TEA-CO_(2) Laser-Induced Shock Wave Plasma Modulated by Wires and Needles Placed in Front of the Target at Low Pressure

摘要

It has been widely reported in the literature that shock wave is induced by focusing laser light on a solid surface. However, the possible role of shock wave excitation of the ablated atoms had not been studied until it was demonstrated experimentally that secondary plasma favorable to spectrochemical analysis could be produced when a high-power pulsed laser was focused on a solid target in reduced air pressures. Based on subsequent studies using time-resolved emission spectrometry, we have proposed a model in which shock wave plays an important role in explaining the characteristics of the secondary plasma emission. The most salient ingredients in this model are the compression process in the shock wave front, which converts the kinetic energy of the propelling atoms to thermal energy responsible for the atomic excitation in the secondary plasma in low pressure gas, and hence the associated plasma emission. Other possible models proposed to explain the excitation mechanism in the secondary plasma are the electron collision model and the electron-ion recombination model. Recently, we have produced experimental evidence to support our shock wave model from the following two experiments. One is the simultaneous detection of the density jumps due to the propagation of shock wave and the emission front of the secondary plasma emission. The other experiment involves the simultaneous measurement of time profiles of charge current and the spatially integrated emission of the secondary plasma.