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MICRO/NANOSCALE HEAT TRANSFER—PART I

Relativistic Molecular Dynamics Simulations of Laser Ablation Process on the Xenon Solid

[+] Author and Article Information
Yun-Che Wang, Jing-Wen Chen

Department of Civil Engineering, Materials Program, National Cheng Kung University, 1 University Road, Tainan, Taiwan, R.O.C.

Lun-De Liao, Hong-Chang Lin

Department of Engineering Science, National Cheng Kung University, 1 University Road, Tainan, Taiwan, R.O.C.

Chi-Chuan Hwang1

Department of Engineering Science, National Cheng Kung University, 1 University Road, Tainan, Taiwan, R.O.C.chchwang@mail.ncku.edu.tw

1

Corresponding author.

J. Heat Transfer 131(3), 033112 (Jan 27, 2009) (10 pages) doi:10.1115/1.3056607 History: Received March 11, 2008; Revised October 16, 2008; Published January 27, 2009

The phenomena of Coulomb explosion require the consideration of special relativity due to the involvement of high energy electrons or ions. It is known that laser ablation processes at high laser intensities may lead to the Coulomb explosion, and their released energy is in the regime of kEV to MeV. In contrast to conventional molecular dynamics (MD) simulations, we adopt the three-dimensional relativistic molecular dynamics (RMD) method to consider the effects of special relativity in the conventional MD simulation for charged particles in strong electromagnetic fields. Furthermore, we develop a Coulomb force scheme, combined with the Lennard-Jones potential, to calculate interactions between charged particles, and adopt a Verlet list scheme to compute the interactions between each particle. The energy transfer from the laser pulses to the solid surface is not directly simulated. Instead, we directly assign ion charges to the surface atoms that are illuminated by the laser. By introducing the Coulomb potential into the Lennard-Jones potential, we are able to mimic the laser energy being dumped into the xenon (Xe) solid, and track the motion of each Xe atom. In other words, the laser intensity is simulated by using the repulsive forces from the Coulomb potential. Both nonrelativistic and relativistic simulations are performed, and the RMD method provides more realistic results, in particular, when high-intensity laser is used. In addition, it is found that the damage depth does not increase with repeated laser ablation when the pulse frequency is comparable to the duration of the pulse. Furthermore, we report the time evolution of energy propagation in space in the laser ablation process. The temporal-spatial distribution of energy indirectly indicates the temperature evolution on the surface of the Xe solid under intense laser illumination.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of the physical problem of laser ablation. The radius of the laser spot is 8 Å. The atoms that are assumed to be ionized are in the hemispherical region under the laser spot.

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Figure 2

Relationship between the interatomic force and atom distance for the combination of the Lennard-Jones and Coulomb potentials for the xenon solid. The inset shows the Lennard-Jones potential for neutral xenon atoms with σ=4.3 Å and ε=0.0197 eV.

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Figure 3

Total energy versus time for the Xe system under laser ablation with Charge +1, Charge +2, and Charge +3. The short dashed-line (black) labeled with no laser indicates the baseline case that no laser energy is applied into the Xe system.

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Figure 4

Side-view morphology under the Charge +1 bombardment, a dimple (50±3) Å in diameter and (50±3) Å in depth is formed. (a) Nonrelativistic MD case and (b) relativistic MD case. Laser spot size is 8 Å in diameter. Insets show the morphology at a later time.

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Figure 5

Side-view morphology under the Charge +2 bombardment, a dimple (100±3) Å in diameter and (100±3) Å in depth is formed. (a) Nonrelativistic MD case and (b) relativistic MD case. Laser spot size is 8 Å in diameter. Insets show the morphology at a later time.

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Figure 6

Side-view morphology under the Charge +3 bombardment, a dimple (150±3) Å in diameter and (150±3) Å in depth is formed. (a) Nonrelativistic MD case and (b) relativistic MD case. Laser spot size is 8 Å in diameter. Insets show the morphology at a later time.

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Figure 7

Relationship with the depth of the dimples and the difference positive charge under the bombardment of a single laser pulse

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Figure 8

Effects of repeated bombardments on the ablation depth. The transmissivity is defined as the ratio of the ablation depths between two consecutive laser illuminations.

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Figure 9

Mean speed of the particles in the nonrelativistic and relativistic models. The speed is in units of multiples of light speed.

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Figure 10

Time evolution of energy distribution for the Charge 1 case. (a) t=1 ps, (b) t=5 ps, (c) t=10 ps, (d) t=15 ps, (e) t=20 ps, and (f) t=25 ps.

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Figure 11

Time evolution of energy distribution for the Charge 2 case. (a) t=1 ps, (b) t=5 ps, (c) t=10 ps, (d) t=15 ps, (e) t=20 ps, and (f) t=25 ps.

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Figure 12

Time evolution of energy distribution for the Charge 3 case. (a) t=1 ps, (b) t=5 ps, (c) t=10 ps, (d) t=15 ps, (e) t=20 ps, and (f) t=25 ps.

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Figure 13

Spatiotemporal distribution of energy wave at time=1, 5, and 10 fs after laser ablation for the case of Charge +3. The inset shows the schematic of the direction of energy propagation away from the dimple.

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Figure 14

Temperature variations versus time. The inset indicates the location of the temperature measurement that has been taken on the surface of the model.

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Figure 15

Total energy of the system from the beginning to the end of the simulation. At time equal to 40 fs, ion Charge +1 is assigned to atoms affected by the laser pulse.

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