I was reading some papers about mechanism of ultrafast laser ablation on metal specimen.
'Metal direct vapor' or ' plasma ' show up several times [1].
Does that mean that metallic bond has been destroyed due to energy input?
How could it happen?
According to solid state physics, metallic bond formed by 'electron cloud and positive ion' holds metal together.
During laser ablation, photon transfer energy to electron. Energy received will turn into kinetic energy to accelerate electron. Electron will hit other electron or lattice to transfer energy [2].
[1] Momma, Carsten, Boris N. Chichkov, Stefan Nolte, Ferdinand von Alvensleben, Andreas Tünnermann, Herbert Welling and Bernd Wellegehausen. "Short-Pulse Laser Ablation of Solid Targets." Optics Communications 129, no. 1–2 (1996): 134-142.
If you heat metal suddenly to very high temperatures, metal might evaporate without going to liquid phase. It may happen in very short pulse length, because electron-phonon( guess it was ~ps range) recombination time is much larger than pulse duration. This means that fs pulse can give its energy to small area and heat it suddenly which will lead to evaporation.
the energy tranfer betweem photon and electron happens in a very short time.
for energy tranfer between electron and lattice, it would be about 10ps.
but after electron heated, it would run to the inner part of material to pass energy.
Will it happen lik that, pulse energy is so powerful that it would penetrate into the material.
In that case pulse energy will creat a super hot layer on the top of specifimen?
that part will evapor will out going through liquid phase?
Dear Xueqian Zhang, your question is in a sense a matter of definition. What do you call a liquid state? Defining liquid state as a disordered state of matter (necessary, not sufficient condition), it implies that the system requires some time to disorder from the initial perfectly ordered state. If you provide a lot of energy fast enough, so that the atoms acquire enough of energy to break the bonds and evaporate faster than the disorder occurs, you'll have evaporation without first going to a liquid phase.
Generally, laser ablation depth will depend on your focus. But if you have high fluence , than you will have higher penetration depth because of diffusion. I'm not sure about temperature profile.
Check this papers:
Perez, Danny, and Laurent J. Lewis. "Molecular-dynamics study of ablation of solids under femtosecond laser pulses." Physical Review B 67.18 (2003): 184102.
Bulgakova, N. M., and A. V. Bulgakov. "Pulsed laser ablation of solids: transition from normal vaporization to phase explosion." Applied Physics A 73.2 (2001): 199-208.
A similar debate to this occurred in the early 1980's in the area of pulsed laser annealing of Si... with, as I recall, similar inability to reach a consensus. (So I'm with Nikita on this one- it really gets into fundamental definitions.) Some work was done by Compaan, who I think is at U of Toledo now- find his early work and that should link you to the various sides of the debate then. I recall it that reflectivity and raman scattering (both time-resolved) gave contradictory results.
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