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

The appearance of low-energy protons is a well known feature in ion-H 2 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 H 2 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 H 2 O to H + +H + +O x from the residual gas which leads to proton pairs with energies in that range.