Rydberg–Ritz analysis and quantum defects for Rb and Cs atoms on helium nanodroplets

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A Rydberg–Ritz approach is used for the interpretation of Rb–He and Cs–He Rydberg states and Rydberg series. Variations of the quantum defects within a Rydberg series give insight into the interaction between the alkali atom’s valence electron and the superfluid helium droplet. A screening of the valence electron from the alkali atom core by the helium droplet is observed for high Rydberg states. For states with lower principal quantum number, the effect decreases and the quantum defects are found to lie closer to free atom values, indicating an increased probability for the electron to be found inside the alkali atom core. An investigation of the spin–orbit splitting of the Cs–He nP(2Π) components reveals that the splitting of the lowest 2Π states is more atom-like [Hund’s case (c) coupling] than at higher n states [Hund’s case (a) coupling]. In addition, we report a detailed study of the droplet size dependence of Ak–He Rydberg series on the example of the Rb–He D(Δ) series. Higher Rydberg states of this series are strongly redshifted, which is also related to the screening effect.