Mode-specific effects in resonant infrared ablation and deposition of polystyrene

Abstract

Low molecular weight polystyrene (∼10 kDa) was ablated with a free-electron laser at 3.31 and 3.43 μm and deposited as thin films on Si(100) substrates. The vibrational bands at 3.31 and 3.43 μm correspond to phenyl-ring CH and backbone CH₂ modes, respectively. Even though the absorption coefficients of these two modes are nearly the same, the ablation yield was approximately 50% higher for the ring-mode excitation compared with the backbone mode. Based on spectral line width, the ring-mode lifetime is approximately triple that of the backbone mode, leading to a higher spatiotemporal density of vibrational excitation that more effectively disrupts the relatively weak Van der Waals bonds between neighboring polymer chains and consequently to higher ablation efficiency of the ring mode. Molecular weight assays of the deposited films showed that relatively little bond scission occurred and that the average molecular weight of the films was similar to that of the starting material.