Relaxing melt and plasma bubbles in laser-pulsed metals

Melting and ablation of aluminum, chromium, and nickel films by nanosecond laser pulses with fluences of 2.4-13 J/cm2 were tracked by high-speed transmission electron microscopy and selected area (20 μmo) diffraction. Electron exposure time was 8 ns. At fluences causing no detected evaporation, short-range order in the melt, as probed by diffraction, shows up with a time lag of nanoseconds. At lower 'evaporating' fluences, the material is ablated by exploding gas bubbles nucleating all over the melt within 5 ns. At sufficiently high laser fluences, a plasma is produced with an ionization degree >20%, varying on the micrometer scale, as revealed by dark-field imaging