Comments on explosive mechanisms of laser sputtering

Laser sputtering differs from ion sputtering mainly in the important role played by thermal effects. These include not only vaporization from the extreme outer surface and boiling from an extended near-surface region, but also two additional effects which are important at high fluences and short pulse lengths. The first is phase explosion (also termed “explosive boiling”) in the sense analyzed by Martynyuk and by Fucke and Seydel. For high fluences and short pulses the target is unable to boil because the time scale does not permit the necessary heterogeneous nuclei to form. It therefore approaches more or less closely to Ttc (the thermodynamic critical temperature), homogeneous nuclei form at a high rate, and the near-surface region relaxes explosively into a mixture of vapor and equilibrium liquid droplets. An alternative explosive mechanism, the subsurface heating model, has, however, also been postulated for high fluences and short pulses. The idea began in 1972 with work by Dabby and Paek, and continued in a large number of articles to the present day. In this model vaporization from the surface causes the target to lose the ideal exponential temperature profile (T α exp(−μx), μ being the absorption coefficient) and to develop a modified profile such that the target is much hotter (up to 3000 K) just beneath the surface. We point out that the surface conditions chosen by Dabby and Paek are wrong. We then present a numerical solution to the problem and find that the subsurface region is indeed hotter but only by a few degrees. Explosive release of material therefore cannot occur by this mechanism but only by phase explosion.