Double excitation of helium by fast electrons, protons, and C(4-6)+ ions.

The electron emission yield from the doubly excited /(2p/sup 1/)/sup 2/D+(2s2p)/sup 1/P/ states in He excited by impact of fast equivelocity electrons, protons, and carbon ions (charge state 4--6) has been measured at the reduced energy of 1.84 MeV/amu. The data constitute a challenge to the recently proposed models that incorporate electron-correlation effects in atomic collisions.