The effect of ambient pressure on laser-induced silicon plasma temperature, density and morphology

Abstract

Laser-induced breakdown spectroscopy of silicon was performed using a nanosecond pulsed
\nfrequency doubled Nd :YAG (532 nm) laser. The temporal evolution of the laser ablation
\nplumes was characterized under a range of low pressures. Electron densities were determined
\nfrom the Stark broadening of the Si (I) 288.16 nm emission line and were found to be in the
\nrange 2.79 × 1016 cm−3 to 5.59 × 1019 cm−3. Excitation temperatures of 9000–21 000K were
\ncalculated using the Si (I) 288.16 nm emission line to continuum ratio. The morphology of the
\nlaser plume, observed with respect to time, was seen to be strongly dependent on the ambient
\npressure. The density and temperature of the plasma were also found to vary critically with
\nplasma morphology. Three ambient pressure regimes were identified where the plasma
\nevolution was observed to differ markedly. Requirements for the existence of local thermal
\nequilibrium conditions in the laser-induced plasmas are discussed with respect to these results.