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

Laser-induced breakdown spectroscopy of silicon was performed using a nanosecond pulsed frequency doubled Nd : YAG (532 nm) laser. The temporal evolution of the laser ablation plumes was characterized under a range of low pressures. Electron densities were determined from the Stark broadening of the Si (I) 288.16 nm emission line and were found to be in the range 2.79 x 10(16) cm(-3) to 5.59 x 10(19) cm(-3). Excitation temperatures of 9000-21 000K were calculated using the Si (I) 288.16 nm emission line to continuum ratio. The morphology of the laser plume, observed with respect to time, was seen to be strongly dependent on the ambient pressure. The density and temperature of the plasma were also found to vary critically with plasma morphology. Three ambient pressure regimes were identified where the plasma evolution was observed to differ markedly. Requirements for the existence of local thermal equilibrium conditions in the laser-induced plasmas are discussed with respect to these results.