We present an approach to describe the phase matching of high harmonics emitted by laser driven atoms in a nonperturbative regime, for which the atomic response displays an intrinsic intensity-dependent phase. We show that the traditional phase-matching conditions involving conservation of wave vectors should be modified by taking into account the gradient of this atomic phase. We investigate various focusing geometries and interpret the numerical results of Salières et al. [Phys. Rev. Lett. 74, 3776 (1995)]. Within the framework of the two-step model, we demonstrate that the gradient of the intensity-dependent phase can be considered as the canonical momentum gained by the electron in the continuum due to acceleration by field-gradient forces, including in particular the ponderomotive force.

We study the stability of the interaction between a pump wave and a counter propagating second-harmonic wave phase-matched in a periodically poled ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ material using both analytical and numerical methods. In contrast to the more usual copropagating case, backward phase-matched frequency doubling displays richer and more complex behavior, owing to the presence of built-in feedback.

We use the Lewis and Riesenfeld invariant method [J. Math. Phys. 10, 1458 (1969)] to obtain the exact Schrödinger wave functions for a harmonic oscillator with time-dependent mass and frequency. Exact coherent states for such system are also constructed.

The detection strategy proposed previously by the authors for binary pure-state signals, which consists of a unitary transformation and photon counting afterward [Phys. Rev. A 54, 2728 (1996)], can also be applied to the case of on-off keyed signals of coherent states with a small amount of thermal noise. It yields an error level almost the same as the quantum minimum bound in practical parameter region.

Given an electronic system in its ground state, having N electrons moving in the field of nuclei generating a potential ${\mathrm{v}}_0$ and accurate electron density ${\mathrm{\rho }}_0$, it is demonstrated that the exact Kohn-Sham equations result from a minimization with respect to ρ, at some very high temperature θ, of a free energy functional ${\mathrm{A}}^{\mathrm{\theta }}$[ρ,${\mathrm{\rho }}_0$]=${\mathrm{T}}_{\mathrm{s}}$[ρ]+〈ρ|${\mathrm{v}}_0$〉+J[ρ](1-1/N)-θS[ρ,${\mathrm{\rho }}_0$], where S[ρ,${\mathrm{\rho }}_0$]=-〈ρ ln (ρ/${\mathrm{\rho }}_0$)〉. The infinite-θ minimum of ${\mathrm{A}}^{\mathrm{\theta }}$ is, within an error so far found to be less than the correlation energy, equal to the total electronic energy of the system.

We have measured the lifetime of the 3p${\mathrm{}}^2$${\mathrm{P}}_{3\mathrm{/}2}$ level in Na-like ${\mathrm{Kr}}^{25+}$, using beam-foil excitation and cascade-corrected analysis. Our result, τ=45.2±2 ps, has half the uncertainty of previous experimental data for highly ionized Na-like ions, and permits an important test of theoretical methods. Recently, Hutton et al. [Phys. Rev. A 51, 143 (1995)] pointed out a seemingly systematic one-standard-deviation difference between theoretical and experimental f values of 3s${\mathrm{}}^2$${\mathrm{S}}_{1\mathrm{/}2}$-3p${\mathrm{}}^2$${\mathrm{P}}_{3\mathrm{/}2}$ transitions for Na-like ions with Z>20. Our more precise result indicates that these small differences are coincidental and insignificant, confirming the reliability of theoretical calculations for this system.

We set up and solve the problem of optimally partitioning the Coulomb operator 1/r into a sum of two functions ${\mathrm{f}}_1$(r) and ${\mathrm{f}}_2$(r) such that both ${\mathrm{f}}_1$ and the Fourier transform of ${\mathrm{f}}_2$ decay as quickly as possible. The rigorous solution involves a Hermite function, but we find that the conventional Ewald-KWIK partition appears to be only slightly inferior.

The R matrix with pseudostates and convergent close-coupling methods are applied to the calculation of excitation and ionization cross sections for the simplified S-wave model of e-${\mathrm{He}}^{+}$ scattering. Very good agreement is obtained between the results of the two methods. Furthermore, the problem of Rydberg series of pseudoresonances, converging to the unphysical positive-energy states in the continuum, is shown to be almost negligible if large basis sizes or simple averaging procedures are used.

We here present an alternatve way to perform R-matrix calculations in which the eigenchannel parameters of multichannel quantum defect theory are calculated directly. Merits of our R-matrix calculations are twofold: (1) an analytical treatment of perturbed discrete Rydberg states, autoionizing states, and their adjacent continuum to avoid numerical calculations outside the R-matrix box; (2) clear assignment of overlapped resonances, e.g., to analyze the lithium inner-shell photoionization cross section, which has been of considerable interest in recent experimental measurements.

