Direct double photoionization of atomic sodium

Double photoionization of atomic Na has been investigated by combining the results of photoelectron and photoion spectroscopies from synchrotron radiation measurements in the photon energy range between 50 and 135 eV. Distinction with regard to the double-photoionization mechanism (one-step or two-step processes) and to final-state configuration allowed an accurate determination of the relative cross section for one-step direct double photoionization. Above 71 eV photon energy double photoionization is dominated by 2s photoionization followed by Auger decay (two-step process) whereas direct double photoionization accounts for only \ensuremath{\simeq}20% of double photoionization.