Spectral properties of non-hydrogenic atoms in weak external fields

We study how the ionic core in a non-hydrogenic atom modifies the dynamics of a Rydberg electron in the presence of a weak static external field. We show that such a system is neither regular nor chaotic: its energy levels display unusual statistical properties, intermediate between the standard Poisson and Random Matrix ones. The ionic core acts as a scatterer whose size is comparable to the de Broglie wavelength of the electron, inducing specific quantum effects.