<title>Characterization of laser-induced plasmas for fiber optic probes</title>

Abstract

We recently developed a number of fiber-optic probe designs for remote elemental analyses using laser-induced breakdown spectroscopy (LIBS). The LIBS process is complicated by non- linear ablation and excitation of the sample which hinders the reproducibility of the technique. We are conducting dual-pulse LIBS laser studies to separately investigate the ablation and excitation steps. Signal enhancements as high as five-fold are observed using dual-laser pulses over conventional LIBS. The enhancement is very dependent on the timing between the two laser pulses. It also appears that the sample matrix influences the degree of enhancement observed. Factors affecting the LIBS signal are also being investigated using a time-resolved imaging setup which consists of an acousto-optic tunable filter (AOTF) along with a gated intensified charge- coupled device (ICCD) detector. Using simple imaging techniques, we have found for certain samples that the emission intensity increases and the overall shape of the plasma changes with increasing number of laser shots. The size and shape of the plume appears to be related to the geometry and depth of the crater that is formed in the sample. It is hoped that the results of these studies will be useful in making better fiber-optic LIBS measurements.