Millimetre wave spectroscopy of high Rydberg states

We have recently developed high-resolution vacuum ultraviolet laser sources and combined these with millimetre waves in double-resonance experiments to achieve a spectral resolution of up to 60 kHz in the spectra of high Rydberg states. The article describes the main features of our experimentalprocedure and presents studies in which we have used millimetre wave spectroscopy (a) to obtain information on the energy level structure, including the spin±orbit and hyper®ne structure, of atomic Rydberg states at high principal quantum numbers n, (b) to record spectrally resolved spectra of the high Rydberg states (n ¶ 100) detected in pulsed-®eld-ionization zero-kinetic-energy photoelectron spectra, (c) to measure stray electric ®elds and ion concentrations in the gas phase, (d) to test and improve the selectivity of the electric ®eld ionization of high Rydberg states and (e) to observe for the ®rst time the hyper®ne structure in high-n, low-l molecular Rydberg states.