TIME ORDERING IN ATOMIC-COLLISIONS

We present a general treatment for the effect of time ordering in the many-electron processes induced by charged-particle impact. We show th at time ordering is essential in order to obtain odd power Z terms in the cross section which is necessary to get different Values for posit ively and negatively charged projectiles. A second-order calculation i s performed for the double ionization of helium. The electron-electron interaction is taken into account only by the shake-off term. The int erference between the shake-off and second-order term is responsible o nly for a part of the dependence of the cross sections on the sign of the charge of the projectile.