Ejection and deposition of atomic clusters at a liquid copper–vapor interface under conditions of normal evaporation

Molecular dynamics simulations of equilibrium liquid–vapor copper systems and non-equilibrium impacts of copper monomers and dimers on a liquid copper film are performed to reveal the mechanisms of ejection and deposition of clusters from liquid metals under conditions of normal evaporation. A lifetime-based retrospective cluster identification algorithm is developed to identify long-lived stable clusters in the vapor phase. It is found that clusters of multiple sizes are ejected directly from the surface, and the mass fraction of monomers in the evaporation flux can be as small as 65%. This result is explained by collective interaction of atoms in metals, when the ejection of a cluster can be energetically more favorable than the ejection of individual monomers. The velocity distributions of ejected clusters can be approximated by the Maxwell–Boltzmann distributions with the temperature parameter smaller than the system temperature. The rotational and vibrational energy distributions of evaporated clusters are close to equilibrium distributions at the system temperature, indicating the absence of energy equipartition during cluster detachment from the surface. The equilibrium condensation coefficient of monomers varies from 0.74 at 3500 K to 0.68 at 5000 K. In the non-equilibrium simulations, the condensation probability of monomers and dimers is determined for broad ranges of the impact speed, angle of incidence, and surface temperature and then used to calculate the non-equilibrium condensation coefficient. A model of cluster evaporation and condensation that can be used as boundary conditions in gas kinetic simulations is developed. The key findings of this work indicate that the direct ejection of clusters from a liquid surface can be an important source of dimers in metal vapor plumes and jets.