Investigation of the High Energy Behaviours of the Scalar Particle   Scattering Amplitude in the Gravitational Field by Functional Approach
  Closed expressions for the Green function and amplitude of the scalar particle scattering in the external gravitational field $g_{\mu\nu}(x)$ are found in the form of functional integrals. It is shown that, as compared with the scattering on the vector potential, the tensor character of the gravitational field leads to a more rapid increase of the cross section with increasing energy. Discrete energy levels of particles are obtained in the Newton potential. 
Nguyen Suan Han