Convergent close-coupling calculations of two-photon double ionization   of helium
  We apply the convergent close-coupling (CCC) formalism to the problem of two-photon double ionization of helium. The electron-photon interaction is treated perturbatively whereas the electron-electron interaction is included in full. The integrated two-photon double ionization cross-section is substantially below non-perturbative literature results. However, the pattern of the angular correlation in the two-electron continuum is remarkably close to the non-perturbative time-dependent close-coupling calculation of Hu {\em et al} [J. Phys. {\bf B38}, L35 (2005)] 
A. S. Kheifets and I. A. Ivanov