Probing electron correlations in double photoionization of He at intermediate energies

The ratio of double-to-single ionization of He has been measured between 280 and 1210 eV to investigate its behavior in this partially unexplored region. These measurements, compared with the most recent theories of Pan and Kelly (private communication) and of Hino [Phys. Rev. A 47, 4845 (1993)], show the importance of including not only ground-state but also final-state correlations, in contrast to the high-energy behavior discussed by Dalgarno and Sadeghpour [Phys. Rev. A 46, R3591 (1992)] where consideration of final-state correlations proves inessential. Our intermediate-energy results also appear to indicate the importance of including higher-order effects in the theory.