Quasiclassical Green function and Delbruck scattering in a screened Coulomb field

A simple integral representation is derived for the quasiclassical Green function of the Dirac equation in an arbitrary spherically-symmetric decreasing external field. The consideration is based on the use of the quasiclassical radial wave functions with the main contribution of large angular momenta taken into account. The Green function obtained is applied to the calculation of the Delbruck scattering amplitudes in a screened Coulomb field.