By our, now well known, definition, the Coulomb elastic scatter between two charged objects is that in which the sum of the pre-scatter kinetic energies is equal to that of the post-scatter state. In such a case, the projectile and target entities are to be regarded as singular entities without any internal degrees of freedom. Excitation or ionisation channels, which would redirect this energy elsewhere and away from the post-scattering kinetic energies, are either not present or else the magnitudes of the energies extracted through these channels are sufficiently low that they can be ignored.

In Chap.​ 2, we developed the quantum theory of scattering and have emphasised the fact that the observer (i.e. the experimentalist) can only measure the asymptotic forms of the pre- and post-scattering states. This will prove the foundation of our derivations in the following three chapters of Part II.