Addentum to "Millimeter spectroscopy in sodium Rydberg states"

Using well-stabilized microwave sources in the extended frequency range 50-315 GHz, we have observed very narrow millimeter resonances between Rydberg levels of sodium with principal quantum number $n$ ranging from 23 to 41. Measurement of the resonance frequencies yields improved values for quantum defects and $P$-level fine-structure intervals.