Non-Sequential Double Ionization is a Completely Classical Photoelectric   Effect
  We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities ($I \ge 10^{14}$ watts/cm$^2$) purely classical correlation is strong enough to account for all of the main features observed in experiments to date. 
Phay J. Ho, R. Panfili, S.L. Haan, and J.H. Eberly