Relativistic Effects in the Excitation of Triplet Helium States by Electrons

The excitation of triplet He states is a pure rearrangement collision provided that spin-dependent potentials are neglected. The high-energy behavior of this rearrangement cross section is proportional to E^-3. If relativistic spin---other-orbit terms are admitted, they produce contributions to the cross section which go to a constant at moderately high energies. These new terms are incoherent with the old ones and have a relative coefficient of αF⁴, where αF is the fine-structure constant. They become significant at an incident energy of about 8 keV and so are not presently observable. Part of the result obtained here also applies to the case where the incident particle is a proton.