Electron correlation in the double excitation of the helium atom by fast charged particles

Double excitation cross sections to the 2s2p state of the helium atom by fast proton and antiproton impact are calculated by perturbation expansion through second order. The initial and the final states are described by correlated wavefunctions. The importance of different mechanisms and of the electron correlation is analyzed. Cross sections for negative projectiles are found to be higher than those for positive projectiles. The present results are compared with other theoretical calculations and with the experimental data. (orig.)