Angular momentum coherences of helium atoms excited by proton impact

Using electric-field anticrossing techniques, we investigated the coherent excitation ofn=5–8 He I states with different orbital angular momenta by proton impact. These measurements give strong evidence that saddle dynamics of H ₂ ⁺ -like systems are well suited for describing the final phase of 12.5 keV H⁺-He collisions. We conclude that, besides electron promotion via the 2pσ orbital, the collision system undergoes diabatic 1sσ–3dσ transitions during the close encounter and present an explanation of the electric dipole moments measured for excitation of H(n=2) states by H⁺-He and H-He collisions.