No abstract

No abstract

Novel analytic bond-order potentials (BOP's) are derived for the sigma and pi bonds of sp-valent systems that are correctly bounded from above by unity. We show that these BOP's allow the concept of single, double, triple, and conjugate bonds in carbon systems to be quantified, the average error compared to accurate tight-binding predictions being only 1% for the sigma bonds and 15% for the pi bonds. Although molecular dynamics simulations are an order of magnitude slower than with standard Tersoff potentials, these new BOP's provide the first "classical" interatomic potentials that handle both structural differentiation and radical formation naturally within its framework.

No abstract

176Femtosecond (τ L ~ 100 fs) laser pulses incident on a metallic target are absorbed by conduction electrons in the skin layer with a depth of 10-20 nm. Owing to heat conduction, the energy absorbed by the electrons propagates into the target in the form of an electron heat wave [1][2][3][4][5]. Electronic energy is simultaneously transferred to the lattice through electron-phonon heat transfer. As a result, a heated layer with thickness d T ≈ 120 nm is formed in aluminum within the elec tron-ion relaxation time t eq = 2-3 ps. The lattice is melted when the energy flux of the femtosecond laser pulse exceeds the melting threshold for the given material. For aluminum, the absorbed energy at the melting threshold is estimated at ~15 mJ/cm 2 .The pressure in the heated layer increases strongly owing to the supersonic heat conduction for which the acoustic time t s = d T /c s ≈ 20 ps is much longer than the heating time t eq [1, 2, 5]. Acoustic decay of the heated and pressurized layer (d'Alembert's solution) and sub sequent reflection of acoustic waves at the vacuum interface are accompanied by generation of thermo mechanical tensile stresses [1,2,[5][6][7], where the amplitude of the tensile stresses increases with the energy density of the laser pulse F. Such motion results in deformation of the surface layer d T . When F exceeds the ablation threshold F abl , spallation (thermome chanical ablation) of part of the melted layer occurs as a result of the tensile stress exceeding the tensile strength of the liquid metal, and a crater forms on the surface. Our experiments and calculations indicate that, when the energy flux is slightly higher than the ablation threshold, the heated layer first expands to a certain value and then returns back with some residual deformation (see, e.g., Fig. 1).In the described experiments, the deformation of the surface of the target in the heated region was stud ied using femtosecond interference microscopy [8]. To analyze the structure of the surface layer after irradia tion near the ablation threshold, transmission electron microscopy was used.To heat and probe the surface of the target, 100 fs pulses generated by a femtosecond chromium-for sterite laser system were used. The surface of the target was heated by pulses having a fundamental wavelength of 1240 nm at an angle of incidence of 45°. The target was probed by the 620 nm second harmonic pulses with the measured time delay with respect to the heat ing pulse. The spatial distribution of the energy density in the focal spot had the Gaussian formIt has been revealed experimentally that nanocavities remain inside a surface layer of aluminum after action of a femtosecond laser pulse. This result is in agreement with numerical simulation. A detailed picture of melting, formation of expansion and compression waves, and bubble nucleation in the stretched melt has been reconstructed through atomistic simulation. It has been shown that the bubbles do not fully collapse but remain as frozen disk shaped nanocavities upon recrystallization ...