We present superelastic cross sections of ${\mathrm{H}}_2$ by electron impact. Our results, obtained with the Schwinger multichannel method, include cross sections for the c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathrm{u}}$→ X${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathrm{g}}^{\mathrm{}(+)\mathrm{}}$, c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathrm{u}}$→ b${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathrm{u}}^{\mathrm{}(+)\mathrm{}}$, c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathrm{u}}$→ a${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathrm{g}}^{\mathrm{}(+)\mathrm{}}$, and elastic c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathrm{u}}$→ c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathrm{u}}$ transitions. The calculated cross sections are very large, indicating their importance in the modeling of discharge environments. We also investigate the dependence of these cross sections on the internuclear separation of the H atoms and on different approximations of the target wave function. To assess the role of the exchange potential in these processes, we also present the corresponding polarization fractions.

The binding energy and fine-structure splitting of the ground state of the ${\mathrm{Sr}}^{\mathrm{-}}$n(${5\mathrm{s}}^2$5p${\mathrm{}}^2$P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17±0.03 meV (J=3/2) and 52.06±0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89±0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].

We derive the analytical threshold behavior for the triplet cross section for electron-impact ionization in the Temkin-Poet model. The analytical results indicate that the most recent numerical calculations may fail to reproduce the correct threshold behavior in an energy regime below about E=0.1 a.u. We also present an analytical expression for the energy distribution of the two electrons near threshold.

For ensemble of bosons trapped in a one-dimensional harmonic potential well we have found an analytical formula for the canonical partition function and shown that, for 100 trapped atoms, the discrepancy between the grand canonical and the canonical predictions for the condensate fraction reaches 10% in the vicinity of the Bose-Einstein threshold. This discrepancy decreases only logarithmically as the number of atoms increases. Furthermore, we investigate numerically the case of a three-dimensional ``cigar-shape'' trap in the range of parameters corresponding to current Bose-Einstein condensation experiments.

The effect of the electronic exchange in the calculation of the scattering amplitude for inelastic electron-cluster collisions is evaluated. The target cluster initially in the ground state is excited, by means of the Coulomb forces, into a particle-hole state. An antisymmetrized transition amplitude is evaluated in the scope of the distorted-wave Born approximation. A comparison is made with a calculation in which the exchange is taken into account by adding to the direct interaction a Coulomb exchange term in the Slater approximation. The treatments are applied to the closed-shell ${\mathrm{Na}}_8$ cluster for incident energies up to 5 eV.

We present the model accounting for the Coulomb effects that occur in the direct double photoionization of the two-electron atom in an intense field of a circularly polarized plane wave. The model is the strong-field approximation (SFA) (the Reiss version of Keldysh-type models) applied to helium. We improve here our earlier study by inserting the electron-electron correlation in the initial state of the atom, and by inserting the Coulomb correction due to the influence of the nucleus on the final state of ionized electrons' pair. While the former improvement is of little significance, the latter extends the applicability range of the SFA (for typical laser wavelengths), and causes appreciable growth of the total ionization rate.

We examine the dynamics of two or more interfering wave packets. Via an analytical expression for the interfering wave packets, the redistribution of population and the evolution of the wave packets are studied. This analysis is in agreement with recent experimental investigations of interfering wave packets including the excitation of a Schrödinger cat state. The analysis is extended to multiple pulses and offers a simple formula for predicting their behavior.

We show that in a system of two coupled lasers, such that the squeezed light produced by the first is injected into the second, the output from the driven laser may present stronger squeezing than both lasers separately, operating under the same conditions, but with the second laser being driven by a coherent signal.

This paper models recent experiments in which a solid titanium target was illuminated by several joules of combined energy from a nanosecond laser pulse to create a preplasma followed by a picosecond laser pulse to drive the gain. Gains greater than 200 ${\mathrm{cm}}^{\mathrm{-}1}$ are predicted for the Ne-like Ti 3p${\mathrm{}}^1$${\mathrm{S}}_0$→3s${\mathrm{}}^1$${\mathrm{P}}_1$ transition at 326 Å, which is driven by the monopole collisional excitation. High gain is also predicted for the 3d${\mathrm{}}^1$${\mathrm{P}}_1$→3p${\mathrm{}}^1$${\mathrm{P}}_1$ transition at 301 Å, which is driven by a combination of collisional excitation and self-photopumping. We also discuss the possibilities for driving a Ne-like Ge laser using this approach.

We show how to reconstruct the quantum state of completely unpolarized light of type I [J. Lehner, U. Leonhardt, and H. Paul, Phys. Rev. A 53, 2727 (1996)] via optical homodyne tomography. Our analysis can be extended to any two-mode situation with complete two-mode randomization and, moreover, to any two-dimensional harmonic oscillator with SU(2) symmetric states. Interestingly, we have shown that the Laplace operator applied to the quadrature distribution of unpolarized photons is orthogonal on the set of these distributions.