Finite-cluster multiple-scattering theory of x-ray bremsstrahlung isochromat spectra

A real-space finite-cluster multiple-scattering method for the calculation of x-ray bremsstrahlung isochromat spectra is presented. This method makes it possible to calculate the spectra in both the near and the extended energy region even for materials lacking full translational symmetry. Various approximations can be introduced within the multiple-scattering formalism very naturally so that an optimal ratio between the computational effort and the accuracy can be achieved. We illustrate the efficiency of this method by calculating the isochromat spectra of Cu and Pd and comparing them with experiment. Both the dipole and the quadrupole contributions to the bremsstrahlung transition-matrix elements are quantified and the latter are found to be negligible. The choice of the most suitable one-electron potential is discussed. The spectra calculated by using a non-self-consistent Mattheiss potential are very close to the results obtained using a self-consistent potential. The energy-dependent Dirac-Hara exchange term gives better results than the energy-independent Kohn-Sham exchange potential.