The electron-capture process from the free-electron gas of a solid by heavy ions at grazing collisions is studied using the first Born approximation. A proper semiclassical expression is derived from the formal quantum treatment by introducing a classical trajectory to describe the movement of the projectile. When our finding is compared with the scattering amplitude obtained with the usual semiclassical formalism, three important differences are found: first, the generally used specular reflection of the projectile does not hold; second, a new term associated with a sort of projectile translation factor comes out; and third, a multiplicative coefficient ranging between 1/2 and 1 arises from the formalism depending on the trajectory.

We extend our previous perturbative study of the multiphoton detachment of ${\mathrm{H}}^{\mathrm{-}}$ [Phys. Rev. A 48, 4654 (1993)] to stronger fields by considering the intensity-dependent photodetachment rates and threshold behavior. An accurate one-electron model potential, which reproduces exactly the known ${\mathrm{H}}^{\mathrm{-}}$ binding energy and the low-energy e-H(1s) elastic-scattering phase shifts, is employed. A computational technique, the complex-scaling generalized pseudospectral method, is developed for accurate and efficient treatment of the time-independent non-Hermitian Floquet Hamiltonian H${\mathrm{^}}_{\mathit{F}}$. The method is simple to implement, does not require the computation of potential matrix elements, and is computationally more efficient than the traditional basis-set-expansion–variational method. We present detailed nonperturbative results of the intensity- and frequency-dependent complex quasienergies (${\mathit{E}}_{\mathit{R}}$,-Γ/2), the complex eigenvalues of H${\mathrm{^}}_{\mathit{F}}$, providing directly the ac Stark shifts and multiphoton detachment rates of ${\mathrm{H}}^{\mathrm{-}}$. The laser intensity considered ranges from 1 to 40 GW/${\mathrm{cm}}^2$ and the laser frequency covers 0.20–0.42 eV (in the c.m. frame). Finally we perform a simulation of intensity-averaged multiphoton detachment rates by considering the experimental conditions of the laser and ${\mathrm{H}}^{\mathrm{-}}$ beams. The results (without any free parameters) are in good agreement with experimental data, both in absolute magnitude and in the threshold behavior.

The effect of the spatial arrangement of a molecular internuclear axis and its plane of rotation in the multiphoton nonresonant excitation process is investigated. We consider diatomic or symmetric-top molecules; the internuclear axis is regarded to be fixed in space during the excitation time interval. Steric conditions preparing the initial angular distribution of the internuclear axis in the excited state are obtained, both for resolved and unresolved magnetic levels and for linearly or circularly polarized incident radiation. The intensities of individual rotational lines or rotational branches are obtained after averaging over all spatial orientations.

We present an application of the artificial-channel method to the calculation of multiphoton ionization rates under continuous-wave illumination conditions. We use the method to treat above-threshold ionization (ATI) of hydrogen in three dimensions with linearly and circularly polarized light at field intensities ranging from ${10}^9$ to ${10}^{14}$ W/${\mathrm{cm}}^2$. The results are compared with perturbative as well as nonperturbative calculations for short pulses and with experiments. The range of validity of the dipole approximation for ATI processes is also examined. An analysis of both the angular and angular-momentum distribution of the photoelectrons at different intensities is presented. We find that ATI cross sections for circularly polarized light at 532 nm are three orders of magnitude weaker, with electronic kinetic energies peaking at higher values, than in the case of linearly polarized light. This last effect is shown to result from the different ponderomotive potential experienced by the electron, due to the drastically different angular distributions computed for the two cases. Connection is made between the angular distributions, and the angular-momentum constraints affecting it, and the kinetic energy of the ensuing electrons. It is argued that it is difficult to understand the ATI process without taking the three-dimensional aspects of the problem into consideration.

Recently, the progress of experimental techniques has allowed the realization of less-than-a-cycle pulses. Though these pulses are as yet too weak to ionize atoms from the ground state, the rapid progress in experimental methods warrants the investigation of possibilities opened up by these new achievements. In this paper we use the quasiclassical Fermi molecular-dynamics method to study the dynamics of ionization and the electron correlations of initially ground-state He atoms in single-cycle infrared pulses. An interpretation is also made in terms of a quasistatic ionization model.

