Scaled-Energy Spectroscopy of the Rydberg-Stark Spectrum of Helium: Influence of Exchange on Recurrence Spectra

Abstract

We report the first observation of the manifestation of exchange effects in recurrence spectra. Using fast-atom -laser-beam scaled-energy spectroscopy, we have measured the Rydberg-Stark recurrence spectrum of the two spin forms of helium in strong fields. Singlets and triplets are produced by excitation of metastable 1s2s 1,3 S states. We observe pronounced modulations in the triplet spectrum due to interference between hydrogenic and core-scattered combination orbits. The modulations are phase correlated with repeating hydrogenic orbits and not present in singlets or in hydrogen. [S0031-9007(98)06521-1] PACS numbers: 32.60. + i, 03.65.SqA complete theoretical treatment of nonhydrogenic atoms in external fields remains a challenge. Experimental progress has provided substantial information on Rydberg atoms in external fields [1]. Application of the semiclassical description, in the form of closed orbit theory, has been quite successful and is the basis for the field of recurrence spectroscopy [2]. Standard closed orbit theory treats nonhydrogenic effects with quantum defects. Recently, nonhydrogenic effects have been included through core-scattered waves [3] and by an explicit core potential resulting in classical core scattering [4]. These extensions have demonstrated that core scattering gives rise to combination orbits not seen in hydrogen. To date, experimental studies of Rydberg-Stark recurrence spectra have used alkali atoms which contain closed shell ionic cores and have one spin state [5,6]. The present work extends experiment to open shell ionic cores and their role in producing combination orbits. In addition, open shell cores provide polarized spin-state dependent scattering systems that permit experimental access to the influence of electron exchange.The exchange interaction is isolated experimentally by measuring recurrence spectra versus spin state. This provides a tool to examine the role of the Pauli exclusion principle in the semiclassical theory and in the dynamics of Rydberg systems, where it is not well understood. Closed orbits are sensitive to the spin-dependent exchange interaction since the Rydberg electron returns to the nucleus and the charge clouds of the electrons overlap. In helium, when the Rydberg electron samples the core, it moves as if it experiences an attraction or repulsion depending on the relative orientation of the two spins. This introduces a coupling between the spatial variables of the Rydberg electron and the spin state of the atom. How this is revealed in the Stark recurrence spectrum of the prototypical two-electron helium atom is a focus of the present Letter.Extensive data have been obtained for m 1 states at 60 different scaled energies in strong fields ͑20 , n , 30͒ up to scaled action 15. This allows a global view of the spectrum and observation of trajectory evolution as a function of scaled energy. Using this information, we introduce a new analysis technique of integrating recurrence strength of an orbit over its scaled energy range. This permits study of the...

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