DESCRIPTION OF IONIZATION IN THE MOLECULAR APPROACH TO ATOMIC-COLLISIONS

Molecular treatments of atomic collisions have traditionally been rest ricted to low nuclear velocities because of their failure to reproduce the fall of the capture cross sections at higher velocities. The limi tation has recently been seen to be due to their description of ionizi ng processes. This feature is shown here to be a general one for multi charged ion-atom collisions. Its origin and characteristics are descri bed and illustrated for the prototypical Li3+SH(1s) reaction. Ionizati on appears as a result of the inertia of the electron cloud to adiabat ically follow the nuclear motion. This gives rise to nonadiabatic tran sitions, which represent an ionizing flux whenever the nuclear velocit y is high enough that the energy of the traveling molecular orbitals i nvolved is positive in both moving atomic reference frames. Two strong ly connected mechanisms appear, corresponding to the relative translat ional and rotational nuclear motions. Because of the finiteness of the basis, these mechanisms terminate with unphysical trapping effects. W hile interesting per se, knowledge of these features is also useful wi th respect to improving molecular treatments of atomic collisions with the addition of pseudostates.