A monomer unit of PMMA. Coarse-grained beads are grouped by circles and the Norrish type I and II cleavages used in the simulations are denoted.
Yield in MMA equivalents (left ordinate) is plotted as a function of fluence using 100 (red circles) and (blue squares) penetration depths for (a) pure photothermal, (b) Norrish type I, and (c) Norrish type II simulations using phtotons. The corresponding depth of material removed in angstroms is also plotted on the right ordinate. The lines are added as guides.
Snapshots of the ablation plume are shown: (a) photothermal ablation at , (b) Norrish type I ablation at , and (c) Norrish type II ablation at . The red and gray represent particles of the original polymer, yellow particles represent mechanically broken bonds and radicals, green particles represent gaseous particles, and blue particles represent double-bonded carbon species.
Contour plots of the fraction of substrate converted to products as a function of depth and time for a Norrish type I simulation using a fluence of with (a) a penetration depth of and (b) a penetration depth of . The scale of the fraction of transformation is also given.
The total energy added per monomer is plotted as a function of depth for the top of the substrate for (a) pure photothermal, (b) Norrish type I, and (c) Norrish type II simulations at a time of . For each case, three different fluences are plotted per penetration depth. The short dashed line indicates a fluence below the ablation threshold while the long dashed and the solid lines indicate fluences above the threshold. The depth of material removed from Fig. 2 is denoted on plots that exhibit ablation. The calculated critical energy to initiate ejection of a layer is indicated by a horizontal line.
The average critical energy per monomer to initiate ablation is given for each of the Norrish type I, Norrish type II, and photothermal simulations using 7.9 and photons. Error bars showing one standard deviation are also given.
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