Laser-induced phase explosion: new physical problems when a condensed phase approaches the thermodynamic critical temperature
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Three kinds of thermal processes may lead to material loss from a laser-irradiated surface: 1) vaporization, 2) normal boiling, and 3) explosive boiling. The latter is equivalent to phase explosion. It is appropriate, at this point, to exclude “subsurface heating”, as there are strong doubts about its existence. The relevance of the three processes depends on the laser pulse duration as well as on the temperature attained in the irradiated zone. We revisit the three thermal processes by noting that: 1) vaporization is not important for the shortest time-scales (<1 ns). 2) Normal boiling is subject to a major kinetic obstacle in the process of bubble diffusion, such motion being sufficiently slow that it will simply not occur for t<100 ns. This is because the value of the bubble diffusion coefficient leads to distances traveled which are atomically small for both 1 ns and 100 ns, and for both T=Tm and T=2Tm, with Tm being the melting temperature. 3) Phase explosion, notwithstanding the unfavorable time-scale (1–100 ns) advocated by Martynyuk, as carefully analyzed in this paper, is found to be the most efficient mechanism in the ablation process when looking at thermal processes. Here it should be recognized that a new field in the physics of condensed matter may be emerging when looking at physical properties near the thermodynamic critical temperature, Ttc. In fact, laser irradiation experiments probably represent a unique tool to investigate matter under extreme thermodynamic conditions and on very short time-scales (ps or fs).