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Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation
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10.1063/1.2783898
/content/aip/journal/jap/102/7/10.1063/1.2783898
http://aip.metastore.ingenta.com/content/aip/journal/jap/102/7/10.1063/1.2783898
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Schematic sketch of the simulation setup shown for the initial MAPLE target containing of polymer chains. The polymer chains are shown in blue and are superimposed on top of the image of matrix molecules shown in the background. The color of matrix molecules schematically shows the energy density deposition by the laser pulse.

Image of FIG. 2.
FIG. 2.

(Color online) Snapshots from simulations of laser ablation performed for pure matrix (a) and MAPLE targets with polymer concentrations of 1 (b), 3 (c) and (d). The same fluence of is used in all four simulations. The polymer chains are shown in blue and are superimposed on top of the image of matrix molecules shown in the background.

Image of FIG. 3.
FIG. 3.

(Color online) Snapshots taken at after the beginning of the laser pulse in simulations of MAPLE. Laser fluences are shown above the snapshots. Polymer concentrations in MAPLE targets are 1, 3, and in (a), (b), and (c), respectively. The polymer chains are shown in blue and are superimposed on top of the image of matrix molecules shown in the background. The initial positions of the target surface before the laser irradiation are shown in the left frames by red dashed lines.

Image of FIG. 4.
FIG. 4.

(Color online) Total yield of matrix molecules and polymer units (closed symbols) and yield of polymer chains (open symbols) as function of laser fluence in simulations performed for MAPLE targets with polymer concentrations of (red squares), (green circles), and (blue triangles). The black solid line represents prediction of the ablation model, Eq. (2), where the critical energy is assumed to be equal to the cohesive energy of the matrix material, , and the laser penetration depth of pure matrix, , is used. The dashed lines correspond to a percentage of the solid line that equals the amount of polymer in the original target.

Image of FIG. 5.
FIG. 5.

(Color online) Yield of individual matrix molecules as a function of laser fluence in simulations performed for MAPLE targets with polymer concentrations of (red squares), (green circles), and (blue triangles). The data are for after the beginning of the laser pulse. The lines are guides to the eye.

Image of FIG. 6.
FIG. 6.

Evolution of the total molecular yield (a) and the yield of matrix monomers (b) in a simulation performed with a fluence of and polymer concentration of . The total yield is defined as the number of ejected matrix molecules and polymer units (mers). The arrow in (a) corresponds to the separation of a large cluster, shown in the left inset in Fig. 7(a), from the target at .

Image of FIG. 7.
FIG. 7.

(Color online) Evolution of the sizes of three largest clusters ejected in a simulation performed at a fluence of and polymer concentration of . Internal temperatures and polymer concentrations in the clusters at early times of (a) 700 and (b) , as well as at the end of the simulation, , are shown in the figure. The temperature values are expressed through the threshold temperature for the phase explosion, . In (a), the elongated cluster disintegrates into three large and one smaller clusters at , as illustrated by insets showing the cluster at 1 and .

Image of FIG. 8.
FIG. 8.

(Color online) Flow velocities in the direction normal to the surface shown for the ejected matrix molecules and clusters containing polymer molecules as a function of the distance from the initial surface of the target. The velocities are plotted for three different times, 0.1, 0.5, and , during the simulation performed with a fluence of and polymer concentration of . The small black dots show the average velocities of the matrix monomers. The large colored circles show the velocities of individual molecular clusters containing polymer molecules, with color indicating the number of polymer chains in the clusters, from red color used for clusters containing more than ten polymer chains to blue color used for clusters with only one chain.

Image of FIG. 9.
FIG. 9.

(Color online) Angular distributions of the ejected molecules and clusters of different sizes in simulations performed at a fluence of for (a) pure matrix and (b) MAPLE target with a polymer concentration of . To provide a statistically adequate representation of the distributions for large clusters, the distributions are plotted for groups of clusters, as indicated in the legends. The densities of molecules/clusters per solid angle are normalized to the maximum of the distributions. The distributions are shown for after the beginning of the laser pulse.

Image of FIG. 10.
FIG. 10.

Cluster size distributions in the ablation plume at after the beginning of the laser pulse in simulations performed at a fluence of for pure matrix (a) and MAPLE targets with polymer concentrations of 1 (b), 3 (c), and (d). The yields of clusters are normalized to the number of individual matrix molecules shown in Fig. 5. Bimodal power-law distributions are observed in all simulations, where is the number of matrix molecules and polymer units (mers) in clusters.

Image of FIG. 11.
FIG. 11.

(Color online) Fluence dependence of the exponents in the bimodal power-law fits of the cluster size distributions, such as the ones shown in Fig. 10. The exponents for small (up to 20 molecules) and large (more than 20 molecules) clusters are shown for after the beginning of the laser pulse in simulations performed for MAPLE targets with polymer concentrations of (red squares), (green circles), and (blue triangles).

Image of FIG. 12.
FIG. 12.

(Color online) Snapshots of the target surface region taken at the end of the simulations performed for MAPLE targets with polymer concentrations of 1 (a), 3 (b), and (c). Laser fluences are shown above the snapshots and the times the snapshots are taken are shown below the snapshots. The polymer chains are shown in blue and are superimposed on top of the image of matrix molecules shown in the background.

Image of FIG. 13.
FIG. 13.

Evolution of the average temperature and the polymer concentration in the surface region of a MAPLE target with initial polymer concentration of irradiated by a laser pulse at a laser fluence of (a), and the distributions of the polymer concentration and the density of the molecular material at the end of the simulation (b). The values of temperature and concentration in (a) are averaged over the remaining part of the original deep surface region represented in the simulation. The temperature values in (a) are normalized to the threshold temperature for the phase explosion, . The density of the molecular material in (b) is normalized to the density in the original target, . The initial polymer concentration profile is shown schematically by a dash-dotted line, whereas the solid lines are just guides to the eye showing trends in the variation of the polymer concentration in (b). The arrow in (a) corresponds to the separation of a large cluster, shown in the left inset in Fig. 7(a), from the target at .

Image of FIG. 14.
FIG. 14.

(Color online) Schematic representation of the scenario for the formation of the experimentally observed “deflated balloon” surface features suggested based on the results of the simulations discussed in Ref. 32.

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/content/aip/journal/jap/102/7/10.1063/1.2783898
2007-10-11
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation
http://aip.metastore.ingenta.com/content/aip/journal/jap/102/7/10.1063/1.2783898
10.1063/1.2783898
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