Optical Breakdown Plasmas: X-ray Emission and Picosecond Co2 Laser Pulses

front. In this layer the energy conservation can be writ ten as I = jA, where j is the mass flux removed from the target and A is the energy characterising the interaction (damage). For small intensities the absorbed power is spent in atom ionization, A (1) -~ const, and the velocity of the removed substance is small and almost independent o f intensity. Then J ~ jo ~ I, J /E ~ const. (I) and h ] ~ I. For high intensities the absorbed power appears as a plasma kinetic energy, A (kT/m) ~ 1 (where m is the atom mass), / ' ( /) ~ const, and h (I) ~ const. Concerning this model the data for Pb, Zn and AI correspond to the low-inten]ity regime. For Bi both regimes were put into evidence. The relative disposal o f the curves in fig. 2 also agrees with the ionization potentials per gram for these metals. The data permit also the calculation o f plasma parameters. Thus, in the case of Bi, on the damage front the plasma density considerably exceeds the critical density, n* = 1019 cm -1. The radiation is absorbed in a layer where the density decreases by diffusion under the critical value. The inverse Bremsstrahlung absorbtion coefficient in this layer which is higher than 103 cm -1 , ensures on the spot dimension d ~ 5X 10 -z cm a considerable adsorbtion. The recombination radiation in this layer delivers a radiation which can penetrate through and heat effectively a plasma o f such parameters. Further experimental evidences could be also interpreted with this model. Finally we want to point out that the pulsed CO2 laser interaction in the aforementioned intensity range with highly absorbing targets, creates the possibility to study postcritical density plasmas.