Dynamics of plume and crater formation after action of ferntosecond 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 Fin, of incident radiation exceeds a threshold of evaporation (F-inc)(ev). It has been shown previously that the spallation layer is formed at fluence above the ablation threshold (F-inc)(abl). Thus the spallation exists within the limits (F-inc)(abl) < F-inc < (F-inc)(ev). A laser beam has a maximum fluence (F-inc)(c) on the axis of the beam. The threshold F-ev separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if F-abl < F-c < F-ev, and (2) with the shell having an aperture in the center (like the volcano muzzle) if F-c > F-ev. (C) 2007 Elsevier B.V. All rights reserved.