Orbital effects in strong-field Rydberg state excitation of N₂, Ar, O₂ and Xe

Rather than being freed to the continuum, the strong-field tunneled electrons can make a trajectory driven by the remaining laser fields and have certain probability to be captured by the high lying Rydberg states of the parent atoms or molecules. To explore the effect of molecular orbital on Rydberg state excitation, the ellipticity dependence of Rydberg state yields of N₂ and O₂ molecules are experimentally investigated using cold target recoil ion momentum spectroscopy and are compared with their counterpart atoms Ar and Xe with comparable ionization potentials. We found the generation probability of the neutral Rydberg fragment O₂* was orders of magnitude higher than that of Xe* due to the butterfly-shaped highest occupied molecular orbital of O₂. Meanwhile, our experimental and simulation results reveal that it is the initial momentum distribution (determined by the detailed characteristics of orbitals) that finally leads to the tendency that the Rydberg state yield of O₂ (Ar) decreased slower than that obtained for Xe (N₂) when the ellipticity of the excitation laser field is increased.