Optical emission spectroscopy of molecular species in plasma induced by laser ablation of carbon in nitrogen

The results obtained from the study of time-integrated molecular spectra emitted by a plasma induced by excimer laser (XeCl, lambda = 308 nm, tau = 30 ns) ablation of a graphite target in nitrogen atmospheres are presented, The dominant molecular species are found to be CN and C-2. The intensities of CN and C-2 bands have been studied for laser fluences in the range 3-12 J/cm(2), for N-2 pressure in the range 1-250 Pa, and distances along the plasma axis of 1-14 mm. The values of roto-vibrational temperatures of CN radicals were calculated by fitting the experimental spectra with computer-simulated spectra. The self-absorption effects have been considered, and the optical path of the CN radical (the product between CN(X) density and plasma radius, R) was also estimated from the fitting procedure. The formation and the excitation of CN radicals, which could be important in the CNx materials deposition process,are discussed. It is shown that a possible mechanism could be the surface reaction of atomic nitrogen with the carbon clusters or particles existing in the plasma plume. (C) 1998 Elsevier Science B.V.