Charge distribution of He I states excited by 10-50 keV proton impact: a study of electron promotion in an atomic Paul trap

Abstract

Collisional excitation of helium atoms by protons of intermediate energies is investigated experimentally. By measuring the intensities of various spectral lines as functions of an electric field applied parallel or antiparallel to the proton beam, we found that the excited He atoms are left after the collision in transient states with large electric dipole moments directed upstream. The experimental results are explained by assuming that during the collision one electron is promoted on the saddle of the two-centre Coulomb potential of the projectile and target. At intermediate energies saddle dynamics is particularly effective due to the stabilization of the electron's motion on the saddle by the Paul-trap mechanism.