The effect of the long-range Coulomb potential on a very-low-energy final state of ionization is known to be important in a variety of atomic reactions. It is here extended to intense field multiphoton ionization. A trial wave function which is a generalization of the Coulomb-Volkov state is chosen with a parameter which is determined variationally. It is then used in a form of the S matrix and yields ionization in the polarization direction (for linear polarization) which is of the order ${10}^8$ larger than that obtained for the Volkov final state. For larger angles of ionization the Coulomb contribution falls off rapidly but still dominates the total rate. This enhancement is not the result of the usual amplification of the wave function at the origin by the long-range Coulomb potential.

We have developed a correlated Keldysh-Faisal-Reiss theory of laser-induced double ionization of a two-electron atom. The basic N-photon T matrix and the expression for N-photon triple-differential rates or cross sections (TDCS’s) are derived. The theory is applied to investigate the TDCS’s for very-high-order multiphoton double ionization of He with lasers of wavelength λ=248 nm and λ=617 nm. Comparison with the uncorrelated results reveals a dramatic influence of the final-state e-e correlation on the above-threshold TDCS’s to be measured in coincidence experiments in intense laser fields. The limiting case of the TDCS’s for weak-field double ionization of He by a synchrotron photon is also investigated; the results confirm the earlier theoretical findings and recent experimental results in that case.

We study the motion of three-level atoms in a magneto-optical field which consists of a laser field with a polarization gradient and a static magnetic field. The Hamiltonian of the model is diagonalized using band-theoretical methods of solid-state theory. Diffraction of atoms is studied numerically, and the results are compared to the ordinary Kapitza-Dirac effect. Spontaneous emission is accounted for by the use of quantum Monte Carlo techniques.

An alternative formalism that diagonalizes the system Hamiltonian in ever larger boxes is shown to yield, for any Hermitian Hamiltonian and in particular a time-periodic Hamiltonian, all the observables sought by generic non-${\mathit{scrL}}^2$ full-scattering and half-scattering methods. No complex potential, coordinate rotations, or solutions of coupled equations are necessary. All observable quantities are computed from the same set of diagonalizations. A model Floquet problem is treated.

We present an analytical solution to a multilevel model of light-induced continuum structure, in which, instead of a single level, a quasicontinuum of excited levels is laser embedded into a previously structureless atomic continuum. The found solution is fully time dependent and is valid for probing strengths of the structured continuum from the ground level by another laser, which can vary from being much weaker than the embedding one to being as strong as the embedding one. For three different embeddings (weak, moderate, and strong), each one determined by a different relation between the ionization rate γ and the spacing Δ of the quasicontinuum levels (ħγ/Δ≪1, ħγ/Δ≃1, and ħγ/Δ≫1), we study the evolution of the ionization spectrum with increasing probing strength. We observe and interpret such strong-field effects in the spectra as (i) conversion of a Fano-like train of peaks into a train of dips; (ii) peak moving, narrowing, and lowering; and (iii) saturation, population trapping, and stabilization against ionization.

The description of Gaussian noise for single-mode quantum fields is briefly reviewed and applied to calculate the maximal information properties of three quantum communication channels degraded by Gaussian noise. These channels are based on (i) heterodyne detection of coherent states, (ii) homodyne detection of squeezed states, and (iii) photodetection of number states. It is found that each channel can outperform the others for a given noise level, where the optimum channel is essentially determined by the product of the average noise and signal energies.

Recent femtosecond processes [Phys. Rev. Lett. 66, 2464 (1991)] confirm the importance of non-Markovian effects since the dynamical characteristic times of the system become of the same time scale as the reservoir. We analyze here the memory effects in the density-matrix coherence for a driven two-level atom in the stationary regime. The coherence parameter η is written in terms of the detuning, the strength of the external driving force, and the temperature as the reservoir. We verify that the memory effects, present in the density matrix, are enhanced by the external driving force. We introduce the Shanon information entropy and we verify that, while in the Markovian approximation the entropy decreases monotonically as function of the detuning, in the non-Markovian approach it is characterized by a minimum, which enables us to differentiate between the two processes.

