We investigate picosecond laser ablation of metals using a hybrid simulation scheme. Laser energy input into the electron system and heat conduction within it are modeled using a finite-difference scheme for solving the heat conduction equation. Atom motion in the near-surface part ͑72 nm͒ of the sample is modeled using molecular dynamics. Energy transfer between the electronic and atomic subsystems due to electron-phonon coupling is taken into account. For the special case of 0.5 ps UV laser irradiation of copper, we investigate the fluence dependence of the ablation yield, the temperature and pressure evolution in the target, and the ablation mechanism.

In this paper we demonstrate, using density-functional tight-binding theory, that certain classes of siliconbased tubular nanostructures are stable and energetically viable. Specifically, we consider silicide and SiH nanotubes. The structures adopted by these nanotubes are very similar to those of previously reported phosphorus nanotubes. As in that case, the Si-based nanotubes have a semiconducting gap, which in contrast to carbon nanotubes is largely independent of the tube diameter and chirality. We further report on the mechanical properties of the Si-based nanotubes and suggest possible routes towards their synthesis.

Molecular dynamics simulations are used for studying the contact angle of nanoscale sessile drops on a planar solid wall in a system interacting via the truncated and shifted Lennard-Jones potential. The entire range between total wetting and dewetting is investigated by varying the solid-fluid dispersive interaction energy. The temperature is varied between the triple point and the critical temperature. A correlation is obtained for the contact angle in dependence of the temperature and the dispersive interaction energy. Size effects are studied by varying the number of fluid particles at otherwise constant conditions, using up to 150,000 particles. For particle numbers below 10,000, a decrease of the contact angle is found. This is attributed to a dependence of the solid-liquid surface tension on the droplet size. A convergence to a constant contact angle is observed for larger system sizes. The influence of the wall model is studied by varying the density of the wall. The effective solid-fluid dispersive interaction energy at a contact angle of θ = 90° is found to be independent of temperature and to decrease linearly with the solid density. A correlation is developed that describes the contact angle as a function of the dispersive interaction, the temperature, and the solid density. The density profile of the sessile drop and the surrounding vapor phase is described by a correlation combining a sigmoidal function and an oscillation term.

Using molecular-dynamics simulation, we study sputtering of a condensed-gas solid induced by the impact of atomic clusters with sizes 1 ഛ n ഛ 10 4 . Above a nonlinear onset regime, we find a linear increase of the sputter yield Y with the total energy E of the bombarding cluster. The fitting coefficients in the linear regime depend only on the cluster size n such that for fixed bombardment energy, sputtering decreases with increasing cluster size n. We find that to a good approximation the sputter yield in this regime obeys an additivity rule in cluster size n such that doubling the cluster size at the same cluster velocity amounts to doubling the sputter yield. The sputter-limiting energy ⑀ s is introduced which separates erosion ͑⑀ Ͼ ⑀ s ͒ from growth ͑⑀ Ͻ ⑀ s ͒ under cluster impact.

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