Strong-field ionization of linear molecules by a bicircular laser field: Symmetry considerations

Using the improved molecular strong-field approximation, we investigate (high-order) above-threshold ionization [(H)ATI] of various linear polyatomic molecules by a two-color laser field of frequencies r omega and s omega (with integer numbers r and s) having coplanar counter-rotating circularly polarized components (a so-called bicircular field). Reflection and rotational symmetries for molecules aligned in the laser-field polarization plane, analyzed for diatomic homonuclear molecules in Phys. Rev. A95, 033411 (2017), are now considered for diatomic heteronuclear molecules and symmetric and asymmetric linear triatomic molecules. There are additional rotational symmetries for (H) ATI spectra of symmetric linear molecules compared to (H) ATI spectra of the asymmetric ones. It is shown that these symmetries manifest themselves differently for r + s odd and r + s even. For example, HATI spectra for symmetric molecules with r + s even obey inversion symmetry. For ATI spectra of linear molecules, reflection symmetry appears only for certain molecular orientation angles +/- 90 degrees -jr180 degrees /(r + s) (j integer). For symmetric linear molecules, reflection symmetry appears also for the angles -jr180 degrees /(r + s). For perpendicular orientation of molecules with respect to the laser-field polarization plane, the HATI spectra are very similar to those of the atomic targets, i.e., both spectra are characterized by the same type of the (r + s)-fold symmetry.