Optical emission spectroscopic study of plasma plumes generated by IR CO₂pulsed laser on carbon targets

Abstract

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 CO 2 pulsed laser at = 9.621 m, 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 + , C 2+ and C 3+ and molecular features of C 2 (d 3 g -a 3 u ; Swan band system). The medium-weak emission is mainly due to excited atomic N, H, O, ionic fragment C 4+ and molecular features of C 2 (E 1 + g -A 1 u ; Freymark system), C 2 (D 1 + u -X 1 + g ; Mulliken system), CN(D 2 -A 2 ), C 2 (e 3 g -a 3 u ; Fox-Herzberg system), C 2 (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 ), C 2 (A 1 u -X 1 + g ; Phillips system) and CN(A 2 -X 2 + ; red system). An excitation temperature T exc = 23 000 1900 K and electron densities in the range (0.6-5.6) 10 16 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 C 2 and CN electronically excited species under various laser irradiance conditions are made.