SCALED-ENERGY SPECTROSCOPY OF THE RYDBERG-STARK SPECTRUM OF HELIUM - INFLUENCE OF EXCHANGE ON RECURRENCE SPECTRA

We report the first observation of the manifestation of exchange effec ts in recurrence spectra. Using fast-atom-laser-beam scaled-energy spe ctroscopy, we have measured the Rydberg-Stark recurrence spectrum of t he two spin forms of helium in strong fields. Singlets and triplets ar e produced by excitation of metastable 1s2s(1,3)S states. We observe p ronounced modulations in the tripler spectrum due to interference betw een hydrogenic and core-scattered combination orbits. The modulations are phase correlated with repeating hydrogenic orbits and not present in singlets or in hydrogen.