DOUBLE-EXCITATION OF HELIUM BY ION IMPACT .2. EXPERIMENT AND THEORY FOR 2-3-MEV PROTON IMPACT

Experimental and theoretical studies of the double excitation of heliu m by 2-3 MeV proton impact are presented. A detailed angular dependenc e of the lineshapes and intensities of the first 2l2l' resonances is d iscussed. The resonances are characterized by the Shore parameters A a nd B and the Fano parameter Q. Calculations within Born-I approximatio n describe approximately the excitation of the 2s(2) S-1 and 2s2p P-1 resonances whereas they fail to reproduce the experimental findings fo r the 2p(2) D-1 one. On the other hand, close-coupling calculations im prove the description of the excitation of the 2s2p P-1 and explains v ery nicely the 2p(2) D-1 one. Weak discrepancies in the description of the 2s(2) S-1 and 2s2p P-1 excitation in the forward direction are th ought to be the signature of a residual post-collisional effect. It is shown that it does not affect the observed lineshapes in our collisio n velocity range. The integration of the resonance parameters over the emission angle of the electron allows us to deduce total electron yie lds and to connect the resulting profile with photoionization data.