Monte Carlo simulation of the laser-induced plasma-plume expansion under vacuum and with a background gas

The plasma-plume created above a copper target irradiated by a pulsed KrF l aser beam (wavelength = 248 nm, pulse duration = 20 ns, fluence = 17 J/cm(2 )) is investigated by means of a Monte Carlo simulation. The plume expansio n under both vacuum and background gas is followed in time by simulating pa rticle motion and collisions in the gas phase. The expansion under vacuum o f laser-ablated particles is dominated by the laser energy absorption by th e evaporated particles during the laser pulse. A comparison between simulat ed time-of-flight curves and experimental curves obtained by spectroscopic time-of-flight measurements has shown that about 6% of the incoming laser e nergy was contributing to the expansion process through the time-delayed re combination of this energy into kinetic energy. The plasma-plume expansion under a residual argon pressure greater than 50 Pa has been found to be ver y much affected by collisions between laser-ablated and ambient gas particl es. The compression of the ambient gas particles by the ejected particles i n the leading edge of the plume (snowplow effect) is clearly observed. (C) 1999 Elsevier Science B.V. All rights reserved.