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. (C) 2002 Elsevier Science B.V. All rights reserved.