Note on low-energy proton pairs in collision-induced H₂ dissociation

The appearance of low-energy protons is a well known feature in ion-H₂ collisions. Recently even the observation of coincident proton pairs with kinetic energies well below the Coulomb energy has been reported in collisions with 15 keV He⁺. We have studied the dissociation H₂ to H⁺+H⁺ in collisions with 50-350 keV H⁺ and He⁺ ions using a time- and position-sensitive multi-particle detector system. This detector allows the coincident measurement of the momenta of both H⁺ fragments. In this projectile energy range we see no evidence of such low-energy proton pairs. However, we find a strong dissociation channel H₂O to H⁺+H⁺+O^x from the residual gas which leads to proton pairs with energies in that range.