Molecular Dynamics Study of Femtosecond Laser Interactions with Cr Targets

The mechanisms of femtosecond laser melting, spallation and ablation of a chromium target are investigated in simulations performed with a hybrid computational model that combines the classical molecular dynamics method with a continuum description of the laser excitation of conduction band electrons, electron-phonon coupling, and electron heat conduction in the irradiated target. The material response to the irradiation by a 200 fs laser pulse is studied for laser fluences covering the regimes of melting and recrystallization of the surface region of the target, photomechanical separation (spallation) of a liquid layer, and explosive disintegration and ejection of a superheated surface region. The transition from the regime of transient surface melting to the spallation is manifested by a sharp increase of the total amount of ejected material (ablation yield) and decrease in the time required for resolidification of the surface region of the target. The transition from the spallation to the phase explosion regime is characterized by a remarkable change in the composition of the ejected ablation plume (smaller droplets and larger fraction of vapor phase), increase in the time for surface resolidification, and saturation or even decrease of the total ablation yield as compared to the spallation regime. The conditions that control the transitions between the three regimes and the mechanisms of laser melting, spallation and phase explosion are established for the chromium target and related to the results of earlier simulations performed for other metals and molecular systems.