical counterpart and that the quantum wave function is localized along all degrees of freedom, whereas the classical system is globally diffusive. We discuss the connection to the previously studied one-dimensional (1D) model of the kicked Rydberg atom and verify that the 1D model is a good approximation to the 3D quantum case in the limiting case of the most elongated initial states. We further study the quantum phase-space distribution (Husimi distribution) of the eigenstates of the period-one time-evolution (Floquet) operator and show that the eigenstates are localized in phase space. For the most elongated parabolic initial state, we are able to identify the unstable periodic orbits around which Floquet states localize. We discuss the possibility of observing quantum localization in high Rydberg states in $n>\mathrm{}100.\mathrm{}$

We propose a general perturbative approach to quantum-optical models without the rotating-wave approximation. We show that a generic Hamiltonian describing interaction between two subsystems can be represented as a series of operators corresponding to different transitions between bare energy levels of the whole system. Under certain relations between frequencies of interacting subsystems one of these transitions becomes resonant. The rotating-wave approximation leads to separation of the resonant transition and to appearance of the integral of motion, which makes the problem exactly solvable in this approximation. (Different resonance conditions lead to different integrals of motion.) All of the other terms in these expansion can be considered as a perturbation. They result in dynamic Stark shifts and small corrections to the integrals of motion. All possible resonances are classified, and approximate integrals of motion are found for each resonance. Examples of field-field, field-atom, and atom-atom interactions are considered.

We propose a reliable entanglement measure for a two-mode squeezed thermal state of the quantum electromagnetic field in terms of its Bures distance to the set of all separable states of the same kind. The requisite fidelity of a pair of two-mode squeezed thermal states is exactly evaluated. By applying the Peres-Simon criterion of separability, we find the closest separable state. This enables us to establish an insightful expression of the amount of entanglement. We also derive the relative entropy of a two-mode squeezed thermal state with respect to another one. This quantity is exploited in the pure-state case by undertaking a similar approximation for the entropic entanglement of a two-mode squeezed vacuum state. Comparison with previous work lends support to our approach that replaces the reference set of all separable Gaussian states by its subset consisting only of the squeezed thermal ones.

A sufficient condition for entanglement in two-mode continuous systems is constructed with the interference visibility and the uncertainty of the total particle number. The observables to be measured (particle numbers and particle number variances) are relatively easily accessible experimentally. The method may be used to detect entanglement in light fields or in Bose-Einstein condensates. In contrast to the standard approach based on entanglement witnesses, the condition is expressed in terms of an inequality which is nonlinear in expectation values. The condition is constructed using uncertainty relations with the particle number and the destruction operators.

We reexamined charge-transfer cross sections for ${\mathrm{C}}^{4+}+\mathrm{H}$ collisions for energies from 1 meV/amu to 1 keV/amu using the recently developed hyperspherical close-coupling method. Our results agree with several previous theoretical calculations using molecular-orbital expansion. However, these converged theoretical predictions do not agree with total cross sections from the merged-beam experiments.

Two simple schemes are presented for the selective population transfer in systems where the target state is degenerate with a second level. Both states are coupled with the ground state via a one photon resonance. The schemes use a combination of two laser pulses. The first pulse breaks the degeneracy of the levels by inducing a Stark shift. The second pulse is chirped across the new levels. The idea is illustrated by numerical simulations of the time-dependent Schrödinger equation for a four-level system. The efficient population transfer is explained using a dressed state representation. In conclusion, applications for molecules are suggested.

We study the strong-field interaction of a negative hydrogen ion in a short infrared laser pulse. An imaging technique is employed in a fast ion beam to record angular-resolved energy distributions of the photodetached electrons. The spectrum exhibits four excess photon channels. A detailed analysis of the energy and angular distribution is presented, including a comparison with recent theories. From our data we determine elastic-scattering phase shifts of the partial continuum waves in the lowest-order detachment channel. Quantitative agreement with scattering theory is obtained. A quantum interference effect predicted in a recent Keldysh-like theory is observed at energies close to threshold. We propose an analogy to a double-slit model which explains the interference features in a simple and intuitive way. We also verify the applicability of the Wigner threshold law in the strong-field regime and discuss its role in the observed quantum interference.

