Bulk photothermal model for laser ablation of polymers by nanosecond and subpicosecond pulses

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Abstract
The main feature of polymeric materials is the hierarchy of bonds between molecular groups. This feature is explicitly taken into account in a model of photothermal laser ablation of polymers derived in the present work. According to this model the reason for laser ablation is a photothermal bond-breaking reaction within the bulk of material originated from laser heating. In order to address the movement of the interface between gaseous and condensed phases, we change the Stefan-like boundary condition considered previously for the Frenkel–Wilson one. In the latter case the activation energy for elimination of a short enough polymer chain from the surface is proportional to the sum of the energies of weak bonds connecting this chain with the surface. We compare predictions of this model with the previously derived Stefan-like bulk model and with the surface photothermal evaporation model with respect to kinetics and dynamics of laser ablation by both nanosecond and subpicosecond pulses, including ablation by two subpicosecond pulses. This consideration suggests experimental evidence that allows one to distinguish between surface and bulk photothermal ablation mechanisms. The parameters used in numerical calculation correspond to the KrF excimer laser ablation of polyimide. The results of the theoretical modeling are compared with the existing data on kinetics of nanosecond laser ablation as well as with findings of time-resolved UV subpicosecond laser ablation experiments.

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