Theoretical and experimental studies on double excitation in proton–helium collisions are reviewed. Two theoretical approaches, which are common in the treatment of atomic collision processes, are described: perturbative approaches and the close coupling method. Experimentally, double excitation has mainly been studied by spectroscopy of the autoionized electrons emitted by the decay of the doubly excited states and by projectile energy-loss spectroscopy. The results emerging from the theoretical and experimental studies include the following points: first, the coupling of the doubly excited states to the continuum is very important in the electron spectra leading to pronounced interference effects. Second, double excitation mechanisms involving the electron–electron interaction are dominant except for low projectile energies. Third, interference effects between various double excitation mechanisms appear to be insignificant in the cross-sections differential in the electron emission angle, but might be important under certain conditions in the cross-sections differential in the projectile scattering angle.
Doubly Excited States in Helium Close to the Double Ionization Threshold: Angular and Energy Resolved Partial Cross Sections
