Collisional excitation of helium atoms by protons of intermediate energies (v p ∼ 1 au) 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.

Single-electron excitation in He+–He collisions is investigated in the 30–50 keV range, where the excitation cross-sections have resonance-like maxima. By measuring anticrossing spectra, we show that the levels of He I with are populated effectively and coherently in this energy range. The superposition states of the excited electron resulting from the collision process are identified as the parabolic Stark states with the largest electric dipole moments. The results are explained with the Paul-trap model and saddle dynamics.

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 quasimolecular evolution of the collision system. States with dominant 1snp 1 P 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.

We calculated the electric-field splitting of the 1snd 1D and 3D levels of the 3He I spectrum for n = 3,4 and 5 and evaluated the anticrossings of singlet and triplet Stark substates. The anticrossing spectra observable after selective excitation of singlet states with asymmetric charge and current distributions are discussed. The anticrossing spectra of 3He I differ significantly from those of 4He I due to the hyperfine interaction and, therefore, provide a valuable means for investigations on collisional excitation of He atoms.

The anticrossing spectra of the helium line λ(1s4l-1s2p 3 P) = 447.2 nm were measured for 10-30 keV He + -He collisions. The theoretical intensity functions were calculated taking into account cascade processes, the inhomogeneity of the axial electric field in the collision volume and the density distribution of the target He atoms. Comparing the theoretical intensities with the measured ones, the post-collisional states of He atoms were determined. The results indicate that for this projectile-energy range, the electronic charge distributions of the excited-atom states change from almost symmetric to strongly asymmetric ones with large electric dipole moments. This result suggests that for projectile energies of 20-30 keV, Paul-trap promotion becomes the main excitation mechanism.