UV laser ablation of metals

Abstract

The mechanism of energy transfer between an excimer laser beam (A = 308 nm, 248 nm), an AI substrate and the environment around the interaction site is investigated for laser intensities 10^12 - 10^13 WM^-2. Two energy transfer mechanisms are considered: inverse bremsstrahlung absorption due to plasma formation, and scattering by nanoparticles in the plume.

Imaging and optical spectroscopy experiments show that a plasma forms and that the electron density can reach 10^26 m^-3 confirming that inverse bremsstrahlung absorption is present in the plume. This electron density is ~ 100 times larger than can be produced by laser heating of the target indicating that the laser ionizes the plume directly.

Nanoparticles are also shown to be present in the plume. These nanoparticles have sizes in the 2 - 3 nm range and a number density ~ 10^21 m^-2. These nanoparticles appear to be vaporized by the laser resulting in high energy ions.

Measurements on the transmission of the laser pulse through the plume show that nanoparticles are too small to attenuate the laser beam. The attenuation mechanism is strictly inverse bremsstrahlung absorption and electron densities reach 10^26 m^-3 in the first 10 ns of the laser pulse.

These high electron densities are shown to be produced by the ionization of nanoparticles in the plume which can generate large electron and ion densities thus ionizing the plume.