Mass analyzed product ions have been detected in coincidence with the projectile following the ionization of water by proton impact. Measurement of the projectile charge state postcollision enables the different ionization processes to be identified: direct ionization, single electron capture, and double electron capture. A complete set of partial and total absolute cross sections is reported for the direct ionization and electron capture processes initiated by proton collisions at 20–150 keV. The cross sections for the direct ionization of ${\mathrm{H}}_2\mathrm{O}$ by proton impact are compared with previous electron impact results [Straub et al., J. Chem. Phys. 108, 109 (1998)].

• Received 16 December 2002

DOI:https://doi.org/10.1103/PhysRevA.70.062716