Trapping, reflection, and fragmentation in a classical model of atom-lattice collisions

A classical one-dimensional model of the collision of an atom of mass M with a cold, semi-infinite harmonic lattice comprised of identical atoms of mass m is considered. In the model, the interactions between the incident atom (adatom) and the lattice are described in terms of a truncated parabolic potential by which the adatom is harmonically bound to the lattice at short distances but evolves freely when its distance is larger than a critical length R(c). The dynamics of the adatom colliding with an infinitely cold lattice is studied as a function of the initial velocity of the adatom. In order to determine whether the colliding atom is bound or reflected from the lattice in the asymptotic time limit, "secondary" collision events in which the incident atom leaves and reenters the interaction zone of the lattice are carefully considered. It is demonstrated that secondary collisions anticipated to be important for heavy adatoms (mu=m/M<1) also occur in the case of light adatoms (mu > or = 1). It is shown that the neglect of secondary collisions leads to an underestimation of the lower energy bound for adatom reflection of roughly 10% for mu close to 1. By generalizing the model to allow for the breaking of lattice bonds, the phenomenon of collision-induced lattice fragmentation is investigated.