A study of molecule formation during laser ablation of graphite in low-pressure nitrogen

Plasma diagnostics were performed during excimer laser ablation of graphite in a low-pressure Np atmosphere using time-and space-resolved optical emission spectroscopy. The laser fluence and nitrogen pressure were set to values typically applied to pulsed laser deposition of thin films. Under these conditions, optical emission spectra were dominated by continuum emission and spectral lines from carbon ions during the early plasma phase (t less than or equal to 200 ns), when the carbon vapour is located in the vicinity of the target surface, whereas molecular bands of CP molecules and CN radicals were essentially observed at later times when the vapour expands through the ambient gas. Emission spectra of C-2, CN and N-2(+) were recorded as functions of time for various distances from the target. From computer simulations of molecular spectra, we deduced rotational and vibrational temperatures. The acquired information about the kinetics of excited plasma species and gas phase reactions contributes to a better understanding of pulsed laser deposition of carbon nitride thin films.