The impact of multipole and relativistic effects on photoionization and   radiative recombination cross sections in hot plasmas
  It is shown in the framework of the fully relativistic Dirac-Fock treatment of photoionization and radiative recombination processes that taking into account all significant multipoles of the radiative field is of considerable importance at electron energy higher than several keV. For the first time, we show that the relativistic Maxwell-Bolzmann distribution of continuum electrons should be used in hot thermal plasmas. This decreases the radiative recombination rate coefficient up to several multipoles compared to the non-relativistic distribution commonly used. 
M.B. Trzhaskovskaya, V.K. Nikulin, and R.E.H. Clark