Spectroscopy of Rydberg Atoms atn≈500: Observation of Quasi-Landau Resonances in Low Magnetic Fields

We have observed barium Rydberg states with principal quantum numbers up to $n=520\mathrm{}$. Such highly excited states allow quantitative in situ measurements of small electric fields on the order of tens of microvolts per centimeter. Because of the effective suppression of stray electric fields we have observed quasi-Landau resonances in external magnetic fields on the order of tens of gauss. The energy dependence of the spacing between neighboring quasi-Landau resonances was found to be in excellent agreement with calculations involving classical trajectories.