Plasma diagnostics were performed during excimer laser ablation of graphite in a low-pressure 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 , when the carbon vapour is located in the vicinity of the target surface, whereas molecular bands of molecules and CN radicals were essentially observed at later times when the vapour expands through the ambient gas. Emission spectra of , CN and 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.

We demonstrate that chemical reactions leading to the formation of AlO radicals in plasmas produced by ablation of aluminum or Ti-sapphire with ultraviolet nanosecond laser pulses can be predicted by the model of local thermodynamic equilibrium. Therefore, emission spectra recorded with an echelle spectrometer and a gated detector were compared to the spectral radiance computed for uniform and nonuniform equilibrium plasmas. The calculations are based on analytical solutions of the radiation transfer equation. The simulations show that the plasmas produced in argon background gas are almost uniform, whereas temperature and density gradients are evidenced in air. Furthermore, chemical reactions exclusively occur in the cold plume periphery for ablation in air. The formation of AlO is negligible in argon as the plasma temperature is too large in the time interval of interest up to several microseconds. Finally, the validity of local thermodynamic equilibrium is shown to depend on time, space, and on the elemental composition. The presented conclusions are of interest for material analysis via laser-induced breakdown spectroscopy and for laser materials processing.

Femtosecond laser ablation of Ti, Zr and Hf has been investigated by means of in-situ plasma diagnostics. Fast plasma imaging with the aid of an intensified charged coupled device (ICCD) camera was used to characterise the plasma plume expansion on a nanosecond time scale. Time-and spaceresolved optical emission spectroscopy was employed to perform time-of-flight measurements of ions and neutral atoms. It is shown that two plasma components with different expansion velocities are generated by the ultra-short laser ablation process. The expansion behaviour of these two components has been analysed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ultra-short laser ablation.

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