High-energy photon splitting in a strong Coulomb field

The helicity amplitudes of the high-energy photon splitting in the external Coulomb field are obtained exactly in the parameter $\al$. The cases of screened and unscreened potentials are investigated. The consideration is based on the quasiclassical approach, valid for small angles between all photon momenta. New representation of the quasiclassical electron Green function is exploited. General expressions obtained are analyzed in detail for the case of large transverse momenta of both final photons compared to the electron mass.