Negative absorption by one ionized gas due to Coulomb scattering

A possible effect connected with the process of stimulated Bremsstrahlung emission and absorption by free electrons is the negative absorption of an electromagnetic wave in an electron gas with drift velocity relative the to scattering centers. Starting with the Fokker-Planck equation some authors did find a negative conductivity for drift velocity higher than the electron thermal velocity. However, others, using classical arguments, show that the negative absorption due to collisions in a drifted electron plasma does not exist, because a drifted Maxwellian does not satisfy the Boltzman equation. Due to this controversy, in this paper we present a re-examination of this problem in terms of the center of mass approach, using classical and quantum arguments and considering the effects of the electron velocity fluctuations of the center of mass. Our conclusion is that for Coulomb centers the electrical conductivity is always positive. (author)