PLASMA SHIELDING EFFECT IN LASER-ABLATION OF METALLIC SAMPLES AND ITSINFLUENCE ON LIES ANALYSIS

Line emission from plasmas formed during laser ablation of steel in ai r at atmospheric pressure has been measured for varying pulse energies and focusing distances. By using a Nd:YAG laser with pulse energies i n 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 focusi ng distance and the pulse energy is related to shielding effects of th e plasma produced, which depend on the type of absorption wave obtaine d at different power densities during the initial formation process. T he influence of these effects on the elemental analysis by LIBS is stu died by obtaining the precision of nickel content determination in ste el 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 focusi ng the laser beam 12 nm below the sample surface for a 100-nm focal-le ngth lens. (C) 1998 Elsevier Science B.V.