  The physical mechanism for metal ablation induced by femtosecond laser
irradiation was investigated. Results of calculations based on
finite-temperature density functional theory (FTDFT) indicate that condensed
copper becomes unstable at high electron temperatures due to an increase of
electronic entropy at large volume, where the local density of states near the
Fermi energy increases. Based on these results, an electronic entropy-driven
(EED) model is proposed to explain metal ablation with a femtosecond laser. In
addition, a mathematical model is developed for simulation of the laser
ablation, where the effect of the electronic entropy is included. This
mathematical model can quantitatively describe the experimental data in the
low-laser-fuence region, where the electronic entropy effect is determined to
be especially important.
