A 240–380 GHz millimetre wave source for very high resolution spectroscopy of high Rydberg states

A millimetre wave source operating in the 240?380 GHz frequency range and its application to the study of high n Rydberg states are described. The level intervals between fine structure components of ns, np and nd (n = 60?80) Rydberg states of krypton have been determined at sub-MHz resolution and accurate effective quantum defects and binding energies have been derived.