Probing the Floquet structue of highly excited atoms using ac/dc Stark spectroscopy

The complex network of atomic states, Floquet or ‘‘dressed’’ states, that results from application of an external ac field to an atomic system is investigated using precision dc Stark spectroscopy of Rydberg atoms. Transitions to the Floquet sidebands of isolated Stark levels, both those possessing a permanent dipole moment (field‐free degenerate) and those with no dipole moment (field‐free non‐degenerate), are found to have absorption intensities in good agreement with theory. In addition, it is found that the density of these states that is associated with a manifold of hydrogenic dc Stark states contains gaps at certain values of this field. The gaps manifest themselves in the Stark spectrum. The observed effect is most easily visualized using a classical model of the atom in which the near‐Keplerian orbit of the valence electron is treated as the dynamical entity, rotating about the applied dc field.