Spectral properties of Atoms in Fields: A Semiclassical Analysis

We develop a semiclassical theory for the spectral rigidity of non-hydrogenic Rydberg atoms in electric fields and evaluate the significant deviations from the well-known Poissonian behaviour in the hydrogenic case. The resulting formula is shown to be in excellent agreement with the exact quantal result. We also investigate diamagnetic atoms; we find that, in contrast to the classically integrable atoms, diffraction has a small effect on the spectral rigidity in the classically chaotic atom. We show our predictions can also be of use in the mixed phase space regime.