Strong-field ionization of homonuclear diatomic molecules by a bicircular laser field: Rotational and reflection symmetries

Abstract

We investigate above-threshold ionization (ATI) of homonuclear diatomic molecules by the so-called bicircular field using the improved molecular strong-field approximation. Bicircular field is a two-color laser field having coplanar circularly polarized counter-rotating components of frequencies $r\ensuremath{\omega}$ and $s\ensuremath{\omega}$, with $r$ and $s$ integers. Our analysis includes the high-energy part of the corresponding spectra, i.e., high-order ATI (HATI). The obtained molecular (H)ATI spectra are more complicated than the corresponding atomic spectra. We have identified four symmetries which are satisfied in (H)ATI of homonuclear diatomic molecules. Two of these symmetries are general rotational symmetries valid both for direct and rescattered HATI electrons. The remaining two symmetries are reflection symmetries valid only for the direct ATI electrons. Analytical proof of these symmetries is also given. These symmetries are illustrated using numerical examples of HATI spectra of the ${\mathrm{N}}_{2}$ molecule for various molecular orientations.