Double excitation of helium by fast protons and antiprotons

Abstract

The charge sensitivity of this two-electron process is investigated as a tool for understanding correlation. The helium wave function is approximated by a sum of pair-products of one-electron wave functions, with the coefficients chosen by diagonalizing the fully correlated two-electron Hamiltonian. Thus, spatial correlation is included in both the asymptotic and scattering regions by using these configuration interaction (CI) wave functions for initial, intermediate and final states. Use of CI wave functions also allows the first-order contributions to be expressed in closed, analytical form. Both the energy-conserving and energy-nonconserving parts of the second-order amplitude are found. The former (a correlated generalization of the independent electron approximation) is analytical and the latter is a one-dimensional integral. It is found that the double excitation cross sections are sensitive to the sign of the projectile charge, but that the energy region where this sensitivity is of the same order as for double ionization is 0.1 to 0.5 MeV/amu, whereas the latter has peak charge sensitivity at 1.5 MeV/amu. Comparison is made with the experimental results of Pedersen and Hvelplund [1] and Giese et al. [2].