Study of Doubly Excited States of Helium –Excitation of 1/3S Metastable Helium Atoms

Doubly excited states of helium are a prototype system to study electron correlation in atoms. While first evidence of their existence dates to the early 20th century, only their spectroscopic characterization by Madden and Codling [1] and the accompanying interpretation of the spectrum by Cooper et al. [2] have been subject to various systematic experimental [3] and theoretical studies [4]. In the nineties, development of synchrotron radiation facilities and soft x-ray monochromators have permitted high resolution studies of Rydberg series converging to higher thresholds below the doubleionization limit. In this work, the photoionization spectra of doubly exited states of helium atom following excitation of triplet and singlet metastable atoms are presented for the first time. The first members of three dipole allowed 1/3Po series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s 3S) and singlet (from 1s2s 1S) manifolds. The transitions from metastable states show a drastically different intensity ratio with respect to transitions from the ground state. Theoretical predictions are in general agreement with theory, when corrected for fluorescence decay, with some exceptions.