The production of sub-micron sodium nitrate particles by laser ablation

Laser ablation is a useful source of particles for chemical analysis by ind uctively coupled mass spectroscopy in many applications. Optimum particle t ransport and ionization requires particles with sizes in the range 0.1-2 mu m. Significant questions remain as to the mechanisms behind particle produ ction by laser ablation. In this work, we collect and observe particles pro duced from single-crystal sodium nitrate during irradiation at 1.06 mu m as a function of fluence and explore the possibility that laser-induced fract ure may produce suitable particles. At fluences between 3 and 4 J/cm(2) sin gle laser pulses incident on cleaved samples produce large numbers of fract ure particles as a result of the fracture of undercut cleavage steps. Polis hed samples, lacking cleavage steps, yield few, if any, particles. As the f luence is raised to 5 J/cm(2), a small breakdown plume is observed and larg e melted droplets (similar to 10 mu m in diameter) are produced from both c leaved and polished targets, presumably by spallation of a thick melted lay er. Particle generation by fracture has the potential to produce particles whose composition closely matches the local sample composition.