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The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies ( >3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold ( >2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

We employ grazing-incidence femtosecond x-ray diffraction to characterize the coherent, femtosecond laser-induced lattice motion of a bismuth crystal as a function of depth from the surface with a temporal resolution of 193 8 fs. The data show direct consequences on the lattice motion from carrier diffusion and electron-hole interaction, allowing us to estimate an effective diffusion rate of D 2:3 0:3 cm 2 =s for the highly excited carriers and an electron-hole interaction time of 260 20 fs.

We present a theoretical study of ultrafast phase transitions induced by femtosecond laser pulses of arbitrary form. Molecular-dynamics simulations on time dependent potential-energy surfaces derived from a microscopic Hamiltonian are performed. Applying this method to diamond, we show that a nonequilibrium transition to graphite takes place for a wide range of laser pulse durations and intensities. This ultrafast transition (ϳ100 fs) is driven by the suppression of the diamond minimum in the potential-energy surface of the laser excited system. ͓S0163-1829͑99͒50730-3͔

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A femtosecond-laser pulse can induce ultrafast nonthermal melting of various materials along pathways that are inaccessible under thermodynamic conditions, but it is not known whether there is any structural modification at fluences just below the melting threshold. Here, we show for silicon that in this regime the room-temperature phonons become thermally squeezed, which is a process that has not been reported before in this material. We find that the origin of this effect is the sudden femtosecond-laser-induced softening of interatomic bonds, which can also be described in terms of a modification of the potential energy surface. We further find in ab initio molecular-dynamics simulations on laser-excited potential energy surfaces that the atoms move in the same directions during the first stages of nonthermal melting and thermal phonon squeezing. Our results demonstrate how femtosecond-laser-induced coherent fluctuations precurse complete atomic disordering as a function of fluence. The common underlying bond-softening mechanism indicates that this relation between thermal squeezing and nonthermal melting is not material specific.

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