Optical emission spectroscopic study of plasma plumes generated by IR CO2 pulsed laser on carbon targets

Optical emission spectroscopy studies, in the spectral range ultraviolet–visible–near infrared (UV–Vis–NIR), were performed to investigate thermal and dynamical properties of a plume produced by laser ablation of a graphite target. Ablation is carried out using a high-power IR CO2 pulsed laser at λ = 9.621μm, power density ranging from 0.22 to 5.36 GW cm−2 and air pressures around 4 Pa. The strong emission observed in the plasma region is mainly due to electronic relaxation of excited C, ionic fragments C+, C2+ and C3+ and molecular features of C2(d 3 g–a 3 u; Swan band system). The medium-weak emission is mainly due to excited atomic N, H, O, ionic fragment C4+ and molecular features of C2(E 1 + g –A 1 u; Freymark system), C2(D 1 + u –X 1 + g ; Mulliken system), CN(D 2 –A 2 ), C2(e 3 g–a 3 u; Fox–Herzberg system), C2(C 1 g–A 1 u; Deslandres–d’Azambuja system), OH(A 2 +–X 2 ), CH(C 2 +–X 2 ), NH(A 3 –X 3 −), CN(B 2 +–X 2 +; violet system), CH(B 2 +–X 2 ), CH(A 2 –X 2 ), C2(A 1 u–X 1 + g ; Phillips system) and CN(A 2 –X 2 +; red system). An excitation temperature Texc = 23 000 ± 1900 K and electron densities in the range (0.6–5.6)× 1016 cm−3 were estimated by means of C+ ionic lines. The characteristics of the spectral emission intensities from different species have been investigated as functions of the ambient pressure and laser irradiance. Estimates of vibrational temperatures of C2 and CN electronically excited species under various laser irradiance conditions are made. (Some figures in this article are in colour only in the electronic version)