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.