Planar laser induced fluorescence imaging and Monte Carlo simulations of pulsed laser ablation

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. 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 < 5 mus. In-flight production of SiO along the contact front 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.;Finally, the DSMC technique is employed to study the chemical reaction Si + O2 → SiO + O, allowing rotational and vibrational excitation the diatomic molecules O2 and SiO. SiO first appears along the sides of the expanding plume and later along the contact front between the Si and O2, a result which compares well will PLIF data. The O 2 rotational temperature indicates that the background gas is thermally heated.;Collectively, these simulations demonstrate the potential of the DSMC method to provide important quantitative information about reactive PLA in rarified background gas mixtures and the subsequent particle flux onto a substrate.;The expansion of Si into Ar is investigated using the technique of Direct Simulation Monte Carlo (DSMC), using a simple ablation model to generate the Si plume. The plume is observed to divide into two components, one which travels nearly collisionlessly toward the substrate and one which interacts more strongly with the gas. At low pressure, a fast (15 eV), collisionless Ar component is created on a ns time scale, impacting the substrate shortly after the fast Si component arrives. At higher pressure, energy transfer from the plume to the Ar gas occurs on a mus time scale and may be regarded as thermal heating. The energy of the fast Ar component also decreases to about 5 eV.