Scaled-energy spectroscopy of a |M| = 1 Rydberg barium atom in an electric field

We observe strong energy-dependent quantum defects in the scaled-energy Stark spectra for |M|=1 Rydberg states of barium atoms at three scaled energies: \varepsilon = -2.000, \varepsilon = -2.500 and \varepsilon = -3.000. In an attempt to explain the observations, theoretical calculations of closed orbit theory based on a model potential including core effect are performed for non-hydrogenic atoms. While such a potential has been uniformly successful for alkali atoms with a single valence electron, it fails to match experimental results for barium atoms in the 6snp Rydberg states with two valence electrons. Our study points out that this discrepancy is due to the strong perturbation from the 5d8p state, which voids the simple approximation for constant quantum defects of principle quantum number n.