Resonant-holographic-interferometry for absolute measurements of excimer laser-ablated neutral-atom plume line-density profiles

Abstract

Experiments have been performed to measure Al neutral atom absolute line‐density profiles using resonant‐holographic‐interferometry. The ablation source is a KrF excimer laser with a per‐pulse energy of about 0.8 J. Targets are either pure aluminum or Al2O3. Aluminum ground‐state neutral atom line‐densities are probed by a dye laser tuned near the 394.401 nm line. A double‐pulse interferometry technique is employed in which one pulse includes the laser ablation plume and a second pulse generates reference fringes on a holographic plate by rotating a mirror. Holograms are reconstructed to give interference fringes on film by using a helium‐neon laser. Interferograms of laser ablated Al metal give maximum Al neutral plume line‐densities in the range of 4‐10×104 cm−2. Aluminum neutral line‐densities from Al2O3 targets are as much as 10‐20 times larger than from Al metal targets. The sensitivity of this resonant diagnostic is 4 to 5 orders of magnitude higher than nonresonant neutral‐particle interferometry and may be adjusted by tuning the dye laser wavelength shift away from the resonance line. This diagnostic has been demonstrated in vacuum, gas (14 mTorr ‐ 35 Torr) and RF plasma (30 mTorr ‐ 1 Torr) environments. At higher gas pressures, the nonresonant gas‐dynamic fringe shift can be subtracted to give the resonant fringe shift and species density.