Above-threshold ionization of diatomic molecules in an intense laser field: Molecular alignment effects

In this work, we study the photoelectron spectra of the diatomic molecule O2 in the direction perpendicular to the laser polarization. Compared with atoms, molecules have additional parameters, e. g., the orientation of the molecule and the symmetry of the initial electronic state, and thus give rise to richer strong-field phenomena. Our simulations show that for parallel and perpendicular alignments of the laser polarization with respect to the molecular axis, the photoelectron spectra display above-threshold ionization (ATI) peaks with a specific 2 (h) over bar omega. separation in which the absorption of an even number of photons is absent. However, for other alignment angles, the even-order ATI peaks appear. According to our analysis, the physical mechanism of the alignment effect of the even-order ATI peaks can be attributed to the symmetry of the molecular ground-state wave function with respect to the photoelectron emission direction.