Laser ablation sampling near the breakdown threshold

Laser-induced plasma spectroscopy (LIPS) is a new analytical method for materials characterization that uses the laser as the sampling and the excitation source. In LIPS, a short-pulsed laser is tightly focused onto a sample surface. During the pulse of the laser, the surface is heated beyond the latent heat of vaporization and material is ablated. The vaporized material develops into a hot plasma of high electron number density. The light emitted from the plasma is collected, dispersed and detected. Several laser and sample properties determine the threshold for ablation and subsequent plasma formation. The laser properties include laser irradiance, excitation wavelength, pulse duration and spot size. The sample properties include reflectivity, density, specific heat and latent heat of vaporization.; Requiring minimal or no sample preparation, LIPS can analyze any phase of matter. Requiring only optical access, LIPS has possible applications to online process monitoring. In the semiconductor industry, LIPS can provide spatially-resolved elemental identification. Depth-resolved analysis also is possible if the laser probes a sample repetitively in one location.; In this work, laser ablation and plasma formation are studied to further develop LIPS as a technique for spatially-resolved materials characterization. The laser is used to probe pure samples of copper, silver, gold and silicon. The first chapter of this dissertation provides an overview of materials characterization and presents LIPS as a useful alternative to the methods already in practice. Chapters two and three develop the theory behind the threshold for laser ablation and plasma formation and review previous research on the determination of ablation thresholds. Chapter four describes the instrumentation used. The time-resolved emission of the laser-induced plasma is studied in chapters five and six. The emission is collected at 5 laser irradiances at two laser wavelengths, 532 nm and 1064 nm. The final experimental chapter (Chapter 7) explores the effects of laser irradiance and wavelength on sample ablation volume. The ablation threshold is calculated for each sample studied.