The Effect of Selective Vaporization on TEA CO2 Laser Induced Shock Wave Plasma.

Abstract

TEA CO2 laser (130mJ, 100ns) and Nd-YAG laser (50mJ, 8ns) pulses were employed to generate secondary plasma from brass samples in air under a reduced pressure of 1 Torr. The vaporization effects on Zn and Cu were studied in terms of their emission characteristics for the two cases. In the case of TEA CO2 laser, the emission intensities of copper spectral lines are extremely low, compared to the zinc spectral lines, indicating a highly selective-vaporization effect. The time-profiles of the emission intensities of ZnI 481.0nm and CuI 327.4nm spectral lines observed near the target surface clearly showed that vaporization of Cu atoms occurred later and continued for a long time at a relatively low gushing speed, while the vaporization of Zn atoms took place sooner with a higher gushing speed. It was shown that only Zn atoms form a shock front, and Cu atoms are left behind the shock-wave and are not excited. This selective vaporization process does not occur at a laser power density considerably higher than the threshold for the plasma generation as demonstrated by the use of Nd-YAG laser of higher power density. This phenomenon of selective vaporization described in the present paper also supports our laser-induced shock wave model.