Different escape modes in two-photon double ionization of helium
  The quadrupole channel of two-photon double ionization of He exhibits two distinctly different modes of correlated motion of the photoelectron pair. The mode associated with the center-of-mass motion favors a large total momentum which is maximazed at a parallel emission. However, the mode associated with the relative motion favors an antiparallel emission. This difference is manifested in a profoundly different width of the angular correlation functions corresponding to the center-of-mass and relative motion modes. 
A. S. Kheifets, I. A. Ivanov and Igor Bray