Dynamics of gelatin ablation due to free-electron-laser irradiation

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Abstract
We have carried out simultaneous, time-dependent measurements of the free-electron-laser (FEL)-induced stress transients and ablation plume in gelatin, which serves as a model system for collagenous tissues. The Mark-III FEL is tunable in the mid-IR (2–10 μm) and produces macropulses of microsecond duration comprised of picosecond micropulses separated by 350 ps. The macropulse duration was shortened with a broadband, IR Pockels cell, producing pulse durations as short as 60 ns and energies in the range of 0.1–1 mJ. The IR beam was focused to a diameter of 112–210 μm, depending on the wavelength, and measurements were made at 3.0, 3.36, and 6.45 μm. For fluences below the ablation threshold, stress transients were measured and accounted for with a standard thermoelastic mechanism. Of particular interest were the measurements with fluences above the ablation threshold, where two classes of dynamics were observed. A cw HeNe beam monitors the plume: at 3.0 μm a single maximum of the 'shadow' is observed, while at 3.36 μm and 6.45 μm a second maximum also was resolved at later times. In addition, at 3.36 μm and 6.45 μm the duration of the momentum recoil is about twice as long as that observed for comparable exposure parameters at 3.0 μm.

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