Attosecond electron-electron collision dynamics of the four-electron   escape in Be close to threshold
  We explore the escape geometry of four electrons a few eV above threshold following single-photon absorption from the ground state of Be. We find that the four electrons leave the atom on the vertices of a pyramid instead of a previously-predicted tetrahedron. To illustrate the physical mechanisms of quadruple ionization we use a momentum transferring attosecond collision scheme which we show to be in accord with the pyramid break-up pattern. 
A. Emmanouilidou and H. Price
physics.atom-ph