Suppose we have an electron $e$ and a proton $p$ colliding to form a hydrogen atom that is the reaction $e+p\rightarrow H + \gamma$ where $H$ is an hydrogen atom and $\gamma$ is some photon.
The scattering amplitude should be I think $A=\langle \gamma , \psi |S|e,p\rangle$
Here $\psi$ is a Hydrogen bound state.
Is there any book or paper that treat this problem?