Photodouble ionization of helium at an excess energy of 40 eV

Angular correlation in the two-electron continuum of the He double photoion ization has been studied, both experimentally and theoretically, for equal and unequal energy sharing conditions at the photon energy of 40 eV above t he threshold. The triple differential cross sections have been measured in the plane perpendicular to the photon direction using the multi-coincidence detection technique of Reddish et al at the Daresbury Synchrotron Radiatio n Source. Recent modifications to the bending-magnet beam line allowed an e ffective cancellation of the circular polarization at the target, leaving a relatively high degree of linear polarization (Stokes parameters: S-3 = S- 2 congruent to 0,S-1 = 0.8). The measured cross sections are compared with the calculations using the 3C method of Maulbetsch and Briggs and the conve rgent close coupling method of Kheifets and Pray. Good agreement between th eory and experiment has been found in most cases, except for the unequal en ergy sharing when one of the escaping electrons is detected in a direction close to the polarization axis.