Note on low-energy proton pairs in collision-induced H₂dissociation

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++Ox from the residual gas which leads to proton pairs with energies in that range.