Melting of Al by ultrafast laser pulses: dynamics at the melting threshold

Using molecular-dynamics simulation, we investigate the melting of a thin Al slab by ultrafast laser irradiation. We employ a laser energy, which is just around the melting threshold. While the equilibrium electron–phonon coupling is well understood, we investigate the influence of the early (i.e., prior to electron thermalization) electron-lattice energy transfer. To this end, as a model study, we vary the fraction of the laser energy, which is directly given to lattice atoms vs. that given to the electronic system. We find that the melting process depends sensitively on the early electron-lattice heating rate. The pressure build-up within the still solid parts of the slab is identified as the main agent which delays the melting transition. The changes in the simulated structure factor data suggest that X-ray measurements of thin films performed just around the melting transition—even if performed long after electron thermalization—may provide information on the early electron-lattice energy coupling process.