Plasma chemistry produced during laser ablation of graphite in air, argon, helium and nitrogen

Abstract

Laser-induced plasma chemistry produced during the ablation of graphite targets at atmospheric pressure in air, argon, helium and nitrogen was studied through time-resolved atomic and molecular emission spectroscopy. The plasma plume and plasma chemistry were generated by focusing a 6-mm diameter, 212 mJ, 1064-nm nanosecond Nd:YAG laser to a spot size of about 1 mm diameter over graphite samples of 99.9% pureness. The atomic emissions C I 247.86 nm, N I 821.50 nm and O I 777.19 nm, and the molecular bands C2 (473.71 nm) and CN (359.04 nm and 388.30 nm) were monitored as a function of time (0.2 to 220 μs). While the C I and C2 emissions followed a near-exponential decay, the CN emission in air and nitrogen showed an emission behavior characterized by two local maxima at 0.2 μs and 20–30 μs after the plasma ignition. The first maximum was explained by the early plasma chemistry produced by the ablated carbon species and the confining background gas, whereas the second maximum was attributed to atomic recombination and shock wave-induced excitation of the plasma plume. Two main effects were observed when the ablation was produced in different background gases. First, the presence of oxygen (≈21%) in air had a negligible effect on atomic lines; however, the CN emission intensity and lifetime were significantly reduced in comparison with an atmosphere of 100% nitrogen. This was attributed to the reduction of nitrogen species as reaction partners during the plasma chemistry in air. Secondly, due to the assumed higher plasma temperature in Ar, this gas favored the emission intensity and lifetime of atomic species but hindered the formation of C2 species. Because the collection optics of the emission spectroscopy system was configured in backscatter mode, which integrates over the entire plasma volume and gate time without spatial resolution, the time-resolved behavior of the plasma emissions were only related to the number density of emitters but not to specific locations in the plasma volume.