Effect of symmetry on exchange of one or two electrons in atom collisions

One- or two-electron charge exchange in collisions between an atom and a similar atom but with two less electrons is investigated. It is shown that the probability for one-electron exchange on intersection of the initial term with the ground final term is two times smaller than the usual value due to symmetry, viz. the identity of the atom and ion. The presence of term intersections in the system under consideration alters the physical pattern of two-electron exchange. A channel of step-by-step exchange of two electrons opens up: the first electron is emitted on the first passage of the pseudointersection and the second electron on the second passage. In the case of intersections with excited state terms for such exchange it is necessary that during the time of double passage of a given pseudointersection transfer of the excitation should occur. It is shown that the experimental cross section for two-electron exchange in collisions between negative hydrogen ions and protons can completely be ascribed to this channel