Circular Rydberg-state spectroscopy

We report the first observation of a resonance induced by a submillimeter wave between circular atomic Rydberg states of maximum angular momentum ($n=25\mathrm{}$, $m_l=24\mathrm{}\to n=24\mathrm{}$, $m_l=23\mathrm{}$ transition in lithium). This experiment is the first step towards a possible measurement of the Rydberg constant directly in frequency units. The Stark shift of the resonance has also been studied and its quadratic character put in evidence.