Temporal measurements of photofragment attenuation at 248 nm in the laser ablation of polyimide in air

A 248-nm laser pulse of 35 ns duration was split spatially into two beams: a strong beam used to completely ablate in a single pulse 1000- and 5000-Å-thick polyimide films on fused silica substrates, and a weak, time-delayed probe beam directed almost perpendicularly onto the sample through the ablated fragments in order to measure their transmission versus time. When ablating the 1000-Å-thick film at 1.1 J/cm2, a weak attenuation of the probe beam with a maximum value of 10-12% was observed during the laser pulse. Ablation of the 5000-Å-thick film at 4.2 J/cm2 showed a strong attenuation of 83% which lasted for 90 ns from the beginning of the ablating laser pulse and then dropped to 50% at 130 ns. These results indicate increased attenuation of the laser beam as the laser fluence increases, thus explaining the reduced ablation efficiency at high fluences.