Phase explosion and its time lag in nanosecond laser ablation

This work investigates interface kinetics during nanosecond pulsed excimer ablation of a metal. During laser heating, the surface can reach a temperature higher than the normal boiling point, resulting in a superheated, metastable state. Phase explosion occurs as the temperature approaches the thermodynamic critical point, which turns the melt into a mixture of liquid and vapor. However, for phase explosion, there is a certain time needed for a vapor embryo to grow to a critical nucleus, called the time lag of nucleation. This time lag becomes important in ablation induced by nanosecond or shorter pulsed lasers. This paper discusses experiments for investigating non-equilibrium phase change phenomena during nanosecond excimer laser ablation of a metal. Evidences of the metastable state in liquid and phase explosion are presented. The surface temperature–pressure relation is found to deviate from the commonly used equilibrium Clausius–Clapeyron equation. Also, for the first time, the time lag of nucleation during nanosecond laser ablation is found to be around 5 ns.