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Inverse bremsstrahlung cross section estimated within evolving plasmas using effective ion potentials

In this paper we estimate the total cross sections for field stimulated photoemissions and photoabsorptions by quasi-free electrons within a non-equilibrium plasma evolving from the strong coupling to the weak coupling regime. Such transition may occur within laser-created plasmas, when the initially created plasma is cold but the heating of the plasma by the laser field is efficient. In particular, such a transition may occur within plasmas created by intense VUV radiation from a FEL as indicated by the results of the first experiments performed at the FLASH facility at DESY. In order to estimate the inverse bremsstrahlung cross sections, we use point-like and effective atomic potentials. For ions modelled as point-like charges, the total cross sections are strongly affected by the changing plasma environment. The maximal change of the cross sections may be of the order of 60 at the change of the plasma parameters (inverse Debye length, kappa, and the electron density, rho_e), in the range: kappa=0-3 A^{-1} and ρ_e=0.01-1 A^{-1}. These ranges correspond to the physical conditions within the plasmas created during the first cluster experiments performed at the FLASH facility at DESY. In contrast, for the effective atomic potentials the total cross sections for photoemission and photoabsorption change only by a factor of 7 at most at the same plasma parameter range. Our results show that the inverse bremsstrahlung cross section estimated with the effective atomic potentials is not much affected by the plasma environment. This observation validates the previous estimations of the enhanced heating effect. This is important as this effect may be responsible for high energy absorption within clusters irradiated with VUV radiation.