Deep Ablation and Depth Profiling by Laser-Induced Breakdown Spectroscopy (LIBS) Employing Multi-Pulse Laser Excitation: Application to Galvanized Steel

The potential of a multi-pulse (MP) laser excitation scheme for deep stratigraphy of electrolytically galvanized steel using laser-induced breakdown spectrometry (LIBS) has been evaluated. For this purpose, a commercial electro-optically (EO) Q-switched Nd:YAG laser was employed, where by reducing the delay between the Q-switch opening and the flash lamp, a train of pulses (up to 11) separated by approximately 7.40 mu s was generated during one lamp flashing. Plasma emission after each individual laser pulse of the MP sequence was detected by a spectrograph equipped with an intensified charge-coupled device (iCCD) detector. With NIP excitation, the ablation efficiency was increased tenfold on iron sample and 22.5-fold on zinc material with respect to dual-pulse or single-pulse excitation. The LIBS signal generated by MP excitation shows an analogous enhancement. Although the total energy per shot delivered to samples was only 60 mJ, it was possible using LIBS to measure the sample stratigraphy up to depths of 90 mu m on zinc-coated steel sheets. A satisfactory agreement between the Zn thickness determined by the MP-LIBS system and data from the manufacturer has also been obtained.