Intra-atomic Electron-Electron Scattering in $\mathit{p}$-He Collisions (Thomas Process) Investigated by Cold Target Recoil Ion Momentum Spectroscopy

We have performed a kinematically complete experiment for the transfer ionization reaction $0.15–1.4\mathrm{MeV}p+\mathrm{He}\to {\mathrm{H}}^0+{\mathrm{He}}^{2+}{+e}^{-}$ by measuring the three-dimensional momentum vector of the ${\mathrm{He}}^{2+}$ ion in coincidence with the scattering angle and the plane of the ${\mathrm{H}}^0$. In the measured fourfold differential cross section we can clearly distinguish between the independent two-step process of capture plus ionization and the correlated $e$- $e$ Thomas scattering. We find a $v_P^{-7.4\mathrm{}\pm 1}$ scaling for the total cross section of the $e$- $e$ Thomas process.