Depth-resolved analysis by laser-induced breakdown spectrometry at reducedpressure

The 581 nm output from a dye laser in a fluence range between 2.86 and 11.4 7 J cm(-2) was used to ablate pure Zn and Fe foils. The average ablation ra te (AAR, pm per shot) was calculated for different experimental variables ( buffer gas, pressure, laser fluence and focal conditions). Deposition of pr eviously ablated material in the ablation crater results in large variation of tbe observed AAR values. This effect was observed in air and argon buff er gases at atmospheric pressure. The situation is largely alleviated at re duced pressure due to free expansion of the ablated material. Under these c ircumstances the capability of laser-induced plasmas to resolve interfacial structures is improved. The effect on depth-resolved studies was checked w ith a commercial Zn-coated steel sample. Doe to the Gaussian-like energy di stribution of the incident laser beam, the material is ablated to produce a conical crater. This Pact ensures that the Zn signal remains for a longer time because the ablated region spreads over the edge gradually. At low pre ssure the emission peaks are better defined and the background becomes flat . However, these conditions produce also the lowest net intensities and som e peaks are not detected. An Ar atmosphere produces more intense spectral l ines at both pressure levels. Best analytical results were obtained at redu ced pressure, with a slight improvement in depth resolution in the presence of Ar. (C) 1998 John Wiley & Sons, Ltd.