Simultaneous excitation and ionisation of helium by fast projectiles

The cross sections for simultaneous excitation and ionisation in helium following impact of fast electrons, protons and C q+ (q=3-6) ions have been investigated by measuring the vacuum-ultraviolet radiation from transitions in HE II (np-1s) in the wavelength region 220-315 AA. The cross sections obtained with 500-2000 eV electrons are found to be larger by a factor of 1.4-2.3 than those obtained with protons of the same velocity (0.92-3.76 MeV). The difference is discussed in terms of an interference between two excitation-ionisation mechanisms.