PROBING ELECTRON CORRELATIONS IN DOUBLE PHOTOIONIZATION OF HE AT INTERMEDIATE ENERGIES

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