Thermodynamic and spectroscopic properties of Nd:YAG–CO2 Double-Pulse Laser-Induced Iron Plasmas

a r t i c l e i n f o Keywords: Laser-induced breakdown spectroscopy Double pulse LIBS Laser ablation Laser heating Laser-induced particles Double-Pulse Laser-Induced Breakdown Spectroscopy of iron using both Nd:YAG and TEA–CO 2 lasers has been investigated to better understand mechanisms of signal enhancement. The signal dependence on the delay between the two laser pulses shows an enhanced signal when the CO 2 laser pulse interacts with the sample before the Nd:YAG pulse. Signal kinetics and a simple model of sample heating by the CO 2 pulse show that the enhancement during the first 700 ns is due primarily to sample heating. Images of the sample surface after ablation as well as time-integrated pictures of the plasma suggest that particles are ejected from the surface during the first microseconds after the arrival of the CO 2 pulse and provide fuel for the subsequent plasma created by the Nd:YAG laser. Optical emission spectroscopy of laser-induced plasmas is a powerful spectroscopic diagnostic technique for sensing and other applications because of its ability to identify all types of materials (solid, liquid, gas), at a close distance as well as in a stand-off configuration. Despite the increasing popularity of this technique (also called LIBS — Laser-Induced Breakdown Spectroscopy) its sensitivity and precision are relatively poor compared to other well established analytical techniques. This is because of the uncertainty in laser energy coupling to the sample, significant matrix effects and relatively high background signal in the atomic and ionic spectra, among other reasons. One approach to improving sensitivity (by increasing the signal-to-continuum background ratio) is the use of a double-pulse configuration (DP– LIBS). The aim of DP–LIBS is to improve the coupling of the laser energy to the target and to the ablated material, leading to a higher number of emitters from the analyzed sample in the plasma. Since the first demonstration of double-pulse LIBS in 1969 by Piepmeier and Malmstadt [1], it has been demonstrated that two laser pulses for LIBS lead to enhanced emission intensities, longer plasma lifetimes and higher plasma temperatures. To optimize these effects, several combinations have been proposed involving various geometries (angle between the laser beams), different laser wavelengths and/or pulse durations and interpulse delay, as well as different pulse energies. A collinear approach combines two pulses following the same path and focused upon the same point of the sample. The orthogonal configuration uses multiple optics to …