Plasma shielding effect in laser ablation of metallic samples and its influence on LIES analysis

Line emission from plasmas formed during laser ablation of steel in air at atmospheric pressure has been measured for varying pulse energies and focusing distances. By using a Nd:YAG laser with pulse energies in the range of 25-250 ml, values of the power density up to 710 GW/cm(2) are obtained. The variation of emission intensities with the focusing distance and the pulse energy is related to shielding effects of the plasma produced, which depend on the type of absorption wave obtained at different power densities during the initial formation process. The influence of these effects on the elemental analysis by LIBS is studied by obtaining the precision of nickel content determination in steel samples. At each pulse energy, a focusing position below the sample can be found that produces maximum intensity and higher precision. A limit of detection of 64 ppm of nickel in steel was obtained by focusing the laser beam 12 nm below the sample surface for a 100-nm focal-length lens. (C) 1998 Elsevier Science B.V.