Absolute cross sections for ionization of the Na-like ions ${\mathrm{Ti}}^{11+}$ and ${\mathrm{Cr}}^{13+}$ due to electron impact have been measured over the energy ranges 300–800 eV and 500–1500 eV, respectively. These measurements confirm predictions by Griffin, Pindzola, and Bottcher [Phys. Rev. A 36, 3642 (1987)] concerning the magnitude of indirect processes for these ions.

We study multiphoton ionization for a one-dimensional atom model (with only one bound state) solving numerically the Schrödinger equation. The intensity dependence of the cross section of the process is related to different ponderomotive thresholds. For very-low-intensity fields and very short times ‘‘non-energy-conserving’’ terms are dominant. The validity of perturbation theory has also been tested.

It is shown that the peak-switching phenomenon can occur in free-free transitions in intense laser fields. To this end we have carried out an exact numerical analysis of the one-dimensional Schrödinger equation.

We have investigated the dynamical behavior of a Co:${\mathrm{MgF}}_2$ laser with modulated losses by using rate equations for a four-level laser. The system displays both regular and chaotic behavior similar to that of a laser with modulated pump intensity. We have shown that there is an important difference between the laser with modulated losses and that with a modulated pump: The former corresponds to an oscillator with an external driving force, and the latter is a parametric oscillator with an external force, the amplitude of which depends on frequency.

In a recent paper [A. B. Volkov, Phys. Rev. A 39, 4406 (1989)] the square of the Dirac Hamiltonian matrix built within a finite-basis-set representation is used to obtain variational solutions to the Dirac equation. We show in this Comment that no advantages are gained by the use of this method that is in fact equivalent to the diagonalization of the nonsquared Dirac Hamiltonian. Most importantly one does not avoid the problems of variational collapse and spurious roots. A simple example is given showing that the method proposed in this work fails to provide bounds to the ground state of hydrogenic ions.

For the Volkov solution in general, and for the Keldysh approximation in particular, it is shown that three-dimensional behavior and one-dimensional behavior are very different.

In a recent publication [M. Schulz et al., Phys. Rev. Lett. 62, 1738 (1989)] experimental evidence for a new process two-electron transfer with excitation, was reported. In this paper we describe a higher-resolution version of the experiment, in which two-electron transfer with excitation was not observed.

Measurements of single-ionization cross sections for antiproton impact on helium atoms are reported for impact energies ranging from 40 keV to 3 MeV. It is found that the measured cross sections are in good agreement with recent theoretical estimates based on the continuum-distorted-wave approximation. From a comparison with similar proton data, the ratio between antiproton and proton results is obtained. The energy dependence of this ratio is compared with various theoretical estimates and explained as a result of polarization and binding effects.

Using an exact formulation of impulse approach (IA) to atom-diatom collisions, we assess its internal consistency. By comparing the cross sections in the forward and reverse directions for the vibrational-rotational inelastic processes, using the half-on-the-shell (post and prior) models of the two-body t matrix, we show that in both cases the IA leads to a violation of the semidetailed balance (SDB) condition for small scattering angles. An off-shell model for the two-body t matrix, which preserves SDB, is shown to have other serious shortcomings. The cross sections are studied quantitatively as a function of the relative translational energy and the mass of the incident particle, and criteria discussed for the applicability of IA.

We report theoretical partial widths for the ${}_{}{}^1{}_{}{}^{}$P°(3s3p) doubly excited state of the He atom, calculated with a method recently proposed by Martin et al. [Phys. Rev. A 40, 4245 (1989)]. Good agreement with the experimental values obtained by Lindle et al. [Phys. Rev. A 36, 2112 (1987)] from photoemission spectra of the He 1${\mathit{s}}^2$→3s3p autoionizing resonance is found.

The first observation of ${\mathrm{Ne}}_2^{2+}$ quasimolecular ion obtained by charge stripping of a 900-keV ${\mathrm{Ne}}_2^{+}$ beam in an Ar gas target is reported. This molecular ion has no bound states in the electronic ground state according to previous theoretical calculation.

The second-order Oppenheimer-Brinkman-Kramers approximation for relativistic electron capture is formulated, and rigorously calculated K-K cross sections are presented for the first time. Theoretical cross sections for electron capture in collisions of ${\mathrm{C}}^{6+}$-Au at 400 MeV/u and ${\mathrm{Ar}}^{18+}$-Ag at 400 and 1050 MeV/u projectile energy are compared with the experimental data and other approximate second Born results. It is shown that exact second Born cross sections grossly overestimate the experimental data.

Near threshold the microwave ionization rates of Na ‖m‖=0,1 Rydberg states within a Stark manifold n show a strong dependence on the location of the state within the manifold. In particular, the rate-limiting step for ionization of states with the smallest Stark shifts (‖k‖≪n) is different from the field-limited step of making the n→n+1 transitions at threshold fields ${\mathit{F}}_{\mathit{T}}$≃1/3${\mathit{n}}^5$ (a.u.). This variation in the ionization rates as a function of manifold location or Stark shift may be understood in terms of the variation in the transition probability to the bluest Stark states from the other members of the manifold.

The dynamic behavior of a coherently pumped ring laser with a homogeneously broadened four-level medium is analyzed theoretically, considering linearly polarized pump and generated laser beams. The laser is shown to be much more stable when these polarizations are parallel than when they are orthogonal. In the latter case the instability pump threshold can be as low as four times the first laser threshold, and the laser dynamics shows Lorenz-type features.

Bistable and dynamically unstable emission from a single-mode homogeneously broadened Raman laser is shown to arise from laser- and pump-field induced distortion and displacement of the dispersion profile associated with the laser gain. These features occur for broad cavity tuning to the low-frequency side of gain center, the threshold for dynamical behavior, being as low as the first lasing threshold. Results of this analytic treatment are in exact agreement with earlier numerical analysis.

A nonlinear, kicked quantum optical system, which classically exhibits regions of regular and chaotic motion, is proposed as a possible experimental test of quantum chaos. The mean photon number is shown to undergo regular collapse and revival in the regular region of phase space and irregular revivals in the chaotic region.

We present a simple two-state model illustrating the fact that the most prominent features observed in high-order harmonic generation are generic to strongly driven systems. We also address two important questions that arise in the comparison of theory and experiment.

We have measured the emission spectrum of two-level-like Ba atoms driven by two strong, equal-amplitude fields with frequency separation 2δ. The spectrum consists of a series of peaks with an essentially constant spacing δ and alternating linewidths. These features differ qualitatively from the characteristic triplet spectrum observed in the case of strong monochromatic excitation. Certain features of the observed spectrum such as its comblike structure can be motivated in terms of the energy spectrum of atom–bichromatic-field product states. Other features, such as the alternating linewidths, require more subtle analysis.

We show that pulses in the Lorenz-Haken model can be observed well below the well-known second threshold by using the anomalous switching technique. More specifically, if the laser system initially operates stably above the first threshold, the pulse motion can be set in when we abruptly switch the pump to a new value, which is much smaller than the second threshold. This phenomenon is due to the existence of multistable attractors and anomalous switching between these attractors. By using the anomalous switching, three physically distinguishable attractors, namely, two time-dependent and one stationary, are numerically shown to coexist for some parameters.