  We calculate various differential and double differential characteristics of
ionization by a single photon for H$^-$, He and for the two-electron ions with
Z=3,4,5 in the region of the so-called quasi-free mechanism (QFM) domination.
We employ highly accurate wave functions at the electron-electron coalescence
line where coordinates of both ionized electrons coincide. We trace the Z
dependence for the double differential distribution. For all considered targets
we discuss the dependence of the photoelectron energy distribution on the
photon energy. Our calculation demonstrated the rapid decrease of QFM
contribution with increase of the difference in energy of two outgoing
electrons, and with decrease of the angle between two outgoing momenta. As a
general feature, we observe the decrease of QFM contribution with nuclear
charge growth.
