Molecular dynamics simulation of subpicosecond double-pulse laser ablation of metals

Subpicosecond double-pulse laser ablation of metals is simulated using a hybrid model that combines classical molecular dynamics and an energy equation for free electrons. The key advantage of our model is the usage of the Helmholtz wave equation for the description of the laser energy absorption. Applied together with the wide-range coefficients of optical and transport properties of the electron subsystem, the model gives the possibility to correctly describe the second pulse absorption on an arbitrary profile of the nascent plasma plume produced by the first pulse. We show that the integral absorption of the second pulse drastically increases with the delay between pulses, which varies in the simulation from 0 to 200 ps. As a result, the electron temperature in the plume increases up to three times with the delay variation from 0 to 200 ps. Thus the results of simulation resemble the previous experimental observations of the luminosity increase in the double-pulse irradiation for the delay interval from 100 to 200 ps. Besides, we bring to light two mechanisms of suppression of ablation responsible for the monotonic decrease of the ablation crater depth when the delay between pulses increases.