Mechanism of resonant infrared laser ablation of polystyrene

Although resonant infrared (RIR) pulsed laser deposition (PLD) was discovered in 2001 and has since been applied to produce technologically interesting thin films and rudimentary opto-electronic devices , the mechanism governing RIR-PLD has been primarily a matter of conjecture. Understanding these mechanisms at the microscopic level has significant implications for process control in thin-film deposition, by linking laser parameters to film characteristics such as surface roughness. This paper demonstrates new experiments and calculations on the model material polystyrene that facilitate quantitative conclusions about these issues. Finite-element modeling of the thermal evolution of the target in RIR-PLD suggests that the initial phase of RIR ablation is spinodal decomposition (phase explosion) of a superheated surface layer. This event is accompanied by a spontaneous structural degradation of polymer chains that are able to enter the gas phase only by a reduction in molecular weight. Subsequently recoil momentum generated by the expanding vapor plume drives the ejection of melted material at non-normal angles, a characteristic signature of recoil-induced ablation.