Laser ablation of carbon at the threshold of plasma formation

The ionization of laser-ablated vapours with lasers producing ns duration p ulses at various wavelengths has been studied in order to understand the me chanisms of the vapour-plasma transition. It has been established that ther e are several regimes characterizing the laser-target interaction which dep end on laser intensity, wavelength, and pulse duration. The range of laser intensities for optimal laser evaporation is determined by the condition of transparent vapours. The intensity range is upper-limited by the opaque pl asma formation due to vapour optical breakdown. Results are given for laser evaporation of graphite with Nd:YAG laser (1.06 4 mu m), KrF laser (248 nm) and ArF laser (193 nm). For the UV laser wavele ngth the regime of skin-effect interaction was proposed as the mechanism of ion acceleration, and the range of validity of the skin-effect mode was es tablished. With UV lasers the interaction has a bimodal nature: the interac tion may proceed initially in the skin effect regime, resulting in a few hi gh-energy ions, until hydrodynamic expansion begins at a later stage. The s kin-effect interaction at the initial stage of the UV laser pulse gives the first, to our knowledge, explanation for the acceleration of ions up to si milar to 100 eV at low laser intensities of 10(8)-10(9) W/cm(2) and ns-rang e pulse duration.