Translational characteristics in the ablation of chlorobenzene van der Waals solids at 248 nm

Time-of-flight quadrupole mass spectrometry was employed for the characteri zation of the desorbate translational distributions in the ablation of cond ensed C6H5Cl films with KrF excimer laser nanosecond and subpicosecond puls es. In the nanosecond case, the desorbate most probable velocities are foun d to exhibit distinctly different dependences on the laser fluence in the a blative and the sub-ablative regimes. Most importantly, the change from the one regime to the other is found to result in a sharp increase in their va lue. This abrupt change provides strong evidence for the existence of a wel l-defined fluence threshold for UV ablation of van der Waals films to occur . The implications of the results for material ejection mechanisms and post -desorption collisional dynamics in the sub-ablative and ablative ranges ar e discussed. Examination of the corresponding process with 500 fs pulses in dicates that the desorbate translational distributions are not very depende nt on the laser pulse width.