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Coupled molecular dynamics-Monte Carlo model to study the role of chemical processes during laser ablation of polymeric materials
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Image of FIG. 1.
FIG. 1.

Monomeric unit of the PMMA polymer is shown here. Each atom or group of atoms represents a coarse grained bead in the simulation. Norrish type I corresponds to break in C–CO bond, whereas Norrish type II is a break in bond.

Image of FIG. 2.
FIG. 2.

Distribution of the nonbonding interaction energies among PMMA beads at a density of and , computed using sample (b). The dashed curve (squares on the bottom axis corresponds to the intermolecular interations while the solid curve atoms (diamonds) on the tip axis is for the intramolecular interactions. Both the energy scales correspond to eV.

Image of FIG. 3.
FIG. 3.

Grüneisen coefficient , computed as a function of density (in ), from simulations using sample (a), performed over a temperature range of . The dashed curve is added as a guide.

Image of FIG. 4.
FIG. 4.

Pressure vs local velocity computed from impulse simulations. The data shown correspond to a constant downward impulsive force of applied over in the largest sample (c). The data are collected over time scale, from the time of formation of the pressure wave near the top surface to just before it interacts with the bottom surface. Only points used for data fitting are shown.

Image of FIG. 5.
FIG. 5.

The flow chart for the Monte Carlo chemical reaction scheme embedded within the MD time integration. Some flow lines are dashed for visual clarity.

Image of FIG. 6.
FIG. 6.

The set of chemical reactions taking place in a PMMA substrate upon laser induced bond breaks. For simplicity, the complete bonding structure is not shown for some molecules.

Image of FIG. 7.
FIG. 7.

(Color online) The evolution of yield, expressed in number of monomeric (MMA) units, as a function of time for (a) simulations at (circles ●), (diamonds ◆), and (squares ∎) fluences, and for (b) simulations at (deltas ▴), (gradients ▾), and (left triangles ◀) fluences. The corresponding values of average sizes are given in (c) for and in (d) simulations, respectively.

Image of FIG. 8.
FIG. 8.

(Color) The snapshots for two of the simulations: (top) at fluence and (bottom) at fluence. The depth scale (in Å) at zero represents the original top surface. The colors represent original polymer beads (gray), gaseous species (green), and doubly bonded carbon beads (blue). The red colored beads represent oxygen in the original and the transformed substrate.

Image of FIG. 9.
FIG. 9.

(Color online) The distribution of energy (in eV) in the substrate, for at fluence, as a function of time, in terms of energy consumed by bond breaks (triangles ▴), energy added as heat by reactions (diamonds ◆), and total energy added as heat by both laser irradiation and reactions (squares ∎). The total energy and the energy added by reactions overlaps after the end of the laser pulse.

Image of FIG. 10.
FIG. 10.

(Color online) The total number of photoproducts and broken bonds produced in the sample as a function of time. The lines correspond to, from top to bottom, (a) the number of double bonded carbons, i.e, , (b) the number of molecules of , (c) the number of molecules of , (d) the total number of broken bonds, (e) the number of molecules of CO, and (f) the number of molecules of . The lines (e) and (f) are almost overlapping. The simulation parameters are the same as those given in Fig. 9.

Image of FIG. 11.
FIG. 11.

(Color online) The kinetic temperature (in K), measured using lateral velocities, as a function of time. The values correspond to the depths of (a) , (b) , (c) , and (d) below the original surface, respectively. The line represents a least squares exponential fit to the temperature. The simulation parameters are the same as those given in Fig. 9.


Generic image for table
Table I.

Activation and formation energies for all chemical reactions occurring in a PMMA substrate (in eV).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Coupled molecular dynamics-Monte Carlo model to study the role of chemical processes during laser ablation of polymeric materials