Double ionization of water molecules by proton impact: the role of the direct ionization mechanism Dahbia Oubaziz, Rachida Boulifa, Zakia Aitelhadjali and Christophe Champion Eur. Phys. J. D, 72 6 (2018) 119 Published online: 21 June 2018 DOI: 10.1140/epjd/e2018-80307-9