We investigate high-order above-threshold ionization of diatomic molecules and their companion atoms by linearly polarized strong laser field using improved molecular (atomic) strong-field approximation. The most noticeable feature of the molecular spectra is the existence of minima that are absent in atomic case. We have derived an analytical formula for their position which is independent of the molecular symmetry.

We investigate high-order above-threshold ionization (HATI) of diatomic molecules having different symmetries by an elliptically polarized laser field using the modified molecular strong-field approximation. The yields of high-energy electrons contributing to the plateau region of the photoelectron spectra strongly depend on the employed ellipticity. This is more pronounced if the major axis of the polarization ellipse is parallel or perpendicular to the molecular axis and at the end of the high-energy plateau. For the ${\text{O}}_{2}$ molecule (characterized by ${\ensuremath{\pi}}_{\text{g}}$ symmetry) the maximum yield is observed for some value of the ellipticity $\ensuremath{\epsilon}$ different from zero. On the other hand, in the same circumstances, the ${\text{N}}_{2}$ molecule $({\ensuremath{\sigma}}_{\text{g}})$ behaves as an atom, i.e., the yield is maximum for $\ensuremath{\epsilon}=0$. These characteristics of the photoelectron spectra remain valid in a wide region of the molecular orientations and laser peak intensities. The symmetry properties of the molecular HATI spectra are considered in detail: by changing the molecular orientation one or other type of the symmetry emerges or disappears. Presenting differential ionization spectra in the ionized electron energy-emission angle plane we have observed similar interference effects as in the HATI spectra governed by a linearly polarized field.

When an electron from a diatomic molecule undergoes tunneling-rescattering ionization, a novel form of destructive interference can be realized that involves all four geometric orbits that are available to the electron when it is freed, because both ionization and rescattering may take place at the same or at different centers. We find experimentally and confirm theoretically that in orientation-averaged angle-resolved high-order above-threshold ionization spectra the corresponding destructive interference is visible for O_{2} but not for N_{2}. This effect is different from the suppression of ionization that is well known to occur for O_{2}.

The strong-field approximation for ionization of diatomic molecules by a strong laser field [D. B. Milo\ifmmode \check{s}\else \v{s}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{}, Phys. Rev. A 74, 063404 (2006)] is generalized to include rescattering of the ionized electron wave packet off the molecular centers (the electron's parent ion or the second atom). There are four rescattering contributions to the ionization rate, which are responsible for the high-energy plateau in the electron spectra and which interfere in a complicated manner. The spectra are even more complicated due to the different symmetry properties of the atomic orbitals of which a particular molecular orbital consists. Nevertheless, a comparatively simple condition emerges for the destructive interference of all these contributions, which yields a curve in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. Here $\ensuremath{\theta}$ is the electron emission angle and ${E}_{{\mathbf{p}}_{f}}$ is the electron kinetic energy. The resulting suppression of the rescattering plateau can be strong and affect a large area of the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane, depending on the orientation of the molecule. We illustrate this using the examples of the $3{\ensuremath{\sigma}}_{g}$ molecular orbital of ${\mathrm{N}}_{2}$ and the $1{\ensuremath{\pi}}_{g}$ molecular orbital of ${\mathrm{O}}_{2}$ for various orientations of these molecules with respect to the laser polarization axis. For ${\mathrm{N}}_{2}$, for perpendicular orientation and the equilibrium internuclear distance ${R}_{0}$, we find that the minima of the ionization rate form the curve ${E}_{{\mathbf{p}}_{f}}\phantom{\rule{0.2em}{0ex}}{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}={\ensuremath{\pi}}^{2}∕(2{R}_{0}^{2})$ in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. For ${\mathrm{O}}_{2}$ the rescattering plateau is absent for perpendicular orientation.

We investigate the laser-wavelength dependence of high-order above-threshold ionization and high-order harmonic generation rates. For both processes we have found pronounced enhancements in the spectra for those wavelengths for which the channel-closing condition is satisfied. These enhancements are explained in two complementary ways. One is in terms of the constructive interference of a large number of long quantum orbits, while the second interpretation is based on threshold anomalies. There are two types of these enhancements, which leave very different footprints in the spectra. When the ground-state parity of the atom changes, the two types are interchanged.

The strong-field approximation for ionization of diatomic molecules by an intense laser field is generalized to include rescattering of the ionized electron off the various centers of its molecular parent ion. The resulting spectrum and its interference structure strongly depend on the symmetry of the ground state molecular orbital. For N2, if the laser polarization is perpendicular to the molecular axis, we observe a distinct minimum in the emission spectrum, which survives focal averaging and allows determination of, e.g., the internuclear separation. In contrast, for O2, rescattering is absent in the same situation.

The very pronounced intensity-dependent enhancements of groups of peaks of high-order above-threshold-ionization spectra of rare-gas atoms are investigated using an improved version of the strong-field approximation, which realistically models the respective atom. Two types of enhancements are found and explained in terms of constructive interference of the contributions of a large number of long quantum orbits. The first type is observed for intensities slightly below channel closings. Its intensity dependence is comparatively smooth and it is generated by comparatively few (of the order of 20) orbits. The second type occurs precisely at channel closings and exhibits an extremely sharp intensity dependence. It requires constructive interference of a very large number of long orbits (several hundreds) and generates cusps in the electron spectrum at integer multiples of the laser-photon energy. An interpretation of these enhancements as a threshold phenomenon is also given. An interplay of different types of the threshold anomalies is observed. The position of both types of enhancements, in the photoelectron-energy--laser-intensity plane, shifts to the next channel closing intensity with the change of the ground-state parity. The enhancements gradually disappear with decreasing laser pulse duration. This confirms the interpretation of enhancements as a consequence of the interference of longmore » strong-laser-field-induced quantum orbits.« less