We calculated near-threshold photodetachment cross sections for ${\mathrm{Cs}}^{-}$ in the presence of a dc electric field using three different approaches: the frame-transformation method with and without rescattering effects and the Kirchhoff-integral approach. Radial wave functions for the electronic motion were obtained using the Pauli equation method with a model potential describing the effective electron-atom interaction. Our results demonstrate the inadequacy of the frame-transformation method in the ${}^{3}P$ resonance region even for weak fields $(E<\mathrm{}10\mathrm{}\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}).\mathrm{}$ We show that the triplet and singlet contributions to the total cross section can be manipulated by varying the external electric field. This can enhance spin-orbit effects in the photodetachment process and create more favorable conditions in the experiment for the observation of the lowest ${}^{3}P$ resonance in ${\mathrm{Cs}}^{-}.$

We investigate the existence of quantum quasi-phase-transitions for an ensemble of ultracold bosons in a one-dimensional optical lattice performing exact diagonalizations of the Bose-Hubbard Hamiltonian. When an external parabolic potential is added to the system quasi-phase-transitions are induced by the competition of on-site mean-field energy, hopping energy, and energy offset among lattice sites due to the external potential and lead to the coexistence of regions of particle localization and delocalization in the lattice. We clarify the microscopic mechanisms responsible for these quasi-phase-transitions as a function of the depth of the external potential when the on-site mean-field energy is large compared to the hopping energy. In particular, we show that a model Hamiltonian involving a few Fock states can describe the behavior of energy gap, mean particle numbers per site, and number fluctuations per site almost quantitatively. The role of symmetry on the gap as a function of the depth of the external trapping potential is elucidated. We discuss possible experimental signatures of quasi-phase-transitions studying the single-particle density matrix and explain microscopically the occurrence of local maxima in the momentum distribution. The role of a thermal population of the excited states on the momentum distribution is discussed.

We study the effects of many-body interactions on the interference in a Mach-Zehnder interferometer for matter waves of ultracold Bose atoms. After switching off an axial trapping potential, the thermal initial wave packet expands, and subsequently interference fringes may be observed in a circular one-dimensional trap. These are computed for axial harmonic or $\delta$-function traps, and for interaction strengths from the Thomas-Fermi regime to the Tonks-Girardeau limit. It is shown that many-body correlations in a realistic setup destroy interference to a large degree. Analytical expressions allowing to infer the observability of phase coherence and interference are provided.

Ab initio calculations demonstrate that specific zwitterionic molecules which have a $\sigma$-bonded donor-acceptor system show an enhanced hyperpolarizability $\left(\beta \right)$ value when the effects of electron correlations are considered. Ab initio second-order Møller-Plesset perturbation theory (MP2) through-space (TS)–through-bond (TB) interaction (TB denotes orbital interactions through directly bonded atoms in a molecule, and TS denotes all interactions other than TB interactions) analysis proposes that the $\sigma$ conjugation between acceptor and donor through bonds causes the dependence of the perturbation term between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (and their vicinal molecular orbitals) in MP2 calculations on electric fields, making the hyperpolarizability $\left(\beta \right)$ value remarkably large.

We study the far-field spatial distribution of the quantum fluctuations in the transverse profile of the output light beam generated by a type-II optical parametric oscillator below threshold, including the effects of transverse walk-off. We study how quadrature field correlations depend on the polarization. We find spatial Einstein-Podolsky-Rosen entanglement in quadrature-polarization components. For the far-field points not affected by walk-off there is almost complete noise suppression in the proper quadratures difference of any orthogonal polarization components. We show the entanglement of the state of symmetric, intense, or macroscopic, spatial light modes. We also investigate nonclassical polarization properties in terms of the Stokes operators. We find perfect correlations in all Stokes parameters measured in opposite far-field points in the direction orthogonal to the walk-off, while locally the field is unpolarized and we find no polarization squeezing.

In a three-level atomic system coupled by two equal-amplitude laser fields with a frequency separation 2δ, a weak probe field exhibits a multiple-peaked absorption spectrum with a constant peak separation δ. The corresponding probe dispersion exhibits steep normal dispersion near the minimum absorption between the multiple absorption peaks, which leads to simultaneous slow group velocities for probe photons at multiple frequencies separated by δ. We report an experimental study in such a bichromatically coupled three-level Λ system in cold ${}^{87}\mathrm{Rb}$ atoms. The multiple-peaked probe absorption spectra under various experimental conditions have been observed and compared with the theoretical calculations. © 2003 American Institute of Physics.

We investigate the problem of an electron placed in the potential of a finite dipole and confined by an impenetrable prolate spheroidal box. The critical cage sizes at which successive bound states are ionized as the box becomes smaller are found to be the roots of associated Legendre functions of complex order. We find the asymptotic behavior for a large cage. The lowest few roots are computed numerically. Comparison is made with the case of an electron bound by an ideal dipole plus short-range repulsion, confined in a spherical box.