| After almost 50 years of development, laser ablation has become an important technique for thin film deposition, micromaching and chemical analysis. In the last decade, researchers have utilized laser ablation for nano-material production, laser surgery and chemical imaging. Many of the recent applications of laser ablation require precise control of the laser ablation process. Experimental studies have been conducted to understand the dependence of the laser ablation phenomena on the laser wavelength, pulse duration, background gas conditions and other adjustable parameters. Much effort has also been made theoretically to understand the detailed mechanisms of laser ablation. Laser ablation is a multi time scale problem which causes major difficulty in the study of the laser ablation process. Consequently, in this work, the laser ablation process is divided into three time stages; namely, plume expansion, cooling and condensation after the laser pulse. In each stage, important mechanisms of laser ablation are studied both experimentally and theoretically. Based on this approach we are able to carry out analyses which yield good agreement with the phenomena observed in each time stage.;In the early vapor plume expansion stage (from a few picoseconds to hundreds of nanoseconds after the laser pulse), studies are made of the energy transfer from the vapor plume to the background gas, the propagation of shockwaves in the vapor plume and the background gas, and the evolution of the physical properties (density, pressure, and temperature) of the vapor plume. In the vapor plume cooling stage (from a few microseconds to tens of microseconds after the laser pulse), the energy loss from the vapor plume by radiation, the change in the temperature of the vapor plume, and the evolution of the size of the vapor plume are studied. In the vapor plume condensation stage (from tens of microseconds to a few miniseconds after the laser pulse), the development of particles in the vapor plume by the condensation process along with the variations of the temperature and shape of the vapor plume are studied.;It is demonstrated, experimentally and theoretically, that the laser ablation process is dominated by the amount of laser energy transferred to the vapor plume (E), the amount of sample vaporized (M), and the type of the background gas. E and M can be adjusted by changing the laser wavelength, laser fluence and the background gas. Once the amount of laser energy transferred to the vapor plume (E), the amount of sample vaporized (M), and the type of the background gas for a laser ablation process are specified, the entire laser ablation process is determined (from vapor plume expansion and cooling to condensation). |
