Planar Laser Induced Fluorescence Imaging and Monte Carlo Simulations of Pulsed Laser Ablation

Citation

Capewell, Dale L. (1997) Planar Laser Induced Fluorescence Imaging and Monte Carlo Simulations of Pulsed Laser Ablation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1bah-m913. https://resolver.caltech.edu/CaltechETD:etd-01102008-095243

Abstract

The first reported planar laser-induced fluorescence (PLIF) images depicting the relative ground-state, neutral density of Si within a pulsed laser ablation (PLA) plume as it expands into vacuum and 10 mTorr Ar are presented. Because of the high flow speeds in PLA plumes and the narrow bandwidth of typical lasers, several images acquired with the laser detuned incrementally must be superimposed to produce images which compensate for Doppler shifts. Density profiles of Si along the axis of symmetry as a plume expands into Ar at pressures of 0-150 mTorr demonstrate the influence of a background gas and substrate on the expanding plume in the interval 0 < t < 5μs. In-flight production of SiO along the contact as an Si plume expands into 1.0 Torr air is demonstrated in the first reported images of a reactive intermediate species produced during PLA.

The expansion of Si into both 10 mTorr and 100 mTorr Ar is investigated using the technique of Direct Simulation Monte Carlo (DSMC), with a simple ablation model used to generate the Si plume. As a consequence of the rapid displacement of background gas, i.e. the snowplow effect, the plume divides into two components, one which travels nearly collisionlessly toward the substrate and one which interacts more strongly with the gas. At 10 mTorr, collisions between fast component Si atoms and Ar atoms during the period t < 250 ns are responsible for most of the energy transfer from Si to Ar. This results in the creation of a fast (15 eV), collisionless Ar component which impacts the substrate shortly after the fast Si component arrives, but does not result in thermal heating. At 100 mTorr, the energy transfer from the plume to the Ar gas occurs on a μs time scale and is attributed primarily to thermal heating. The energy of the fast Ar component decreases to about 5 eV.

Finally, the DSMC technique is employed to study the expansion of Si into both 10 mTorr and 100 mTorr O₂. The chemical reaction Si + O₂ → SiO + O is investigated, including rotational and vibrational excitation the diatomic molecules O₂ and SiO. At both pressures, SiO first appears along the sides of the expanding plume and, at 100 mTorr, appears only along the contact front between the Si and O₂. This result compares well will PLIF data. An examination of the O₂ rotational temperature indicates that the background gas is heated along a conical surface whose tip is located at the ablation spot as the plume expands.

Collectively, these simulations demonstrate the potential of the DSMC method to provide quantitative information about the reactive expansion of a wide variety of PLA plumes into rarified background gas mixtures, and study the particle flux onto a substrate when parameters such as background gas pressure and composition are varied.

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