Structure of Sodium Rydberg States in Weak to Strong Electric Fields

We present results of an experimental survey of the $n=15\mathrm{}$ state of Na in electric fields from 0 to 4 kV/cm. Both linear and higher-order Stark effects play important roles. States with low $\mathrm{}\left|m\right|\mathrm{}$ show marked differences from hydrogenic behavior, particularly in the vicinity of level crossings. Computed values for the energy levels are in good agreement with the data. The experimental method provides a simple technique for populating selected Stark or angular momentum levels of a Rydberg state.