Plasma plume induced during ArF laser ablation of hydroxyapatite

Plasma plume induced by ArF excimer laser ablation of a hydroxyapatite (Ca10(PO4)6(OH)2) target was studied during expansion into a vacuum or water vapour. The ArF laser operated at a wavelength of 193 nm with a pulse energy of 300-350 mJ and a 20 ns pulse duration. The emission spectra of the plasma plume were registered with the use of a spectrograph and an ICCD camera. The expansion of the plasma plume was studied using the time of flight method. The time-dependent radiation of the Ca I and Ca II lines was registered with the use of a monochromator and photomultiplier at various distances from the target. The dynamics of the plasma plume was also imaged by means of fast photography. It was found that during expansion into a vacuum, the plasma front moved with a constant velocity of 1.75 × 104 m s−1, while in the case of ambient water vapour at a pressure of 20 Pa, velocities of 1.75 × 104-1.5 × 103 m s−1 were found depending on the distance from the target. Electron densities of 1.2 × 1024-4.5 × 1021 m−3 were determined from the Stark broadening of the Ca II and Ca I lines at distances of 1-25 mm from the target. Temperatures of 11,500-4500 K were determined from the relative intensities of carbon lines and continuum radiation at distances of 4-29 mm from the target. The results allowed the estimation of thermal and kinetic energies of ablated particles. During expansion into a vacuum, the kinetic energies of Ca, P and O atoms were 64, 49 and 25 eV, respectively. During expansion into water vapour, kinetic energies dropped to 0.47, 0.36 and 0.19 eV, respectively at a distance of 25 mm from the target and were comparable to the energies of thermal motion.