We demonstrate that accumulated photon echoes can be generated by synchrotron radiation. This is an experiment of a nonlinear optical effect carried out by using the radiation from an electron storage ring. A dye-doped polymer film was excited by the visible light from the storage ring, and the accumulated photon echoes from the sample were observed by employing a phase-modulation technique. The use of synchrotron radiation facilitates the extension of the spectral range where we can observe photon echoes with ultrahigh time resolution.

An alternative mechanism for Smith-Purcell radiation is proposed. This mechanism may have relevance to recent reports of higher radiation power. The electron beam excites resonant transitions of atomic quantum levels in the optical grating material by the fields of the traversing electrons. The dipole moments of all the atoms which are excited by the same electron radiate in phase with each other and produce ‘‘super-radiant radiation.’’ To calculate the radiant intensity due to this process we first calculate the dipole moments of the atoms excited by the classical electrical field of the traversing electron. Assuming that the dipole oscillations are dominated by a collision time ${\mathit{T}}_2$ we calculate the classical radiant intensity from the optical gratings due to this process. Sample numerical calculations based on a ruby grating result in substantial radiation levels.

We investigate numerically the dynamical bifurcations that occur in detuned degenerate four-wave mixing as the pump intensity is varied. We find several types of gluing bifurcations and symmetry-breaking bifurcations. A sequence of symmetric-breaking and gluing bifurcations leads to Lorenz-like chaos.

The thermal coherent states considered previously by the present authors represent an alternative mixed-state generalization of the usual pure-state coherent states. They describe displaced harmonic oscillators in thermodynamic equilibrium with a heat bath at nonzero temperature. We show how they provide a ‘‘random’’ (or ‘‘thermal’’ or ‘‘noisy’’) basis on a quantum-mechanical Hilbert space scrH. Their usefulness rests on the fact that the corresponding statistical density operator provides a probability operator measure on scrH. We thereby show how the thermal coherent states permit a generalization to nonzero temperatures of the well-known P and Q representations of operators in scrH. Particular emphasis here is placed on imbedding the formulation in the Bargmann or holomorphic representation of scrH. We examine the corresponding Bargmann representations of both state vectors and operators, and show how the former relate to the usual position and momentum representations and the latter to the usual P, Q, and W (or Weyl) representations. A particularly important and unexpected result is that the present temperature-dependent generalized P and Q representations are the analytic continuations to negative temperatures of each other. The usual Q and P representations thus represent the limits as the temperature approaches zero along the positive and negative real axes, respectively, of the enlarged generalized Q representation, suitably analytically continued to negative temperatures. We discuss the possible physical applications of the present thermal coherent states to both quantum optics situations involving coherent signals in the presence of thermal noise and to signal and image processing.

An atomic interference method is presented that can be used to construct arbitrary superpositions of coherent states with equal mean photon number in a single-mode cavity by sending only one atom through the apparatus. The method is suitable to generate any quantum state that has a one-dimensional coherent state representation on a circle in phase space. This method is demonstrated in the case of generating Fock states.

Resonant cw degenerate four-wave mixing signals usually are not observed at noncycling transitions in alkali-metal vapors due to population depletion from optical pumping. We demonstrate that frequent atom-wall collisions can overcome the effects of optical pumping when atomic vapors are contained in thin cells. Significantly enhanced degenerate four-wave mixing signals are observed at noncycling transitions, and they are comparable in magnitude to the signals observed at cycling transitions in thin cells. This technique extends the utilization of degenerate four-wave mixing to noncycling transitions in atoms or molecules.

We investigate the amplitude and intensity spatiotemporal coherence and the photon coincidence rates of far-field down-converted light resulting from the interaction of a pump wave of finite spectral width with a nonlinear crystal of finite spatial width. We examine the interplay between energy and phase mismatches and their effects on the intrabeam and interbeam spatial and temporal coherence of the down-converted beams. We show that the down-converted light is spatially incoherent in the second order, and that the signal-idler fourth-order coherence extends over finite entanglement angles. We also study the effect of the apertures through which the down-converted light is collected, when they are centered at phase-matched or misaligned directions.

We analyze an electromagnetically induced transparency near an autoionizing state, which is at the same time stabilized against ionization. We show that the effect is fairly general as long as only one continuum is involved. We compare the efficiency for third-harmonic generation in this system with that in an intermediate two-photon resonance system, and discuss the advantages of the former. Explicit calculations are presented for a specific autoionizing resonance in Ca.