PHYSICAL REVIEW B 92, 174104 (2015) Molecular dynamics simulation of subpicosecond double-pulse laser ablation of metals Mikhail E. Povarnitsyn, 1 Vladimir B. Fokin, 1 Pavel R. Levashov, 1, 2 and Tatiana E. Itina 3 1 Joint Institute for High Temperatures RAS, Izhorskaya 13 Bldg 2, Moscow 125412, Russia 2 Tomsk State University, 36 Lenin Prospekt, Tomsk 634050, Russia 3 Laboratoire Hubert Curien, UMR CNRS 5516/Universit´ e de Lyon, Bˆ at. F, 18 rue du Prof. Benoit Lauras, 42000 Saint-Etienne, France (Received 29 June 2015; revised manuscript received 1 September 2015; published 6 November 2015) 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. DOI: 10.1103/PhysRevB.92.174104 PACS number(s): 79.20.Eb, 64.60.Q−, 64.70.D−, 68.03.Fg I. INTRODUCTION Ultrashort subpicosecond double-pulse (DP) laser ablation of solids is used for a precise treatment of materials, to increase the spectral line intensities in laser-induced break- down spectroscopy (LIBS) [1–4], for investigation of DP plume dynamics [5], optimization of the nanoparticle size distribution in vacuum [6,7] and liquids [8,9], and modification of optical properties [10,11]. During the last decade, the numerical modeling of ultrashort laser ablation of metals was focused on understanding the basic mechanisms responsible for the process of ablation [12,13] and dynamics of laser- irradiated nanoparticles [14,15]. In addition, efforts were aimed at optimizing the temporal laser pulse shape [16], the hydrodynamic simulation of a single pulse (SP) [17,18] and DP laser ablation [19,20], molecular dynamics (MD) simulation of SP [21–25] and DP laser ablation [26], as well as the study of ablation efficiency of a target wetted by a thin liquid film [27]. It is known that the absorption of laser energy in the optical range by polished metals is typically on the order of 10% at room temperature, and the rest of energy is reflected from the target surface. Thus the absorption of a subpicosecond SP can be approximated by an effective absorption coefficient that defines the energy distribution inside the skin layer of a metallic target. At the same time, a careful simulation of either multiple femtosecond interactions or even of an SP nanosecond laser ablation requires more sophisticated models. In particular, these models should account for laser energy absorption in an extended plume with a strongly nonhomogeneous distribution of thermodynamic parameters. An atomistic simulation seems to be a very effective tool in the investigation of laser ablation since it allows to consider fluctuations of thermodynamic parameters on nanoscales, defects of atomic structure, kinetic processes of metastable phase nucleation, lattice overheating, evaporation and conden- sation as well as kinetics of homogeneous and heterogeneous melting and solidification. Difficulties in the application of this method to the simulation of laser-mater interaction arise when one needs to take into account the dynamics of an electron subsystem with a temperature higher than that of the ionic subsystem. In many early investigations, this nonequilibrium state was described by the continual two-temperature model (TTM) [28,29]. Ivanov and Zhigilei proposed an approach that accounts for the conduction band (CB) electrons in this two-temperature case [22] combining the MD method and the TTM model. In their model, the simulation of the SP laser energy absorption was based on the Beer’s law that allows to consider only relatively weak laser pulses. Improvements of this approach [22] in successive works include the introduction of an electron temperature to the MD potential [30] as well as the inclusion of the blast force of delocalized electrons to the MD equations [24,31]. By using such modifications, however, it is hard to meet the energy conservation law, [32] and they are not suitable for the DP laser ablation considered here. The key physical factors in the last case involve the complex absorption of the laser radiation both in the skin layer and in the extended region of the subcritical plasma, wide-range models of electron thermodynamics, electron thermal conductivity, and electron-phonon/ion coupling, and the interaction of shock and rarefaction waves generated by both pulses. To improve our understanding of the basic mechanisms of the DP laser ablation of metals, we develop a hybrid model that is based on the approach described in Ref. [22]. Then, we extend the model by a procedure of calculation of the laser energy absorption using the solution to the Helmholtz wave equation for each pulse. The dielectric function as well as the electron-phonon/ion coupling coefficient and the electron thermal conduction coefficient have a wide-range form described in detail in our previous paper [33]. The article is organized as follows. In Sec. II, we present the details of our hybrid MD-TTM model. Section III deals with the discussion of the results of the study that are summarized in Sec. IV. 1098-0121/2015/92(17)/174104(10) 174104-1 ©2015 American Physical Society