The interaction of KrF excimer laser pulses with polyimide, poly(ethylene terephthalate), polystyrene, poly(methyl methacrylate), and polyethylene has been studied by timehyphen;resolved reflectivity. The width, fluence, and peak intensity of a KrF (248 nm) laser pulse reflected from the polymer surface was determined over a large range of incident laser fluences, 1ndash;3000 mJ/cm2. The reflected pulse was truncated once the incident fluence exceeded a critical value (threshold), characteristic for each polymer. Above the threshold the pulse decayed exponentially with time constants ranging from 2 to 6 ns. For polyethylene, such a critical value appears to lie above the highest fluence available in the present experiments. The threshold fluences for pulse truncation are usually somewhat lower than the ablation thresholds measured photoacoustically. The mechanism of the process is discussed in terms of timehyphen;dependent absorption and scattering from particles emerging from the surface during ablation. Approximate calculations for ablation under our experimental conditions (ns wide excimer pulses) suggest that large particles are ejected initially which are continuously reduced in size during the laser pulse.
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