Time ordering in atomic collisions

We present a general treatment for the effect of time ordering in the many-electron processes induced by charge-particle impact. We show that time ordering is essential in order to obtain odd power Z-terms in the cross section which is necessary to get different values for positively and negatively charged projectiles. A second-order calculation is performed for the double ionization of helium. The electron-electron interaction is taken into account only by the shake-off term. The interference between the shake-off and second-order term is responsible only for a part of the dependence of the cross sections on the sign of the charge of the projectile. (author)