  It is shown that negative ions are ejected from gas-phase water molecules
when bombarded with positive ions at keV energies typical of solar-wind
velocities. This finding is relevant for studies of planetary and cometary
atmospheres, as well as for radiolysis and radiobiology. Emission of both H-
and heavier (O- and OH-) anions, with a larger yield for H-, was observed in
6.6-keV 16O+ + H2O collisions. The ex-perimental setup allowed separate
identification of anions formed in collisions with many-body dynamics from
those created in hard, binary collisions. Most of the ani-ons are emitted with
low kinetic energy due to many-body processes. Model calcu-lations show that
both nucleus-nucleus interactions and electronic excitations con-tribute to the
observed large anion emission yield.
