  Molecular dynamics simulations have been used to study the driving force of
ion irradiation induced interfacial mixing in metal bilayers in which the
relative mass of the constituents is considerable. We find no apparent effect
of chemical forces, such as heat of mixing or cohesive energy up to 8 keV ion
energy, although a considerable number of liquid and high energy particles (hot
atoms) persist up to even 20 ps during the thermal spike. This result is in
direct conflict with the widely accepted theory of thermal spike mixing
(chemical interdiffusion model). The supersaturation of vacancies also occurs
and which induces a thermally activated intermixing of the lighter constituent
of the bilayer. The delay and the decoupling of the intermixing of the light
constituent is explained as a backscattering effect at the interface: the
interface acts as a diffusional barrier for high energy particles due to the
large difference in atomic masses. The heavier atoms are predominantly ejected
to the overlayer at the beginning of the thermal spike while the light atoms
are injected to the bulk at the beginning of the cooling period (in Ti/Pt) or
during the thermal spike with some time delay (Al/Pt).
