The dependence of the strength of the electron-phonon coupling and the electron heat capacity on the electron temperature is investigated for eight representative metals, Al, Cu, Ag, Au, Ni, Pt, W, and Ti, for the conditions of strong electron-phonon nonequilibrium. These conditions are characteristic of metal targets subjected to energetic ion bombardment or short-pulse laser irradiation. Computational analysis based on first-principles electronic structure calculations of the electron density of states predicts large deviations ͑up to an order of magnitude͒ from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling. These thermophysical properties are found to be very sensitive to details of the electronic structure of the material. The strength of the electron-phonon coupling can either increase ͑Al, Au, Ag, Cu, and W͒, decrease ͑Ni and Pt͒, or exhibit nonmonotonic changes ͑Ti͒ with increasing electron temperature. The electron heat capacity can exhibit either positive ͑Au, Ag, Cu, and W͒ or negative ͑Ni and Pt͒ deviations from the linear temperature dependence. The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with laser interaction with metals.

The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast nonequilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces ͑melting fronts͒ from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within ϳ2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short р10 ps laser pulses and strong electron-phonon coupling ͑Ni films͒, the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to TϷ1.05T m , and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to ϳ150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and more numerous clusters with higher ejection velocities are produced in the regime of stress confinement as compared to the regime of thermal confinement. For monomer molecules, the ejection in the stress confinement regime results in broader velocity distributions in the direction normal to the irradiated surface, higher maximum velocities, and stronger forward peaking of the angular distributions. The acoustic waves propagating from the absorption region are much stronger in the regime of stress confinement and the wave profiles can be related to the ejection mechanisms.

Recent applications of the breathing sphere model for molecular dynamics simulations of laser ablation of organic solids have yielded detailed microscopic data of the processes involved. The results to date include a prediction of a fluence threshold for ablation, an explanation for the presence of clusters in the plume and a consistent analytical description of the velocity distribution for both matrix molecules and heavier analyte molecules in matrix-assisted laser desorption. In this paper we review the approach and the basic physical picture that emerges from the simulations, present new results, and discuss future prospects for microscopic simulations of laser ablation.

The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron-phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase explosion is signified by an abrupt change in the composition of the ejected plume (from liquid layers and/or large droplets to a mixture of vapor-phase atoms, small clusters and droplets), and results in a substantial increase in the duration of the melting process. In simulations performed with longer, 50 ps, laser pulses, when the condition for stress confinement is not satisfied, the spallation regime is absent and phase explosion results in smaller values of the ablation yield and larger fractions of the vapor phase in the ejected plume as compared to the results obtained with a 1 ps pulse. The more vigorous material ejection and higher ablation yields, observed in the simulations performed with the shorter laser pulse, are explained by the synergistic contribution of the laser-induced stresses and the explosive release of vapor in phase explosion occurring under the condition of stress confinement.

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