Excitation of autoionizing states of helium by 100 keV proton impact: theory and experiment

A joint theoretical and experimental study of the excitation of the autoionizing (2s(2))S-1, (2p(2))D-1 and (2s2p)P-1 states of helium by 100 keV proton impact is presented for the first time. The role of the three-body Coulomb interaction in the final state between the ejected electron, the scattered proton and the recoil helium ion is emphasized. Calculations have been carried out with inclusion of the three-body Coulomb interaction and within an expansion of a two-electron excitation amplitude in powers of projectile-target interaction up to the second order. A new parametrization is proposed to describe resonance profiles distorted by the Coulomb interaction in the final state (CIFS). New high-resolution (up to 68 meV) measurements of electron emission spectra made it possible to resolve the near-lying (2p(2))D-1 and (2s2p)P-1 resonances and reveal an evident distortion of the resonance profiles by CIFS for forward electron ejection angles below 40 degrees. Processing of the experimental spectra has been done both with the new parametrization, with allowance for CIFS, and with the Shore formula. Considering the complexity of the problem, reasonable agreement is achieved between experiment and theory.