Analysis of plume following ultraviolet laser ablation of doped polymers: Dependence on polymer molecular weight

Abstract

This work investigates the effect of polymer molecular weight MW on the plume characteristics of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in vacuum at 248nm. Laser-induced fluorescence probing of the plume reveals the presence of ArH products (NapH and PhenH from, respectively, NapI- and PhenI-doped films). While a bimodal translational distribution of these products is observed in all cases, on average, a slower translational distribution is observed in the low MW system. The extent of the observed dependence is reduced as the optical absorption coefficient of the film increases, i.e., in the sequence NapI∕PMMA, PhenI∕PMMA, and PS-doped films. Further confirmation of the bimodal translational distributions is provided by monitoring in situ the temporally resolved attenuation by the plume as it expands in vacuum of a continuous wave helium–neon laser propagating parallel to the substrate. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires that a critical number of bonds are broken.