  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.
