RESONANT HOLOGRAPHIC-INTERFEROMETRY MEASUREMENTS OF LASER-ABLATION PLUMES IN VACUUM, GAS, AND PLASMA ENVIRONMENTS

Resonant holographic interferometry and dye-laser-resonance-absorption photography 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 (approximate to 0.1 cm(2), 2-6 J/cm(2)). 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 interferograms has been extended to account for Doppler shift effects, the diagnostic laser bandwidth, and the selective absorption of the laser beam. Absolute line densities in the range 4.3x10(13)-1.0x10(15) cm(-2) have been measured in the ablation plumes, which imply measured Al neutral densities of up to 1X10(15) cm(-3). The total number of Al neutral atoms in a plume has been measured to be approximate to 3x10(14), which corresponds to a surface etch rate of approximate to 1 nm/pulse. Expansion velocities in the range 1.1-1.4 cm/mu s were measured for the pressures 210 mTorr, while approximate to 0.3 cm/mu s was measured for 1 Torr and approximate to 0.08 cm/mu s was measured for 35 Torr. Ablation plume expansion into a 1 Torr rf argon plasma environment was compared with the expansion into a 1 Torr argon gas. The ablation plume appeared to expand and dissipate slightly faster in the plasma.