EVOLUTION OF THE PLUMES PRODUCED BY LASER-ABLATION OF A CARBON TARGET

The time evolution of plumes produced by laser ablation of a graphite target has been followed by means of intensified CCD imaging. The imag es were obtained in the same conditions used for the deposition of mic rocrystalline diamond. An ArF (193 nm, 23 ns) excimer laser beam was f ocused onto a pyrolytic graphite target with a fluence around 3 J cm(- 2). The study was performed in high vacuum and under a hydrogen or hel ium pressure of 1 mbar. Each image was obtained after a single laser p ulse and acquired with aperture times from 5 ns to several microsecond s depending on the delay time. Spatially resolved optical spectroscopy studies of the plume were also carried out. In vacuum, the images sho w a free expansion of the plume with a front velocity of 4 x 10(6) cm s(-1). Spectroscopy results indicate the presence of C-2. At tens of m icroseconds, the trails of some particulates emitted from the target c an be observed. With a background pressure of 1 mbar, the plume decele rates and a shock wave structure is developed. In the case of H-2, spe ctroscopy reveals CH formation in the expansion front.