Pulsed laser ablation of binary compounds: effect of time delay in component evaporation on ablation plume expansion

Abstract

Pulsed laser ablation of compound materials is often occurs with delayed evaporation of a less volatile component, however, the effect of the delay on the ablation plume expansion remains virtually unexplored. Here we have performed an experimental and theoretical study of the delayed evaporation effect by the example of the plume produced by nanosecond laser ablation of a gold-silver alloy in vacuum and compared with ablation of pure gold and silver targets. The plume expansion dynamics is investigated by time-of-flight mass spectrometry and direct simulation Monte Carlo (DSMC) while the laser-induced target evaporation is analyzed using a thermal model. A dramatic effect of the delay time on the average kinetic energy of the plume particles, especially for the less volatile gold, is demonstrated and the main collisional processes governing the two-component plume expansion under the conditions of delayed evaporation are revealed. Based on comparison of experimental and DSMC data, the delay of the gold evaporation onset is estimated as app. 0.6 ns. The delayed evaporation is therefore an important factor for correct interpretation of time-of-flight measurements in ablation plumes with components of different volatilities.