Comparative study on atomic and molecular Rydberg-state excitation in strong infrared laser fields

Rydberg-state excitation of atoms in strong infrared laser fields provides a new complementary aspect of the perspective of atom--strong field interactions. In this article, we perform an experimental and theoretical study on the corresponding process of diatomic molecules, ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$. We show that neutral molecules can also survive strong 800-nm laser fields in high Rydberg states, while their behavior is remarkably different in comparison with their companion atoms, Ar and Xe. The Rydberg excitation of ${\mathrm{N}}_{2}$ generally behaves similarly to Ar, while that of ${\mathrm{O}}_{2}$ is more significantly suppressed than the ionization compared to Xe in a high intensity region, which can be understood in the frame of a semiclassical picture, together with their different structures of molecular orbitals. However, distinct quantum features in the Rydberg excitation processes that are apparently beyond the semiclassical picture have been identified, i.e., the less suppressed probability of ${\mathrm{O}}_{2}$ at low intensity and the oscillation behavior of the ratio between ${\mathrm{N}}_{2}$ and Ar, indicating that our understanding of the relevant physics is still far from complete.