Photoabsorption of Nonhydrogenic Rydberg Atoms in a Magnetic Field: Effects of Core-Scattered Classical Orbits.

The physics of multielectron atoms in strong external fields is substantially more complex than that of hydrogen. In particular, puzzling new resonances were recently observed in experimental photoabsorption spectra of diamagnetic helium and in quantum mechanical calculations of diamagnetic Rydberg atoms. We compute spectra in the framework of periodic-orbit theory and show that the new resonances can be quantitatively explained by different classes of core-scattered classical orbits.