Simulations of laser ablation of poly (methyl methacrylate): Fluence versus number of photons

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.