Plasma plume formation at the surface of several natural silicates by mid-infrared gas laser radiation

The influence of a laser’s wavelength on the laser ablation of natural silicates by CO2 laser (λ ∼ 10 μm) and CO laser (λ ∼ 5 μm) irradiation was investigated. High-speed photography was used. For a CO laser, a short and wide-angle conical plume was formed. For a CO2 laser, two types of plumes were observed. For a CO2 laser by lower laser fluency (0.15 kJ/cm2), a short and wide-angle conical plume was formed. At higher fluency Q/S (>0.30 kJ/cm2), one of the two distinct types of plumes was observed during the action of a single laser pulse. The plume of the first type was long (∼20 mm) and narrow with the onset close to the start of the laser pulse. The second type was a conical plume with the onset delayed by a few microseconds from the laser pulse. The first type of plume expands along the laser beam, whereas the second one is directed normally to the surface. Comparing the results for both lasers at the wavelengths of ∼10 and ∼5 μm, we conclude that a long narrow plume is formed due to the intensive resonant absorption of CO2 laser radiation, which leads to breaking of Si-O bonds and to plasma creation in a thin layer of the target surface. The plume expansion is due to laser-induced acceleration of the particles in the direction of a laser beam and is connected with the ponderomotive forces acting on the plasma particles. The wide conical plume is due to thermal heating of the surface.