Dynamics of laser ablation in gaseous and plasma environments.

Abstract

This dissertation experimentally studies the hydrodynamics, gas-phase and plasma phase chemical reactions between excimer laser ablated Al neutral atoms interacting with background inert gases/plasmas (Ar and He) and reacting gases/plasmas (air and O$\sb2$). Al metal and Al$\sb2$O$\sb3$ (alumina) ceramic are used as targets. Dye-laser-resonance-absorption photography (DLRAP) was used to obtain initial hydrodynamic expansion velocity of laser ablated Al neutral atoms from alumina versus Al targets. Resonant-Holographic-Interferometry (RHI) was used to measure the line-density and total number of Al neutral atoms ablated from alumina, which were at least 10-20 times higher than the number obtained in Al ablation. No distinguishable depletion in the total number of Al neutral atoms was found in both alumina and Al ablation under non-reacting gas and ($\sim$50 Watts) RF-generated plasma environments, nor in reacting gas (air and 9.92% O$\sb2$, balance argon) and RF-generated plasma environments by using the RHI diagnostic. Al neutral atoms were observed to travel slightly faster in the plasma than in gas. The same emission intensity of AlO molecules was observed by time-integrated emission spectroscopy, when ablated from alumina in RF-generated oxygen plasma or in gas. It is believed that the RF plasma density ($\sim$10$\sp9$cm$\sp{-3}$) was too low to make a significant difference in O$\sb2$ dissociation. Both time- integrated and time-resolved spectroscopy showed that AlO molecular emission intensity was increased in the (higher density) high voltage, high power ($\sim$0.4 MW) capacitive discharge oxygen plasma as compared to oxygen gas; a decrease in Al neutral emission occurred for alumina ablation in the oxygen discharge plasma, suggesting depletion. Similarly, AlO molecular emission was observed in Al metal ablation with oxygen discharge plasma only, but not in oxygen gas nor non-reacting gas and plasma (Ar) environments.