Strong-field approximation for ionization of a diatomic molecule by a strong laser field. III. High-order above-threshold ionization by an elliptically polarized field
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.