Excitation of 2 1S, 2 1P and 3 1P, 3 3P levels of helium in He+ on He collisions at a few hundreds eV

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Abstract
The excitations of Helium on 2 1S, 2 1P and 3 1P, 3 3P levels in He+ on He collisions at low energy are investigated experimentally. The data are interpreted in the frame of a semi-diabatic description of He+2 , using a semi-classical treatment of the collision. 2 1P differential cross sections at 100 and 150 eV (Lab) and angular ion-photon correlation functions at 100 eV (Lab) and laboratory scattering angles of 10.25 and 12.5 deg. have been measured using an ion-photon coincidence technique. These data, together with previous measurements on 2 1S excitation, show that channels 2 1P and 2 1S are coupled by a secondary rotational mechanism. The effects of such mechanisms on the differential and total cross sections of both excited channels are examined in the général case using a 3 state model including two Σg states and one Πg state. 3 3P differential cross sections have been measured at 120 and 150 eV (Lab) by detecting in coincidence the scattered ion and the 3 3P-2 3S photon (λ = 3 889 Å). Coincidence polarization measurements at 150 eV, 13.5 deg. (Lab) show that the symmetry of the final molecular state leading to 3 3P is Σ. In spite of the large number of secondary mechanisms involved in the 3 3P excitation (which produce Rosenthal effects in the total cross section), it is shown that an extremely simplified two state calculation is able to give the général features of 3 3P differential cross section, which are mainly determined by the primary mechanism. A similar coincidence technique operating on the 3 1P-2 1S line (λ = 5 016 Å) has been used to measure the 3 1P differential cross section at 150 eV (Lab). A calculation of the 3 1P excitation using a 3 state model is consistent with the polarization measurements at 150 eV, 12, 13.5 and 15.5 deg. (Lab) which show that the symmetry of the final molecular state is Π