Dynamics of plume and crater formation after action of femtosecond laser pulse

Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluence F of incident radiation exceeds a threshold of "evaporation" ()ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold ()abl. Thus the spallation exists within the limits ()abl<F<()ev. A laser beam has a maximum fluence ()c on the axis of the beam. The threshold F separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if F<F<F, and (2) with the shell having an aperture in the center (like the volcano muzzle) if F>F.