Imaging and modeling of pulse laser induced evaporation of metal films

Evaporation triggered with nanosecond laser pulses in self-supporting aluminum films was imaged by high-speed transmission electron microscopy. This unconventional method provides up to three images from a single transient process with exposure times ⩾5 ns and at intervals ⩾25 ns with a spatial resolution of 100 nm. The chronological order of ablation was observed on the nanosecond time scale by successive shifting of the moment of exposure. Information was obtained about the mode and duration of evaporation. The ablation reveals a threshold behavior. Above a laser energy density of 5–6 J/cm2 the irradiated film region is completely evaporated during the laser pulse. Below this threshold evaporation is marginal and the film disintegrates mainly by liquid flow. The experimental results are compared with a new model comprising two different evaporation mechanisms, surface and volume evaporation. The ablation as observed cannot be explained by surface evaporation only. The strong rise of the evaporation rates above the threshold is associated with the onset of volume evaporation. It is also shown that the vaporization enthalpy must be considered as a function of temperature for a correct modeling of evaporation by short laser pulses.