In the strong-field ionization of atomic and molecular systems, the photoelectron is exposed to the long-range Coulomb force which is neglected in the standard theories based on the strong-field approximation (SFA). We introduce an ansatz which takes into account the Coulomb effects and at the same time is as simple as the standard SFA. Our Coulomb distorted plane wave-Volkov approximation provides analytical expressions for the relevant matrix elements. We also present a generalization of this approximation taking into account first-order term of an expansion in the atomic potential. Similarly as in the standard improved SFA, this generalized approximation describes well the rescattering plateau and the cutoff observed in the photoelectron spectra. Our new approximation is illustrated with numerical examples of strong-field ionization of the hydrogen atom exposed to linearly and circularly polarized laser pulses. The spectra obtained are slightly flatten in comparison with the SFA spectra and this effect is stronger for shorter laser wavelengths.

Nondipole effects occurring in the process of atomic ionization by an intense, mid‐infrared, counter‐rotating bicircular laser field are investigated using the strong‐field approximation with leading‐order nondipole corrections. The time integrals appearing in the expression for the differential ionization rate are computed in two ways: numerically, and by applying the saddle‐point approximation. The nondipole corrections introduce an asymmetry in the photoelectron momentum distribution along the field propagation direction. The asymmetry is quantified by the partial average value of the propagation‐direction momentum component of the photoelectrons and by the normalized difference of the differential ionization rates computed including and excluding the nondipole corrections. Using the saddle‐point approximation, it is investigated how the nondipole corrections change the solutions for direct photoelectrons and how this affects the momentum spectra. The impact of nondipole corrections increases with increasing photoelectron energy. Analysis of the complete photoelectron spectra including both direct and rescattered photoelectrons shows that, in the low‐energy region, a shift against the propagation direction occurs. The partial average of the propagation–direction momentum component in the rescattering region exhibits a plateau structure and also a local minimum structure that was recently observed in an experiment with a linearly polarized laser field (Lin et al., Phys Rev. Lett. 128, 023201 (2022)).

Using the CO molecule as target, we investigate high-order harmonic generation by a bichromatic elliptically polarized laser field. This field consists of two elliptically polarized components with the commensurable frequencies and mutually orthogonal semi-major axes. Both odd and even harmonics are emitted and their ellipticity can be large depending on the values of the laser-field parameters. It is often the case that the ellipticity of subsequent odd and even harmonics is substantially different so that, in order to produce a series of high-order harmonics with similar ellipticity, it is beneficial if the emission of odd or even harmonics is suppressed. In this paper we explore how this can be achieved using the ellipticity of the laser-field components and the relative phase as control parameters. For some values of these parameters it is possible to produce a comb of odd or even harmonics with similar ellipticity. These harmonics can later be employed for various applications the example of which is the generation of an elliptically polarized attosecond pulse train.

Nondipole effects in processes assisted by a THz field having the strength of a few MV/cm can be significant due to its long wavelength. We illustrate this for strong-laser-field-induced ionization assisted by a THz field. To this end, we generalize our strong-field-approximation theory so that it includes the first-order term in a 1/c expansion of the vector potential. We show that in this case, in addition to a shift of the maximum of the photoelectron momentum distribution, the differential ionization probability as well as the cutoff energy can be significantly increased. For an explanation of these unexpected results we use the saddle-point method adjusted to include nondipole effects.

We address ionization of a diatomic molecule by a bichromatic elliptically polarized field with co-rotating components. Using the strong-field approximation we investigate symmetry properties of the photoelectron momentum distribution and explore the minima which appear in the photoelectron spectra. We distinguish two types of minima: (i) two-center interference minima which appear due to the destructive interference of the contributions of two electron wave packets emitted from the two centers of the diatomic molecule and (ii) the one-center minima which are caused by the interference of the parts of the wave packet emitted from the same atomic center at different times. The position of the two-center interference minima depends on the molecular orientation. When a molecular orbital is modelled using the atomic orbitals of a specific parity, the position of the two-center interference minima does not depend on the ellipticity of our driving field. However, when a molecular orbital consists of both odd and even atomic orbitals the interference of their contributions and the position of the minima depend on the ellipticity. The position of the interference minima in the photoelectron momentum plane is confirmed using the saddle-point method. The position and the number of the one-center minima do not depend on the molecular orientation, but they strongly depend on the ellipticity of the field components. Finally, comparing the photoelectron spectra of the CO molecule with the spectra of homonuclear molecules and the NO molecule we show that the electron probability density distribution plays a significant role for the high-energy rescattered electrons.