We describe theoretically the generation of ultrashort (subfemtosecond) pulses using high-order harmonics of a laser pulse with a time-dependent degree of ellipticity. The single-atom response is calculated by using a low-frequency strong-field approximation. Propagation effects are taken into account using a method going beyond the slowly varying envelope approximation. Propagation modifies significantly the results obtained in the single-atom response and, in certain conditions, makes the generation of one attosecond pulse possible. We discuss prospects for the observation of these ultrashort pulses. [S1050-2947(97)09411-0].

Analytical and numerical results for the T-matrix and the total cross section for scattering by a Yukawa potential in a strong low-frequency laser field are presented. They are based on our recently derived off-shell low-frequency approximation. The conditions under which the results by Daniele and co-workers can be applied are analysed. Expressions for the T-matrix and for the total cross section for scattering in a laser field in the eikonal approximation are derived. The total cross section is presented in the form of an eikonal multiple scattering series as a product of two factors. One of these factors only depends on the type of the scattering potential, while the other depends on the parameters of the scattered particle and the laser field. The results obtained are in good agreement with our earlier findings derived by using the [1,1] Pade approximation.

Inelastic electron-hydrogen atom collisions with the excitation of the 2s and 2p states in the presence of a bichromatic laser field are considered. Resonant and exchange effects are neglected and the problem is treated within the first Born approximation. The interaction of the scattered electron with the laser field is taken into account exactly, while the interaction of the laser field with the hydrogen atom is treated by first-order time-dependent perturbation theory. The contribution of the target intermediate states to the scattering amplitude is taken into account using the Sturmian representation of the Coulomb Green's function. The differential cross sections for the inelastic collision processes are analysed as a function of the relative phase between the two laser field components and it is shown that the influence of the variation of the relative phase is more pronounced than for the elastic scattering analysed earlier (1997 J. Phys. B: Ar. Mol. Opt. Phys. 30 1061). Furthermore, these effects appear at lower laser field intensities and can also be important for larger scattering angles. The symmetry properties of the differential cross section (DCS) as a function of the relative phase are also analysed and the minima of the DCS as a function of the scattering angle are explained by destructive interferences between the undressed part of the scattering amplitude and that part which corresponds to the dressing of the excited state.

We investigate the influence of the off-shell effects on the scattering processes in the presence of a laser field in view of the recent experiments by Wallbank and Holmes. In these experiments a large discrepancy between the observed data for small-angle scattering and the on-shell Kroll - Watson formula was reported. We show in the present work that the off-shell effects are not responsible for this discrepancy by presenting results for the differential cross sections obtained, using the off-shell low-frequency approximation and the [1,1] Pade approximation. We model the scattering from helium and argon atoms by the scattering on a Yukawa-type potential. We show, using some examples, that higher laser field frequencies and intensities are necessary for obtaining observable off-shell effects. We also comment on possibilities of explaining the Wallbank and Holmes results. For one of these possibilities - double scattering - we compute, using our simple model, the relative differential cross sections and obtain a qualitative agreement with the experiments of Wallbank and Holmes, both for helium and argon.

We calculate the harmonic spectrum and the time profile of the emission of harmonics emitted by a hydrogen atom exposed to a low-frequency and moderate-intensity laser pulse. This calculation has been performed by solving numerically the corresponding three-dimensional time-dependent Schrodinger equation. The results are compared to those obtained with the strong-field approximation model. We show that the semiclassical interpretation based on the existence of one or several return times is valid for harmonics at the end of the plateau or beyond the cutoff. We also demonstrate that it is possible to control in time the emission of one given harmonic by using an external field whose polarization depends on time.

Potential scattering in a strong elliptically polarized multicolour laser field is considered. The first result is the off-shell low-frequency approximation for scattering with emission (absorption) of L photons. A simple result for the total cross section is obtained using the optical theorem. In the case of a linearly polarized multicolour laser field a generalization of the on-shell Kroll-Watson-type low-frequency approximation is derived. Numerical results, obtained using the [1, 1] Pade approximant to the off-shell low-frequency T matrix for scattering on the Yukawa potential in a bichromatic laser field, are compared with the Varro and Ehlotzky results. The symmetry and asymmetry properties of the scattered electron spectra, as well as the symmetry breaking at higher laser field intensity, are analysed. It was observed that the differential cross section for scattering in a bichromatic laser field as a function of L. can have pronounced maxima. The positions of these maxima are determined by the generalized Bessel function properties and can be controlled by changing the laser field intensity and the relative phase between the laser field components.

We present analytical and numerical results for the T matrix and cross sections (differential and total) for scattering in a strong low-frequency laser field. Presented results, based on recently derived expressions for the off- and on-shell low-frequency T matrices are beyond the first Born approximation results of Daniele et al. (1986) for scattering on the Yukawa potential in a laser field. An analytical expression for the total cross section is obtained using the (1,1) Pade approximant to the off-shell low-frequency T matrix. This result explicitly shows departure from the sum rule and it is analysed as function of the laser field intensity and polarization angle. If the laser field is so strong that the laser field induced scattering particle impulse eA0 is comparable with the initial impulse pi this departure has a maximum. For an ultra-strong laser field (eA0>>pi) the total cross section is small in comparison with the field-free section, i.e. the influence of the scattering potential is weak. The results for different values of the screening radius are also presented and it is observed that the departure from the sum rule is larger for long-range potentials (when the interplay between the potential and the intense radiation field is important). The departure is strong for small values of the polarization angle ( Theta i<20 degrees ) and weak for 45 degrees < Theta i<135.

We report results of a Monte Carlo study of the kinetics of random sequential deposition of line segments (mostly dimers) on the 1D lattice substrate, already occupied with point-like quenched impurities at low concentration. The area covered by the placed objects grows with time and finally reaches a jamming limit when no more deposition is possible. The jamming coverage values, obtained by numerical simulations, depend on the segment length and on the previous occupation of the substrate by impurities. The rate of late-stage deposition is not disturbed by presence of forbidden sites when the process of deposition starts (t=0). Numerical results, shown in semi-log scale, confirm that area coverage theta (t) approaches the jamming limit theta ( infinity ) exponentially, with the same exponent factor -1 multiplying scaled time, as in the case of random sequential deposition of line segments on the clean 1D lattice (initially non-occupied).