Relativistic two-photon bremsstrahlung
  An approximate approach for the description of the two-photon bremsstrahlung emitted by a relativistic projectile scattered in a spherically-symmetric field is developed. It based on the accurate treatment of the analytical structure of the singularities in relativistic one-photon free-free matrix elements. For the case of a Coulomb field the analytical expressions for the amplitude and cross section of process are presented. Numerical results obtained within the framework of the proposed approach are compared with the available experimental data and with the results of simpler theories. 
A.V. Korol, I.A. Solovjev