Figure 2. In their electronic ground state alkali atoms and molecules are located at the surface of helium nanodroplets (left). Upon excitation of a high Rydberg state of the atom, the ion core is expected to immerse into the helium droplet while the electron is orbiting outside (right).
Similar to free atoms, Rydberg states on helium droplets follow a systematic behavior which allows an organization of these states into Rydberg series, which gives much more information on the nature of these systems than the investigation isolated transitions [Lackner2011,Lackner2012]. Quantum defects and ionization thresholds obtained within a Rydberg-Ritz approach reveal insight into the screening mechanism that shields the Rydberg electron from the alkali ion core and demonstrate the attractive interaction between ion core and the droplet [Lackner2013].