THE MULTIELECTRON, HIDDEN CROSSINGS METHOD FOR INELASTIC PROCESSES INSLOW ION ATOM-ATOM COLLISIONS/

A new method is described for studying collision dynamics in slow ion/ atom-atom collisions. It is a generalization of the single-electron, t wo-center hidden crossings method to multielectron systems. This appro ach derives from the analytic properties of energy surfaces and wave f unctions of the adiabatic electronic Hamiltonian when the internuclear distance is extended into the complex plane. The collision dynamics i n the adiabatic limit is determined by the topology of the unique mult ivalued electronic energy surface, particularly by its singular points , the square-root branch points. The surfaces described here have been studied using a complex version of the unrestricted Hartree-Fock and configuration interaction method with all single electron excitations, based on a bivariational principle. Although various inelastic proces ses can be calculated, the method is especially useful for the descrip tion of ionization. We have illustrated this through the calculation o f cross sections for ionization of helium by proton and antiproton imp act, as well as for the collision of two hydrogen atoms. (C) 1998 Amer ican Institute of Physics. [S0021-9606(98)02002-9].