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11ALT'22
LM-O-12
LASER-MATTER INTERACTION
Coulomb explosion in metals under the influence of fs-laser pulses
An ultrashort super-powerful laser action on metal targets leads to the appearance of strongly nonequilibrium states in them, characterized by a large difference in the temperatures of the electron Femi gas T and the crystal lattice T, T >> T due to the release of laser radiation energy in the electronic component of the metal. The impact is also accompanied by a powerful transfer of matter, which is characterized by high temperatures, speeds and pressures. It is known that at the metal-vacuum interface there is a thin surface layer of spatially separated electric charges of the opposite sign - a double electric layer (EDL), resulting from the displacement of the electron gas outside the positively charged crystal lattice. EDL is electrically neutral under normal conditions.
The description of the processes of ultrashort fs-ps laser ablation of metals was carried out within the framework of a nonequilibrium combined continuum-atomistic model. The continuum representation was used to describe processes in the electronic component of the metal and EDL in the hydrodynamic approximation [1]. The state of a metal lattice heated by energy exchange with an electron gas was displayed within the framework of an atomistic model. The developed nonequilibrium combined model, which takes into account the influence of the electric field, was used to simulate the fs effects in the Al target.
Mathematical modeling of laser action with duration t = 100 fs and fluence F = 0.5 J cm-2 made it possible to determine two ablation mechanisms, which were noticed earlier in experimental works [2-3]. The first is a fast non-thermal mechanism, the second is a slow thermal ablation mechanism. The appearance of a nonthermal mechanism is related to the fact, as shown by the simulation results, that nonequilibrium laser heating causes a rapid increase in the temperature Te and the electron pressure pe. The excess electron pressure pene, which acts only inside the electron gas, leads to the violation of the quasi-neutrality of the DEL and the generation of an additional Coulomb force Fne, which exerts a stretching effect on the lattice ions. Upon reaching a sufficient value, the force Fne leads to the breakdown of thin surface layers forming a flow of rapidly expanding ions and clusters at a speed of ~ (8-14) km/s. Non-thermal removal of matter occurs and continues during the duration of the laser pulse. With the end of the laser pulse, the degree of thermodynamic nonequilibrium rapidly decreases, which manifests itself in the equalization of temperatures Te ~ T and pressures pe ~ peeq, as well as in the disappearance of the nonequilibrium electron pressure pene and the excess Coulomb force Fne. As the thermodynamic nonequilibrium decreases, the mechanism of fast nonthermal ablation associated with electric fields is replaced by the mechanism of relatively slow thermal ablation, where thermal and HD processes play the main role.
Acknowledgements: This work was supported by Russian Science Foundation (project No. 18-11-00318).
[1] V.I. Mazhukin, M.M. Demin, A.V. Shapranov, A.V. Mazhukin. Appl.Sur.Sct., 530, 2020, 147227
[2]H. Dachraoui, W. Husinsky, G. Betz. Appl. Phys. A, 83(2) (2006) 333-336.
[3]H. Dachraoui and W. Husinsky. Appl. Phys. Lett. 89, (2006) 104102 doi:10.1063/1.2338540
V.I. Mazhukin, M.M. Demin, A.V. Shapranov, A.V. Mazhukin
Keldysh Institute of Applied Mathematics of RAS, Moscow, Russia
