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 helium by 2-3 MeV proton impact are presented. A detailed angular dependence of the lineshapes and intensities of the first 2l2l' resonances is discussed. The resonances are characterized by the Shore parameters A and B and the Fano parameter Q. Calculations within Born-I approximation describe approximately the excitation of the 2s(2) S-1 and 2s2p P-1 resonances whereas they fail to reproduce the experimental findings for the 2p(2) D-1 one. On the other hand, close-coupling calculations improve the description of the excitation of the 2s2p P-1 and explains very 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 thought to be the signature of a residual post-collisional effect. It is shown that it does not affect the observed lineshapes in our collision velocity range. The integration of the resonance parameters over the emission angle of the electron allows us to deduce total electron yields and to connect the resulting profile with photoionization data.