Generation of nanoclusters by ultrafast laser ablation of Al: Molecular dynamics study

The laser ablation of materials induced by an ultrashort femtosecond pulse is a complex phenomenon, which depends on both the material properties and the properties of the laser pulse. A combination of molecular dynamics (MD) and momentum scaling model (MSM) methods is applied to a large atomic system to study the process of ultrafast laser-material interactions, behavior of matter in a highly nonequilibrium state, material disintegration, and formation of nanoclusters (NCs). Laser pulses with several fluences in the range from 500J/m2 to 5000J/m2 interacting with a large system of aluminum atoms are simulated. The response of Al to laser energy deposition is investigated within the finite-size laser spot. It is found that the shape of the plasma plume is dynamically changing during expansion. At several tens of picoseconds it can be characterized as a long hollow ellipsoid surrounded by Al atoms and NCs. The time evolution of NCs in the plume is investigated. The collisions between single Al atoms and generated NCs and fragmentation of large NCs determine the fractions of different-size NCs in the plume. The MD-MSM simulations show that laser fluence greatly affects the size distribution of NCs, their polar angles, and magnitude and direction vectors of NC velocities. These results and predictions are consistent, in many aspects, with the experimental data and previous MD simulations.