Dynamics of ion expulsion in ultrashort pulse laser sputtering of Al₂O₃

The mechanism and dynamics of ion expulsion in near IR, ultrashort pulse laser sputtering, or ablation, of c-Al₂O₃ is studied using a combination of time-of-flight mass spectrometry and femtosecond laser pump-probe techniques. Coulomb explosion (CE) is identified as the dominant mechanism for ion expulsion under low fluence irradiation with a low number of laser shots (`gentle' ablation regime). The momenta of the emitted ions are equal under these conditions. This is explained by invoking impulsive CE from the surface region lasting ca. 1 ps, as confirmed by pump-probe measurements. After sufficient incubation (`strong' ablation regime) the velocity distributions are shifted to lower values, evidence for `phase explosion' is seen and the ions no longer have the same momenta but tend towards the same kinetic energies. Similar low velocity contributions are observed for low fluence pulse durations of 2.4 ps. Pump-probe measurements indicate that this is closely related to the onset of electron-phonon coupling during the laser pulse.