Abstract
Excitation of helium atoms by 50-500 keV proton impact to singly excited states was investigated experimentally. By measuring the intensities of He I spectral lines as functions of an axial electric field applied to the collision volume, we analysed the electric charge distributions of collisionally excited states with principal quantum numbers n = 4, 5 and 7. The experimental results reflect the transition from intermediate- to high-energy processes. States with large electric dipole moments were found in the lower part of the investigated energy range indicating a quasi-molecular evolution of the collision system. States with dominant components and accordingly very small electric dipole moments were found for 400 and 500 keV proton impact, signifying that the high-energy limit is reached.
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