Alignment-dependent ionization of molecules in near-circularly polarized intense laser fields

The alignment-dependent photoelectron spectrum is a valuable tool for mapping out the electronic structure of molecular orbitals. However, this approach may not be applicable to all molecules, such as CO₂, as the ionization process in a linearly polarized laser field involves contributions from orbitals other than the highest occupied molecular orbital (HOMO). Here, we conducted a theoretical investigation into the ionization process of N₂ and CO₂ in near-circularly polarized laser field using the Coulomb-corrected strong-field approximation (CCSFA) method for molecules. In particular, we introduced a generalized dressed state into the CCSFA method in order to account for the impact of the laser field on the molecular initial state. The simulated alignment-dependent photoelectron momentum distribution (PMD) of the two molecules exhibited markedly disparate behaviors, which were in excellent agreement with the previous experimental observations reported in Phys. Rev. A102, 013117 (2020). Our findings indicate that under a near-circularly polarized laser field, the alignment-dependent PMD of molecules is primarily sourced from the HOMO, in contrast to the situation under a linearly polarized laser field. Moreover, a satisfactory correlation between the alignment-dependent angular distribution and the orbital symmetry was observed, which suggests an effective approach for molecular orbital imaging.