DYNAMICS OF PLUME PROPAGATION AND SPLITTING DURING PULSED-LASER ABLATION OF SI IN HE AND AR

A modeling approach for calculating the-expansion of a laser-generated plasma into a background gas has been developed. Although relatively simple in structure, the model gives excellent fits to various experim ental data for Si in background gases of He and Ar, including the prev iously unexplained ''splitting'' of the ablated plume. The model is ba sed on a combination of multiple-scattering and hydrodynamic approache s. It allows the plume to be broken up into components, or scattering orders, whose particles undergo 0, 1, 2,... collisions with the backgr ound. Particles can only be transferred from one order to the next hig her order by collisions. The densities in the individual orders propag ate according to the usual conservation equations to give the overall plume expansion. When Ar is the background gas, there is a non-negligi ble probability that Si plume atoms will reach the detector without un dergoing any collisions. This gives rise to a flux component that is u ndisplaced from that obtained when no background gas is present in add ition to the delayed peak from the scattered flux. In Ar only a few or ders are necessary for convergence. The behavior in the light gas He i s more complex because of the relatively small effect of any one-scatt ering event and the calculations must be carried out in some cases to as high as the 12th scattering order to find agreement with the experi ments. [S0163-1829(98)03827-2].