Mass transfer in the Frenkel-Kontorova chain initiated by molecule impact

Supersonic Motion of Atoms in an Octahedral Channel of fcc Copper

In this work, the mass transfer along an octahedral channel in an fcc copper single crystal is studied for the first time using the method of molecular dynamics. It is found that the initial position of the bombarding atom, outside or inside the crystal, does not noticeably affect the dynamics of its motion. The higher the initial velocity of the bombarding atom, the deeper its penetration into the material. It is found out how the place of entry of the bombarding atom into the channel affects its further dynamics. The greatest penetration depth and the smallest dissipation of kinetic energy occurs when the atom moves exactly in the center of the octahedral channel. The deviation of the bombarding atom from the center of the channel leads to the appearance of other velocity components perpendicular to the initial velocity vector and to an increase in its energy dissipation. Nevertheless, the motion of an atom along the channel is observed even when the entry point deviates from the center of the channel by up to 0.5 Å. The dissipated kinetic energy spent on the excitation of the atoms forming the octahedral channel is nearly proportional to the deviation from the center of the channel. At sufficiently high initial velocities of the bombarding atom, supersonic crowdions are formed, moving along the close-packed direction ⟨1¯10⟩, which is perpendicular to the direction of the channel. The results obtained are useful for understanding the mechanism of mass transfer during ion implantation and similar experimental techniques.

Atom deposition and sputtering at normal incidence simulated by the Frenkel-Kontorova chain

The impact of a molecule of N atoms with a speed of v_{0} on the free end of the Frenkel-Kontorova chain is numerically simulated. Depending on the values of N and v_{0}, different scenarios of the molecule-chain interaction are observed. Molecules with low speed stick to the chain. At somewhat higher speeds, the molecules bounce off the chain. Further increase in v_{0} results in bouncing off a molecule larger than the incident one. At even higher speed, bouncing of the molecule off the chain takes place simultaneously with the formation of a supersonic crowdion (antikink) propagating along the chain. A very high collision velocity leads to the sputtering of atoms from the chain and the formation of single and multiple supersonic crowdions. Interestingly, the sputtering yield Y as the function of v_{0} demonstrates a nonmonotonous dependence. This is explained by the fact that supersonic crowdions can have a discrete set of propagation velocities. When v_{0} is such that supersonic crowdions are effectively excited, the latter transfer energy deep into the chain, and the sputtering is minimal. For some v_{0} ranges, the formation of supersonic crowdions is suppressed. In these cases, the energy transferred from the impact of the molecule to the chain is spent mainly on the sputtering of atoms. The results obtained qualitatively explain the physics of bombardment of a crystal surface by atomic clusters with applications in physical vapor deposition, ion implantation, ion-beam sputtering, and similar experimental techniques.