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

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 {μ}m) 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 {μ}m, depending on the wavelength, and measurements were made at 3.0, 3.36, and 6.45 {μ}m. 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 {μ}m a single maximum of the 'shadow' is observed, while at 3.36 {μ}m and 6.45 {μ}m a second maximum also was resolved at later times. In addition, at 3.36 {μ}m and 6.45 {μ}m the duration of the momentum recoil is about twice as long as that observed for comparable exposure parameters at 3.0 {μ}m.