Nucleation Rate Surface Semiempirical Design, Results and Discussion, страница 2

Fig. 3. Topology for droplet (dark gray) and crystal (gray) particles nucleation.

The particle size distributions as function of the power of laser pulse is measured on the example of Polymethyl Methacrylate laser ablation. We used here a rate difference for size gain in a supersaturated vapor media for the different particle phase states.

Fig. 4 The particle size distribution at laser beam intensity of 0.09 J/cc.

In Fig. 4 one can see the typical particle size distribution which are observed at three values of the power of laser pulse, i.e. 0.03; 0.09, and 0.13 Joule per cc. A clear bimodal particle size distribution is observed. That result illustrates qualitatively true bridging with topologies shown in Fig. 1. Each size distribution is measured by means of Diffusion Aerosol Spectrometer [5]. It is apeared that low energy (0.03 j/cm²) beam produce one mode size distribution. It is reasonable to assume that the mode is associated with supersaturated vapor nucleation which generates near 10 nm (solid) particles the result of solid Polymethyl Methacrylate sublimation under laser pulse. The bigger in energy laser pulse have chance to create the pressure in the light spot area which is enough to make phase transition from solid to liquid states in detectable quantity. Well known that according the phase state diagram at temperature which is upper the triple point the target substance has chance to be in the vicinity of parameters associated with transition of solid to liquid. That situation can be seen in Fig. 1 and it is reason to get melted Polymethyl Methacrylate as well as the mode for droplets at higher energy density for laser beam. One can see in Fig. 4 the second mode associated with droplets.

3. Conclusions

According the nucleation rate surface topology (shown in Fig. 1) the simultaneous vapor and liquid phase formation under laser ablation of crystalline matter is possible at pressures between metastable critical points for vapor-crystal and negative pressure crystal-liquid equilibria. Only vapor phase can be generated down in pressure from the crystal-liquid critical point at negative pressures. Only droplets can be formed under laser ablation at pressure over vapor-crystal critical point. The droplets can be evaporated at that conditions than, i.e. vapor phase can be generated in two-step process in that case. Experiments on laser ablation of Polymethyl Methacrylate illustrate the reasonable agreement with semiempirical consideration.

4. Acknowledgements

Research is under support of the Russian Foundation for Basic Research through grant numbers of 07-08-13529-ofi and 07-03-00587-а.

5. References

[1] M.M. Martynyuk, “Phase explosion of a metastable liquid”, Phys. Combust. Explosions, 13(1), 178 -184 (1977).

[2] Q. Lu, S.S. Mao, X. Mao, and R.E. Russo, “Delayed phase explosion during high-power nanosecond laser ablation of silicon”, Appl. Phys. Letters, 80(17), 3072-3074 (2002).

[3] M.P. Anisimov, P.K. Hopke, D.H. Rasmussen, “Relation of phase state diagrams and surfaces of new phase nucleation rates”, J. Chem. Phys., 109(4), 1435-1444 (1998).

[4] E.R. Buckle, K.J.A. Mawella, P. Tsakiropolous, “Particle condensation in the vapor emitted by a heated source“, J. Colloid Interface Sci. 112, 42 (1986).

[5] R.A. Mavliev, A.N. Ankilov, A.M. Baklanov, B.Z. Gorbunov, N.A. Kakutkina, K.P. Kutsenogiy, S.E. Paschenko, V.I. Makarov, Kolloidn. Zh. (Russian), “Application of screen diffusion battery for determination of the aerosol dispersity“, 46(6), 1136-1138 (1984).

Figure captions

Fig. 1. Topology of nucleation rate surfaces over PT phase diagram for droplet formation (gray) and vapor embryo formation (dark gray) from crystal or glass. Point, t, is triple one.

Fig. 2. The simplified PT phase diagram following article by Anisimov et al. [3].

Fig. 3. Topology for droplet (dark gray) and crystal (gray) particles nucleation.

Fig. 4 The particle size distribution at laser beam intensity of 0.09 J/cc.