Resonant holographic measurements of laser ablation plume expansion in vacuum and argon gas backgrounds.

Abstract

Resonant Holographic Interferometry (RHI) and Dye-Laser-Resonance-Absorption-Photography (DLRAP) have been utilized to investigate the expansion of the laser ablation plumes produced by a KrF excimer laser beam (248 nm) focused onto an aluminum target ($\approx$0.1 cm$\sp2,$ 2-6 J/cm$\sp2).$ Plume expansion was studied in vacuum and in background argon gas pressures of 14 mTorr, 52 mTorr, 210 mTorr, 1 Torr, and 35 Torr. The existing theory for the interpretation of resonant holograms has been extended to account for Doppler shift effects, the diagnostic laser bandwidth, and the selective absorption of the laser beam. Results show that fringe shift interpretation is relatively insensitive to parallel expansion temperatures in the range 0.1 eV-1.0 eV. Absolute-line-densities in the range $4.3\times10\sp{13}\ {\rm cm}\sp{-2}$ to $1.0\times10\sp{15}\ {\rm cm}\sp{-2}$ have been measured in the ablation plumes, which imply measured particle densities of up to $1\times10\sp{15}\ {\rm cm}\sp{-3}.$ The total number of aluminum neutral atoms in a plume has been measured to be ${\approx3}\times10\sp{14}$, which corresponds to a surface etch rate of $\approx$1 nm/pulse. The observed ablation plume expansion in vacuum and for pressures $\le$210 mTorr was consistent with the two-component theory for laser ablation. A fast component of ablated particles, corresponding to ions and charge exchange neutrals accelerated by the ambipolar potential, moved quickly away from the target surface while a slow component of particles lingered near the surface. Expansion velocities in the range 1.05 cm/$\mu$s to 1.41 cm/$\mu$s were measured for the pressures $\le$210 mTorr, while $\approx$0.34 cm/$\mu$s was measured for 1 Torr and $\approx$0.084 cm/$\mu$s was measured for 35 Torr. Perpendicular plume kinetic temperatures were estimated to be in the range 2 eV to 4 eV. Ablation plume expansion into a 1 Torr RF-argon-plasma was compared with the expansion into a 1 Torr argon gas. The ablation plume appeared to expand faster and dissipate faster in the plasma.