Discretized close-coupling calculation of double excitation of He by proton impact

Double excitation of He by impact of high energy protons and bare positive ions has been observed experimentally using electron spectroscopy. The measured electron spectra show resonance structures associated with the doubly excited states populated in the collision. These resonances exhibit typical Fano profiles due to the interference between autoionization following double excitation and ionization by projectile impact. In this the authors present, for the first time close-coupling calculations which reproduce the interference profiles observed experimentally. They have studied double excitation of He(2s21Se, 2s2p1P0, 2p21De) by 150-3000 keV protons using the impact parameter method. They have used a one center expansion of the electronic wave function on the He nucleus. The basis states are defined in the framework of the Feshbach theory: double excited states are obtained in a Q subspace whereas non-resonant (bound and continuum) states are obtained in the complementary subspace P. The Q states are obtained using the pseudopotential Feshbach method and the P states are represented with standard L2 STO bases using a discretization technique. The basis set includes the ground state, the 2s21Se, 2s2p 1P0, and 2p21De doubly excited states (Q subspace), the first excited states and all discretized continuum states of He with E < 500 eV from L = 0 to L = 4 (P subspace). Having solved the corresponding system of 104 coupled equations, P - Q couplings, which are responsible for autoionization, are introduced at the end of the collision. Their results for double differential cross sections, Shore and Fano parameters are in good agreement with recent experimental measurements by the Toulouse group at 2 MeV, and show that excitation to S and D states are second order processes which cannot be described in the Born approximation, even at energies as high as 2 MeV