Laser-pulse sputtering of atoms and molecules .2. Recondensation effects

When solids are sputtered with sufficiently intense laser pulses, particle release takes place, and a terminating expansion then develops. The resulting gas-dynamic processes depend on whether backscattered particles are subject to recondensation or reflection. We here wish to examine the problem further with emphasis on-recondensation effects. (a) We show that, under conditions of recondensation, the surface region permits simple analytical approximations. (b) These simple forms enable the recondensed quantity, Q(rec), to be evaluated as the integral over time of the product of flow velocity and density. Alternatively Q(rec) can be deduced by numerical solution of the flow equations and in either case it is found to increase significantly with the complexity of the particles involved. Thus it ranges from a fraction approximately 0.10 for atoms to approximately 0.43 for large molecules. (c) Recondensation effects are particularly important when the polymer polyimide is laser sputtered, as the recondensation occurs in massive amounts beyond the bombarded spot and the patterns of recondensation show remarkable symmetry changes. (d) Closely related effects occur when targets are laser sputtered to produce films, even if formal recondensation is not involved.