Coulomb logarithm in femtosecond-laser-matter interaction

In femtosecond-laser-matter interaction, collisional absorption plays an important role during the early stages of the interaction, when the laser intensity and the plasma temperature still have moderate values. We propose a cutoff impact parameter b(max) for the Coulomb logarithm lambda=ln Lambda in the electron-ion collision rate which takes into account, for an overdense plasma, the crystalline structure of the ion background. Calculations are presented for a bcc lattice and generalized to sc and fcc lattices. The results are applicable for laser intensities I(l)< or =10(17) W cm(-2) and when the Debye theory is valid, that is to say, when the Landau length is distinctly less than the minimum ion distance, and when the Debye screening is unaffected by the electron quiver motion in the laser field. We present the space angle dependence of the cutoff b(max) as also its spatial average, and we discuss the ensuing corrections to the Coulomb logarithm due to the improved values of b(max). Finally, we present the pertinent values, averaged over a Maxwellian distribution function, which are needed for applications of the ballistic model for collisions.