Ionization-excitation of helium into the np (n = 2-5) states

Cross sections for the simultaneous ionization and excitation to the np sta tes (n = 2-5) of the helium atom by fast proton and antiproton impact have been calculated. We have applied the impact parameter method and have used second-order perturbation approximation. Electron correlation has been take n into account in the initial state but has been neglected in the final sta te. Our cross sections are compared to the experimental data. (C) 1999 Publ ished by Elsevier Science B.V.