Theory of rapidly oscillating electron angular distributions in slow ion-atom collisions

A general expression for the ionization amplitude in slow ion-atom collisions is derived. The expression is inverted to obtain adiabatic electronic wave functions at complex values of the internuclear distance. It is shown that beating between sigma and pi components of electronic wave functions gives rise to rapid oscillations of electron angular distributions with ion velocity nu. These rapid oscillations measure the real part of that eigenvalue whose imaginary part gives the well-known Wannier exponent.