Simulations of Laser Ablation of Poly(methyl methacrylate):  Fluence versus Number of Photons

Abstract

Using molecular dynamics simulations with an embedded chemical reaction scheme, two pathways of chemical decomposition of poly(methyl methacrylate) leading to ablation are independently investigated using two photon energies. The model employed in this study uses a coarse-grained substrate with predetermined reaction channels and a fixed penetration depth. The simulations allow for single-photon absorption per site with the possibility of photochemical bond cleavage. Within these parameters, the absorption of 7.9 and 5 eV photons (equivalent to 157 and 248 nm radiation, respectively) is simulated over a range of fluences to demonstrate the fundamental photochemical effects of the number of photons versus the total energy absorbed. Above the ablation threshold, similar mechanisms of ejection occur using either photon energy, but, at equal fluences, more material is ejected with a larger number of lower energy photons. When more photons are absorbed cleaving a greater number of bonds, more transformation occurs deeper within the substrate, and the amount of ejected material increases. If thermal as well as photochemical processes occur, the ablation threshold is shifted to a higher fluence. The amount of residual photon energy following the bond scission affects the ablation plume composition.

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

“…The choice of chemical reactions and their influence on the substrate are subjected to more detailed investigations reported elsewhere. [72][73][74]…”

“…A detailed investigation into the ablation threshold as a function of laser processing parameters such as wavelength and pulse width will be reported in an upcoming publication. 74 Selected snapshots from two different simulations-for ͑i͒ 8 mJ/cm 2 fluence at = 157 nm and ͑ii͒ 15 mJ/ cm 2 fluence at = 248 nm-are shown in Fig. 8 corresponding to the end of the runs at 450 ps.…”

“…A more detailed study of the effect of this substrate modification will be reported elsewhere. 73,74 The energy generated by reactions continues to heat the substrate for a long time, but on a much smaller scale. Unzipping and hydrogen abstraction reactions will continue until all the radicals are exhausted ͑i.e., terminated or ejected͒ or the substrate cools down substantially.…”

Two-Photon Polymerization of Inorganic-Organic Hybrid Polymers as Scalable Technology Using Ultra-Short Laser Pulses

“…Well-established methods such as laser welding (Sieben & Brunnecker, 2009;El-Bataghy, 1997;Song et al, 2009), laser ablation (Conforti et al, 2007;Baudach et al, 2000;Spyridaki et al, 2003;Blanchet et al, 1993), and direct laser writing (Schweizer et al, 2008;Koechlin et al, 2009) were improved concomitantly with the laser quality (e.g. beam profile) or the laser energy (e.g.…”

Modification of poly(methyl methacrylate) by keV Ar deposition

“…Poly(methyl mathacrylate) (PMMA) has been extensively studied computationally [19][20][21] and experimentally [22][23][24] in regards to wide range processes related to surface modification. During modification, properties ranging from chain scission to cross-linking were observed to be highly dependent on the deposition dose, energy and temperature, when compared to other polymers [23].…”