Rearrangement Collisions—Electron Excitation ofHe(2s3)

There are many formal expressions for the $T$ matrix for a rearrangement collision. First-order approximations to the exact wave functions in the different expressions yield different numerical results for the cross section. We have chosen eight forms of the $T$ matrix and have evaluated the eight cross sections for electron excitation of $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}s\right)$, which is a pure rearrangement collision when spin-dependent forces are neglected. The projection-operator formalism for $T$ yields good agreement with experiment.